Nutrition of Commercial Pecan nut orchards

1. Introduction

The most important environmental factors that influence photosynthesis, flowering, fruit set, fruit growth and fruit quality are light, water and nutrition. Inorganic nutrition of plants was raised for the first time during 1840 when Justus von Liebig published his book on this subject. Since then 17 elements were identified as essential for plant production. Fourteen of these elements, i.e. Nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), sulphur (S), chloride (Cl), iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), boron (B) and molybdenum (Mo), are utilised and taken up from the soil and water. The other three, carbon, hydrogen and oxygen are utilised and taken up from the air and water. The mineral component of plant material is small (less than 15%) and variable and depends on the plant part, age and conditions in which they were produced.

Through the years, in the endeavours to increase plant production, a number of approaches have been tested for inorganic nutrition.

  • Right at the beginning the main objective was to determine, which elements were necessary for plant production.
  • The next step was to identify deficiency symptoms.
  • Next it was endeavoured to improve production by manipulating the inorganic nutrition.

The main result of the use of these endeavours was that plant tissue analysis became important. Leaf analysis has come to the front and has received the most attention. A logical result of leaf analysis was the developing of threshold values.

The next step was to concentrate on certain plant parts and physiological processes. Fruit, vegetative growth, fruit set, cell division and cell growth and their relationship with mineral nutrients were investigated. Nowadays we strive to manipulate physiological processes in order to improve the final result. One example is the accumulation of energy (carbohydrates) during late summer and autumn.

No matter which method is used, plant analysis especially leaf analysis will form an integral part.  Today it is necessary to evaluate the role of each nutrient element in production and to use the most effective method for effective supply.  The best sustainable income will be generated when nutrients are supplied in the most economic way, which ultimately is the goal of a good fertilisation program.

The importance and roll of these essential nutrients are discussed extensively in many publications. This publication will concentrate on the practical aspects that will impact on production of pecan nuts and economical use of fertilisers.

2. Nitrogen

Nitrogen is present in the soil in a number of compounds and forms. The organic fraction in the soils contains the largest portion of the total nitrogen. The nitrogen is present in the form of amines (protein like compounds) which cannot be utilised by the trees. Once the organic matter is decomposed by the microbes, available nitrogen is produced, which can be utilised by the trees. Of all the different nitrogen compounds in the soil, only ammonium- (NH4) en nitrate  (NO3) nitrogen can be utilised by the plants in significant quantities. Plants can only absorb small amounts of urea directly.

Eighty percent (80%) of all nitrogen in the plant is present as protein, ±10% as nucleic acids and the rest as amino and nitrate nitrogen.

Nitrogen deficiencies
A nitrogen deficiency is most critical during the “pre-bloom, fruit set to fruit drop” stage. CO2-assimilation is directly related to the concentration of nitrogen in the leaves (Syvertsen, 1989). In the absence of potassium the assimilated nitrates are not processed completely. A nitrogen deficiency reduces growth in general, limits the branching of roots, results in poor development of the chloroplast and reduces yield drastically.

When the supply of nitrogen through the root is too low to satisfy the demand.

d, nitrogen is relocated from older to young developing plant organs. A mild nitrogen deficiency results in an even yellowing of the complete tree. As the deficiency progresses the yellowing of the older leaves intensify and they are shed. This developed into bare twigs with only leaves at the tip and is quite specific for a continuous nitrogen deficiency.

Table 1. Nitrogen sources

Source
% nitrogen (m/m)
Urea
 46
Ammonium nitrate
34
Other nitrates eg Ca, Mg
 Various
Urea ammonium nitrate
 32*
Limestone ammonium nitrate
 28
Ammonium sulphate nitrate
 27
Ammonium sulphate
 21
Ammonium phosphates
 Various
Ammonium nitrate solutions
 19 to 21*
Nitric acid
 12 to 14*

Nitrogen excess
In it’s simplest form, an excess of nitrogen manifested in trees with dark green leaves growing vigorously. High nitrogen levels during the late summer and autumn will have a detrimental effect on the dormant period, accumulation of carbohydrates and can reduce yield and quality of the next crop. Nitrogen applications at the wrong time will have the same effect on yield and quality as an excessive status. If the required mass of nitrogen is applied during January to April, the trees will experience a nitrogen excess at the period when too much nitrogen will harm the crop. Both the current and the next crop will be jeopardised.

Sources of nitrogen
Plants can only utilise nitrogen in the form of nitrate (NO3-) and ammonium (NH4+) while organic forms and nitrites are only absorbed in very small amounts. A variety of nitrogen sources are available in RSA and they all contain ammonium- and/or nitrate nitrogen or compounds like urea or organic nitrogen that can be converted to these two forms (Table 1). In the RSA the concentration of the nutrient element in the fertilisers are given in mass of the element per kg of fertiliser or % but on a mass-mass-basis (m/m).

These fertilisers are liquids with a density >1,0. Therefore a litre of UAN contains 420g and one litre AN19 215g N and not 320g and 190g respectively.

Urea; Due to the slow rate of hydrolyses of urea to ammonium and nitrate during the winter and under waterlogged (anaerobic) conditions, applications during July and August should not be done. The trees cannot utilise urea directly and under these conditions, a nitrogen deficiency might be induced.

Ammonium nitrate (34%N) is a very popular source of nitrogen worldwide, but it is not available in RSA as a powder.  It contains equal masses of ammonium and nitrate nitrogen in a fairly concentrated form. It is soluble in water and the nitrate part is more subjected to leaching than the ammonium part.

In South Africa ammonium nitrate is available in various other formulations but also as a liquid containing 19 to 21% N (m/m). Limestone ammonium nitrate (LAN) contains 28% nitrogen of which 14% is present as ammonium and 14% as nitrate nitrogen. It also contains about 10% lime which is not meant to counteract the acidification caused by the ammonium nitrogen. The ammonium and nitrate is completely soluble in water but not the lime.

Ammonium nitrogen is oxidised in the soil to nitrite and then nitrate by the nitrifying bacteria. Nitrification is a biological process through which ammonium (NH4) is oxidised by Nitrosomonas sp. to nitrite (NO2) and nitrite to nitrate (NO3) by Nitrobactor spp. During these oxidation processes, acid ions (H+) are produced which acidifies the environment.  The origin of the ammonium has no bearing on the final result. All ammonium nitrogen, whether is from fertilisers or organic material will be nitrified by the bacteria. These processes take about 14 to 21 days to be completed.

Calcium and magnesium nitrates as well as ammonium phosphates are primarily being use to supply the other nutrient in an acceptable form rather that nitrogen.

Nitrogen fertilisation of pecan nut trees.
Nitrogen is the most important nutrient element in fertilisation of pecan nut trees and many other crops. Not because it is required in large amounts but because of its involvement in yield and quality and because the form, the time and quantity applied are all very important.  The nitrogen supply to pecans must be managed to be at its highest during bud break, blossom and fruit set and at a minimum during ripening and harvest.

Splitting of the nitrogen application in conventional irrigation systems (all but single line drippers and open hydroponics) is primarily based on the clay content of the soil. Even with fertigation with microjets and sometimes double line drippers, the spitting is based on the clay content.  In general, as the clay content decreases, the number of applications increases. Table 2 serves as a guideline for the application of nitrogen by means of hand applications, mechanical spreaders and microjets.  The distribution applies to all inorganic sources, organic enriched sources but not for organic material such as compost and kraal manure.

Table 2. Guidelines for the distribution of nitrogen given as a % of the total requirement.

 Clay content of the soil (%)
 % N to be applied in July/Aug
 % N to be applied in Aug/Sept
% N to be applied in Sep/Oct 
% N to be applied in Oct/Nov
<10%
35
25
25
15
10 to 15
50
25
25
16 to 20
 50
 50
21 to 25
 75
 25
>25
 100

The above mentioned schedules only serve as guide line and can further be refined by considering the affectivity of the nitrogen application, leaf status, colour of the leaves and local conditions. The purpose of the distribution is to get the supply to match the requirement. Clay soils have a natural storing capacity for nitrogen and will be able to follow the demand curve with only one single application of nitrogen. A sandy soil has less of a storage capacity and needs more frequent replenishment to supply according to the demand.

Fertigation via microjets requires the same approach as applications by hand. The volume of soil treated by both methods is about the same. Using hand application the fertilisers must be applied where irrigation is applied; making the volumes of soil treated the same. Since Hoagland formulated the first nutrient solution in 1919 no evidence was presented to proof that plants require the nutrients in a narrow concentration range. Only with single line drippers and open hydroponic systems (OHS) the concentration and ratios need to be within certain limits.

Distribution of nitrogen via drip systems is based on other principles. With drippers the volume of soil treated varies from 100 to 500 litres and with microjets from 2000 to 10000litres and even more. An application of 10g N is effective with drippers because it creates a concentration of 20 to 100mg N per litre soil. The same application with microjets represents a concentration of < 5mg N per litre.

Small masses of fertilisers are applied at least at weekly intervals and the distribution is based on physiological processes.

Acidification of the root zone under drip irrigation is more rapid due to the smaller volume of soil treated with the same mass of nitrogen as for microjets. Liming is not effective. The simplest way is to control the pH is to change the ratio of NH4+:NO3- . The higher the NH4+ -component, the more acid will be generated and the lower the pH. A ratio of 20NH4+ to 80NO3- will keep the pH fairly stable around 6,5 to 7,0.  Nitrogen is the only essential nutrient element available to the plants as an anion and a cation and can thus be used in nutrient solutions to manipulate the p

3. Phosphorus

The reaction of tree crops on fertilisation with phosphorus is less spectacular than that of annuals. Trees have the ability to absorbed phosphorus (P) almost all year round. When the demand is less than the absorption, P is stored in the leaves and wood for later use.  Plants can only utilise about 10 to 20% of the P in the soil. The rest is present in an unavailable form (Figure 1) which is continuously supplemented from the unavailable pool.

When the P in the available pool is absorbed the replenishment comes from the unavailable pool but the rate can restrict the supply to the trees.
The most important ionic species of P are HPO4- – and H2PO4- and the specie dominating is determined by the pH of the soil. At a pH(water) of 7,5 the available P will be fixed as tri-calcium phosphate which is also insoluble in water. Liming acid soil or acidifying alkaline soil will mobilise the fixed P. Absorbed P is quickly incorporated in the metabolic processes. The organic forms of P in the plant are very mobile and can be transported up and down in the trees. Young leaves and other organs are supplied with P from older plant organs including the roots as well as freshly absorbed P.

Phosphorus deficiency.
A phosphorus deficiency inhibits a number of processes like the production of energy-rich compounds and components of the electron transfer chain. Without these compounds energy transfer is not possible.

Excess phosphorus
Excessive supply of P has little influence on the condition of the trees or the nut. The major problem is that excessive levels of P will have a negative effect on the availability, absorption and utilisation of copper, zinc, boron, iron and nitrogen.

Figure 1. Cycle of plant available phosphorus in the soil.

Sources of P
In RSA virgin soils contain hardly any available P. However, many phosphate containing deposits are available. This give rise to a variety of phosphate fertilisers (Table 3).  The concentrations of water and citric acid soluble phosphates are used to evaluate the potential of these products for South African agriculture.

Water soluble P is immediately available to plants. Citric acid soluble P needs to be dissolved by the acids in the soil and from the roots before it becomes available.

Table 3.  Phosphate fertilisers available in Southern Africa.

Source P Content* %
WS CAS Total
Super phosphate (single) 10.5 10.5 10.5
Super phosphate (double) 19.6 19.6 19.6
Rock phosphate 0 3.0 12.6
Calmafos 0 9.0 9.5
Phosphoric acid 31.0 31.0 31.0
Mono ammonium phosphates 22.0 22.0 22.0
Mono ammonium phosphates water soluble 26.0 26.0 26.0
Di ammonium phosphate 23.5 23.5 23.5
Mono potassium phosphate 23.5 23.5 23.5

WS  = water soluble phosphorus   CAS    = Citric acid soluble phosphorus
* The actual concentrations of the various P sources may differ a bit from these values.

Fertilisation with phosphorus.

Soil applications.
The efficiency of soil applications of phosphates is restricted by the pH and clay content of the soil.  Fixation by the clay and amorphous materials in the soil is also governed by the pH. One method to reduce the fixation is to apply the phosphorus in a narrow strip of about 5cm wide, below the drip line of the trees where the irrigation water is also applied. This is also called “band placing” or banding of fertilisers. The supers can also be placed in a circle below the drip line. Do not mix the supers with the soil. Only single and double supers can be banded. Ammonium phosphates (MAP and DAP) contains too much ammonium which will scorch the roots when banded. Likewise for potassium phosphates where the potassium is the limiting component. Ammoniated supers can also not be banded.

Figure 2. Illustration of the various methods of banding super phosphates

If the phosphate is spread out evenly over the surface the mass is so diluted and it will be fixed in the top few mm of the soil. Banding is about 80% successful and can be used on alkaline and acid soil and on clayey and sandy soils. On white neutral sands (<5% clay) P will move much quicker into the subsoil and all sources of phosphorus can be spread out.

Fertigation by microjets.
Application of water soluble phosphates like MAP and phosphoric acid are not effective when applied through the microjets. The mass/volume is spread over an area of >20m2 and will be fixed in the top few mm of soil. It is exactly the same kind of problem experienced with hand applications. Successful spreading of phosphates is restricted to neutral sandy soils.

Fertigation by drippers.
An important principle with fertigation and drippers is that water soluble phosphorus must be applied continuously. If the total wetted volume is less than 500 litres, P should be applied once per week for at least 8 months per annum.

Table 3.  Phosphate fertilisers available in Southern Africa.

Source P Content* %
WS CAS Total
Super phosphate (single) 10.5 10.5 10.5
Super phosphate (double) 19.6 19.6 19.6
Rock phosphate 0 3.0 12.6
Calmafos 0 9.0 9.5
Phosphoric acid 31.0 31.0 31.0
Mono ammonium phosphates 22.0 22.0 22.0
Mono ammonium phosphates water soluble 26.0 26.0 26.0
Di ammonium phosphate 23.5 23.5 23.5
Mono potassium phosphate 23.5 23.5 23.5

WS  = water soluble phosphorus   CAS    = Citric acid soluble phosphorus
* The actual concentrations of the various P sources may differ a bit from these values.

4. Potassium

Roll in pecan nut production.
Potassium (K) is involved in activation of many enzyme reactions in the biology of the plant. More than 60 enzymatic reactions require potassium for optimal functioning.  One distinct roll of potassium is in the assimilation of CO2 (photosynthesis) and the relocation of the formed products. Potassium is used in transporting carbohydrates and is directly linked to kernel oil content. Shortages are common and require several years to correct.

Potassium is also involved in the transport of products of photosynthesis from the leaves to the roots. In contrast to Ca, K is completely mobile within the plant. Potassium is present in plant tissue in fairly large concentrations and a deficiency will lead to malfunctioning of many physiological and biochemical processes. Potassium is strongly absorbed by the roots and is easily transported to the meristem tissue. Relocation of K from old to young tissue occurs commonly. That is why it is quite difficult to identify a responds to a foliar spray. Soon after absorption the K is removed to other tissue.   Potassium is also involved in the synthesis processes of protein, cytokines and hence the growth rate of plants. Potassium plays an important roll in the activities of the stomata. Plants that are well supplied with potassium require less water for the same mass of synthesised organic material compared to plant lacking potassium. The transpiration rate is reduced and the opening of the stomata better regulated when potassium is in sufficient supply. Potassium cannot be substituted by other mono-valent cations in these functions.  When potassium is in short supply the stomata will take much longer to close but also to re-open once it were closed in responds to climatic factors.

Translocation of potassium is best when the nitrogen status of the plants is optimal. Translocation also occurs in the phloem vessels and potassium can move up and down in the plant.

Absorption of potassium is being reduced by high concentration of H+, (low pH in the soil), calcium (Ca), magnesium (Mg), sodium (Na) and ammonium (NH4).

Potassium deficiency.
In practise, a K deficiency is first of all expressed in a decrease in fruit size followed by a decrease in number of fruit. Potassium is the element that has a major impact on the manufacturing of sugars, starch and oils. A lack of K will harm these processes and reduce quality.

Excessive potassium levels.
Potassium and magnesium react antagonistic during absorption and even as a responds to foliar sprays. It is important to watch the magnesium status when focussing on increasing the K status to improve nut size. Foliar applications of potassium usually result in a decrease in the concentration of magnesium in the leaves.

Sources of potassium
The most freely available sources of potassium in Southern Africa are potassium chloride (50% K) and potassium sulphate (45% K). Both are also used in preparing mixes. Potassium chloride is the cheapest source but cannot be used on chloride sensitive crops and crops that require chloride free conditions.

Potassium sulphate can be used as a source of both K and sulphur (S) and on soils where the application of chloride is unwanted.
Potassium nitrate is also freely available but is mostly used in foliar sprays and hydroponic mixes. The combination of K and NO3 enhances the absorption of K by the roots and this formulation is an excellent source of both K and N in hydroponics and fertigation with drippers. Due to the cost potassium nitrate is seldom applied by conventional systems to the soil.

Potash-magnesium is a double salt of K and Mg and can be used when both K and Mg need to be applied. It contains 22% potassium and 6% magnesium

Fertilisation with potassium
The reaction to soil applications of potassium is faster/easier on sandy than clayey (>20-25% clay) soils. The volume of the root hairs has a direct relationship with the mass of K absorbed by the roots.

Applications tot the soil.
One method to evaluate an application of K to the soil is to calculate the potassium saturation. K-saturation is the %K in terms of the total cations namely K+Ca+Mg+Na.  The %K should ideally be 5 to 7,5.
When applying potassium chloride or sulphate by hand or mechanically, the fertilisers should be spread out evenly below the canopy where the water is applied.

The total application can be spilt into one or more applications depending on the clay content of the soil.  Not more than 500g potassium chloride per mature tree should be applied to soils containing less than 10% clay. Even if it is spread over 6 to 8m2 around the tree. The increase in the concentration of soluble salts due to potassium chloride or sulphate will damage the roots, could scorch the leaves and cause leaf drop. The maximum for soils containing 15 to 20% clay is 1000g per mature tree per application. Soils containing more than 25% clay can handle more than 1000g potassium chloride per application.

Potassium chloride or sulphate cannot be banded. Temporary salinity created by the concentrated application will damage the roots, leaves and even shoots.

Fertigation by microjets.
Both potassium chloride and sulphate can successfully be applied through the microjets. Even on soils with properties that limit the absorption of K, fertigation can be effective. However, the requirements are low concentrations of K applied over the entire length of the irrigation cycle during the day. During the day when the trees utilises the water enriched with K, the K is absorbed before it is subjected to all the negative forces in the soil.

Fertigation with drippers.
Potassium salts are potentially more saline than calcium salts. This limits the application of potassium chloride and sulphate to fairly low concentrations. Solutions of potassium salts have higher EC’s than calcium salts at comparative concentrations (Table 4).

Table 4. Comparative EC’s of 0,10% solutions of some fertilisers.

 Salt
EC mSm-1
 pH
Potassium nitrate
105
5,25
Potassium sulphate
140
5,35
Potassium chloride
185
5,50
Calcium chloride
150
5,55
Calcium nitrate
120
5,25

Due to the mobility of potassium in the plant, continuous applications are not required. This can therefore be utilised to control the EC of the nutrient solution. For instance if more calcium is required in spring, the concentration of Ca can be increased during this critical period without increasing the EC, by just lowering the concentration of K in the solution. To ensure that the trees will not lack K during this period, K can be stored in the trees for use when the concentration is lowered.

5. Calcium

Roll in pecan nut production.
Potassium (K) is involved in activation of many enzyme reactions in the biology of the plant. More than 60 enzymatic reactions require potassium for optimal functioning.  One distinct roll of potassium is in the assimilation of CO2 (photosynthesis) and the relocation of the formed products. Potassium is used in transporting carbohydrates and is directly linked to kernel oil content. Shortages are common and require several years to correct.

Potassium is also involved in the transport of products of photosynthesis from the leaves to the roots. In contrast to Ca, K is completely mobile within the plant. Potassium is present in plant tissue in fairly large concentrations and a deficiency will lead to malfunctioning of many physiological and biochemical processes. Potassium is strongly absorbed by the roots and is easily transported to the meristem tissue. Relocation of K from old to young tissue occurs commonly. That is why it is quite difficult to identify a responds to a foliar spray. Soon after absorption the K is removed to other tissue.   Potassium is also involved in the synthesis processes of protein, cytokines and hence the growth rate of plants. Potassium plays an important roll in the activities of the stomata. Plants that are well supplied with potassium require less water for the same mass of synthesised organic material compared to plant lacking potassium. The transpiration rate is reduced and the opening of the stomata better regulated when potassium is in sufficient supply. Potassium cannot be substituted by other mono-valent cations in these functions.  When potassium is in short supply the stomata will take much longer to close but also to re-open once it were closed in responds to climatic factors.

Translocation of potassium is best when the nitrogen status of the plants is optimal. Translocation also occurs in the phloem vessels and potassium can move up and down in the plant.

Absorption of potassium is being reduced by high concentration of H+, (low pH in the soil), calcium (Ca), magnesium (Mg), sodium (Na) and ammonium (NH4).

Potassium deficiency.
In practise, a K deficiency is first of all expressed in a decrease in fruit size followed by a decrease in number of fruit. Potassium is the element that has a major impact on the manufacturing of sugars, starch and oils. A lack of K will harm these processes and reduce quality.

Excessive potassium levels.
Potassium and magnesium react antagonistic during absorption and even as a responds to foliar sprays. It is important to watch the magnesium status when focussing on increasing the K status to improve nut size. Foliar applications of potassium usually result in a decrease in the concentration of magnesium in the leaves.

Sources of potassium
The most freely available sources of potassium in Southern Africa are potassium chloride (50% K) and potassium sulphate (45% K). Both are also used in preparing mixes. Potassium chloride is the cheapest source but cannot be used on chloride sensitive crops and crops that require chloride free conditions.

Potassium sulphate can be used as a source of both K and sulphur (S) and on soils where the application of chloride is unwanted.
Potassium nitrate is also freely available but is mostly used in foliar sprays and hydroponic mixes. The combination of K and NO3 enhances the absorption of K by the roots and this formulation is an excellent source of both K and N in hydroponics and fertigation with drippers. Due to the cost potassium nitrate is seldom applied by conventional systems to the soil.

Potash-magnesium is a double salt of K and Mg and can be used when both K and Mg need to be applied. It contains 22% potassium and 6% magnesium

Fertilisation with potassium
The reaction to soil applications of potassium is faster/easier on sandy than clayey (>20-25% clay) soils. The volume of the root hairs has a direct relationship with the mass of K absorbed by the roots.

Applications tot the soil.
One method to evaluate an application of K to the soil is to calculate the potassium saturation. K-saturation is the %K in terms of the total cations namely K+Ca+Mg+Na.  The %K should ideally be 5 to 7,5.
When applying potassium chloride or sulphate by hand or mechanically, the fertilisers should be spread out evenly below the canopy where the water is applied.

The total application can be spilt into one or more applications depending on the clay content of the soil.  Not more than 500g potassium chloride per mature tree should be applied to soils containing less than 10% clay. Even if it is spread over 6 to 8m2 around the tree. The increase in the concentration of soluble salts due to potassium chloride or sulphate will damage the roots, could scorch the leaves and cause leaf drop. The maximum for soils containing 15 to 20% clay is 1000g per mature tree per application. Soils containing more than 25% clay can handle more than 1000g potassium chloride per application.

Potassium chloride or sulphate cannot be banded. Temporary salinity created by the concentrated application will damage the roots, leaves and even shoots.

Fertigation by microjets.
Both potassium chloride and sulphate can successfully be applied through the microjets. Even on soils with properties that limit the absorption of K, fertigation can be effective. However, the requirements are low concentrations of K applied over the entire length of the irrigation cycle during the day. During the day when the trees utilises the water enriched with K, the K is absorbed before it is subjected to all the negative forces in the soil.

Fertigation with drippers.
Potassium salts are potentially more saline than calcium salts. This limits the application of potassium chloride and sulphate to fairly low concentrations. Solutions of potassium salts have higher EC’s than calcium salts at comparative concentrations (Table 4).

6. Magnesium.

Roll in pecan nut production
Magnesium (Mg) is the central ion in the molecule of chlorophyll but more than ±70% of the total Mg content of plants is in a mobile form. The rest is present in chlorophyll, pectin and oxalates.
The main roll of Mg is its participation as co-factor in enzyme reactions in the processes of phosphorylation (energy transfers). Magnesium is therefore important in the energy management of the plant.

The Mg status of the trees can therefore also, like nitrogen, be misinterpreted by using leaf analyses data without orchard information. Due to its mobility, the Mg is relocated from old to new leaves. The old leaves are being shed too early leaving the trees sparsely foliated. The analytical result will reflect the content of magnesium of a tree with a low leaf density.

Magnesium deficiency
A magnesium deficiency will reduce the assimilation rate of CO2 and other processes long before the chlorophyll per se is reduced.  Assimilation of nitrogen into protein is also reduced by a lack of magnesium and an alternate bearing pattern can be triggered.

Magnesium deficiency symptoms will develop on old leaves. The magnesium is relocated from the old leaves and symptoms usually appear during autumn or early spring when the demand from new growth is highest.  These deficient leaves are shed prematurely and photosynthetic capacity is lost.

Magnesium deficiencies occur country wide and are aggravated when potassium nutrition is emphasised. Absorption of magnesium is suppressed by potassium applications to the soil and foliar sprays.

To correct a magnesium deficiency might take a number of years. It is therefore essential to increase the potassium applications with caution while the magnesium status is monitored closely.

Excess magnesium
An excess of magnesium can substitute Ca in the cell walls but the resultant compound has the same qualities.
An excess Mg is mainly of importance due to its inhibiting effect on the absorption of potassium. This will therefore also result in inferior or less fruit.

Sources of magnesium
Magnesium can be supplemented by applying any of the following sources (Table 5). Other factors like pH of the soil and additional elements required, will dictate the most suitable source.

Table 5. Magnesium sources for use in citrus orchards.

Source
% Magnesium
Magnesium oxide
 50-54
Magnesium hydroxide
40-45
Magnesium sulphate
10
Dolomitic lime
15-30
Magnesite
20-30
Magnesium carbonate
15-25
Calmafos
11
Magnesium nitrate
5.3-9.6
Potash-magnesium
6
Mixtures
Varying
Fertilisation with magnesium

Soil applications
The magnesium content of the soil expressed as a % of the four cations, Ca, Mg, K and Na, should be between 15 and 25%. Deficiency levels of magnesium in the soil are usually related to acid soils and can be corrected with dolomitic lime. However, deficiencies in the trees as indicated by foliar analyses and visual symptoms can also be found on soils containing enough magnesium.

Fertigation with microjets.
Although magnesium sulphate can be applied through the microjets, applications of magnesium oxide or hydroxide or even dolomitic lime are preferred for tree crops. The application of magnesium sulphate through microjets has not been tested properly.

Fertigation with drippers.
Magnesium sulphate and nitrate are the most popular sources for application through the drippers. Because Mg is very mobile in the plant, it can be “accumulated” to be utilised when supply is low. Therefore the application of magnesium can also be postpone to control the EC and applied at a later/earlier stage when the demand for Ca is less. It is not required to apply the magnesium continuously every day.

7. Sulpher.

Roll in citrus production.
Notwithstanding the fact that many crops require more sulphur than P, fertilisation with sulphur did not received its rightful measure of attention. Sulphur is applied indirectly through compost, manures and quite a few inorganic fertilisers and is seldom in short supply. However, when highly concentrated fertilisers were being applied extensively, sulphur deficiencies developed and started to draw attention.

Plants absorb sulphur as the sulphate (SO4=) and this process does not depend on the pH of the soil. Sulphur move mainly upwards to the tops and is hardly being relocated.  Therefore young tissue is supplied with sulphur from the roots and not from older tissue. When S is in short supply, the protein content of plants and seeds is reduced. Many processes involved in photosynthesis require sulphur. Sulphur deficiency results in yellowing of leaves and can be mistaken for a nitrogen deficiency by the untrained eye.  Leaf analyses can solve this confusion.

However, the symptom of an S deficiency is quite specific. Contrary to nitrogen, sulphur deficiency develops on the new growth. The new leaves have a “rich” yellow colour on a background of green leaves from the previous flush. Trees with an S deficiency flower poorly and hardly set any fruit.

Sources of sulphur.
Additional sulphur can be supplied by changing the nitrogen and/or potassium source to one that contains sulphur. Table 6 contains a list of the most available sources of S. In all these listed sources except elementary sulphur, sulphur is present as the sulphate (SO4=) which is directly available to the plants.  Elementary sulphur, also known as flowers of sulphur, needs to be converted by the microbes to the sulphate.

Table 6. Sources of sulphur.

Product
%S
Ammonium sulphate
24
Potassium sulphate
18
Calcium sulphate (gypsum)
19
Single Super phosphate
11
Magnesium sulphate
13
Elementary sulphur
95-100
Fertilisation of sulphur

Soil applications.
The sulphate ion moves easily through the soil profile and is subjected to leaching. Applications of sulphur to the soil are very effective to correct a deficiency or to maintain the optimal status. Applications of gypsum in May to July can be used to correct a deficiency without any precaution or disruption of the fertilisation program. Gypsum supplies Ca and SO4= and can be applied at any time.  Maintenance of the S status can be done by applying any of the sources mentioned above, at the appropriate time for the accompanying ion (ammonium, potassium, magnesium, phosphorus or calcium).
 
Fertigation by microjets.
Fertigation of ammonium sulphate or potassium sulphate will be as effective as applications by hand or mechanical spreaders.

Fertigation by drippers.
The sulphates of ammonium, magnesium and/or potassium are used in the program to supply nitrogen, magnesium and potassium but also sulphur. In many instances, the sulphates are used to balance the N and P requirement and more than the required mass of SO4= is applied. Nutrient solutions should contain not less than 30 and not more than 600mg S per litre. Remember to incorporate the concentration of S in the irrigation water into the final concentration in the nutrient solution.

8. Chloride and Sodium.

Sodium (Na).
Sodium is an essential nutrient element but is better known for the toxic effects on plants and destructive habits in soil. Due to the abundance of sodium in soils and waters too little sodium or sodium deficiency is not regarded as a problem in agriculture.

Chloride (Cl).
The element chlorine is present in agricultural soil, irrigation water and fertilisers as the chloride ion (Cl-) and is utilised by the plant as such.  Chloride is an essential nutrient element and deficiency symptom of Cl- resembles that of iron deficiencies. Chloride is involved in splitting the water molecule during photosynthesis and apparently also in the functions controlling the stomata.

However due to its abundance in waters and soils, chloride excesses are more important. At high concentrations, chloride will suppress the absorption of other anions like nitrate and sulphate.

9. Micro Nutrient Elements.

So far boron, copper, iron, manganese, molybdenum and zinc are identified as essential micro nutrients for plants. However, a short supply of nickel is recently identified as causing a deficiency called “mouse ear” in pecan nuts. The micro nutrients are involved in many physiological reactions in the plant and a deficiency will influence almost all aspects of plant production.

Boron is involved in carbohydrate supply to the growing tips (meristem), lignifications of cell walls, nucleic acid synthesis and rate of respiration. Boron is therefore involved in reproduction, growth and maintenance of plant organs. When reproduction, growth and maintenance are inhibited by a lack of B, production is drastically reduced.

Copper is involved as a catalyst in the reduction of molecular oxygen, metabolism of protein and carbohydrates.

Symptoms of iron deficiency appear usually first on the shady or inside of the canopy.  The new leaves have a normal size and the veins, even the very fine ones are green on a light green to yellow back ground of the blade (lamina).

The functions of Mn in the plant are biochemically about the same as that of magnesium. It is involved in many complexing reactions of enzymes and oxidation-reduction reactions in the photosynthetic transfer of electrons.

Molybdenum (Mo) is the nutrient element that is required in the lowest dosage. Nevertheless, without Mo, no plant production is possible. Molybdenum is absorbed as the molybdate (MoO4=) and is an essential component of at least two important enzymes namely, nitrogenase and nitroreductase.  These enzymes are very involved in the assimilation of nitrogen

Nickel (Ni) is utilized in converting urea to ammonia. Therefore, if urea is used in an orchard with nickel deficiency, urea will not be converted properly and will result in toxicity.

Zinc is required to produce serine a precursor of tryptophan. Tryptophan is converted to the growth hormone indole acetic acid that governs the growth rate. Hence the short internodes and small leaves when zinc is deficient.  Trees in soils with low pH will respond to soil-applied Zn. Trees in soils with a higher pH will not respond to soil-applied Zn and require foliar sprays.

Sources of micro nutrients.
The sources of micro nutrients that are currently available are legio, especially those for copper, iron, manganese and zinc. They vary from inorganic salts to organic complexes of various compositions. The efficiency of these sources also varies considerably depending on the formulation and concentration. Please refer to the chapter on foliar sprays.

Fertilisation with micro nutrient elements.
Foliar applications of the nutrient elements, except for iron, are the most effective method to maintain optimum levels or the correct deficiencies. Soil application of boron is also effective but foliar sprays are more manageable and less risky.
Iron deficiencies are seldom corrected efficiently by means of foliar sprays. Soil applications are more effective and pose no hazard.
For effective foliar applications of nutrients (macro and micro) the contact time of the spray solution with the leaves, concentration of the nutrient element in solution and formulation of the chemical are important. Please refer to the chapter on foliar sprays.

10. Other Elements

Humans and animals require more micronutrients for optimal development than plants. Animals require also iodine (I), chromium (Cr), nickel (Ni), selenium (Se), strontium (Sr), vanadium (V) and cobalt (Co).  The influence of many of these elements on plant production has been investigated.

It was found that Co and Se improved yield in a variety of crops, potassium iodide  increased the sugar content of apples and grapes, strontium can supplement Ca, vanadium and gallium stimulate root development and silicon (Si) is known to protect the plant physically against adverse environmental conditions.

Silicon is absorbed by the roots and leaves and precipitates just below the epidermis where it provides protection against the penetration of mycelium. Silicon also forms water insoluble complexes with lignins and stimulates the formation of phenols, which is the plant’s natural protecting against insects and microbes.  Insect find silicon treated plants not tasty. Si is also precipitated in roots, stems, trunks and leaves and supports the plant during periods of colds, heat and drought.

Experience and trials with silicon sprays on pecan nut trees indicate the positive results on yield of nuts by off-setting the impact of dry and hot climatic conditions during fruit set.

Table 7. Effect of two foliar sprays with silicon on the yield of pecan nuts. All in kg nuts-in-shell.

Measurement
2 x 10mg Si per litre
Control
Average yield per ha
807
604
Increase (kg/ha)
203
% increase per ha
34 

11. Foliar applications of Nutrients.

Plants can obtain all their non-gaseous requirements via the root system. However, most plant organs including woody parts can also absorbed nutrients from solutions (Wittwer, 1963). Although the leaves can only absorbed small masses of nutrients, foliar sprays can be used successfully to supplement nutrition. The efficacy of foliar sprays is much higher than that of soil applications, especially regarding the micro-nutrient elements like copper and zinc.

Foliar sprays can be regarded as an aid and are applied for two main reasons namely.  

  • When the supply of nutrients via the root system is not sufficient, foliar sprays are applied to supplement the nutrient supply. The supply might be too low due to a sick root system   (Phytophthora or nematodes), conditions in the soil fixing the available forms of the nutrients (high pH, free lime, clay or a too low pH) or due to limited mobility of the element in the plant.
  • When it is required to manipulate the physiology of the trees for a specific reason, foliar sprays are an effective method. For instance sprays with urea for fruit set and MAP to reduce the acid levels in the fruit.

Although foliar sprays can substitute soil applications too many applications are required rendering foliar sprays for this reason impractical. Usually so many foliar applications will be detrimental due to the additional effects like biuret, even at low concentrations. To apply 250g N per tree, about 5 sprays with 1000g urea at a 100% efficiency are required.  Foliar sprays can therefore only be considered for remedial actions and specific interventions.

Live cells carry a negative charge in relation to the environment. Cations will therefore move towards the inside of the tree until the charge gradient reaches equilibrium.   Cations (K+, Ca++ and Mg++) are actively and anions passively attached to the surface of the leaves. From here on the rate of absorption will depend on the specific element and other factors.   Some of the stress factors that can harm the physiology of the tree and hence yield can be lifted by foliar sprays at the right time. The most sensitive stage for detrimental factors to reduce yields is during budding, blossom and fruit

In commercial horticulture, foliar sprays should be able to increase production under the following conditions.

  • When the incidence of fixing a nutrient element by the soil is high.
  • When it is required to lift stress quickly, especially during the forming phases of the fruit.  The reaction time of foliar sprays is seconds and in most cases the potential absorption is 80% completed within 15 minutes.
  • When the root system is sick and ineffective.
  • When weeds will reduce the absorption of nutrients by the trees.
  • When the supply of an element at a critical stage is too low.
  • To manipulate the physiology of the tree.

The basic requirements for successful foliar sprays are;

  • Mass of the nutrient applied
  • Formulation.
  • Contact time.

Mass required to be absorbed.    
One of the most important issues of foliar sprays is the mass of the nutrient applied per tree or ha.   Evaluating any spray material should start by calculation the potential of the formulation to be successful. A mature pecan nut tree carries on average 15-20kg dried leaf material.
Therefore if the concentration of potassium in the leaves need to be increased by 0,25% a mass of   0,25x20kg = 50g K per tree need to be absorbed.   At 100 trees per ha   5000g K per ha is required. The absorption of K is never 100%. Under orchard conditions absorption efficiency of potassium is only ±35%. Therefore 38kg potassium nitrate needs to be applied per ha.

Secondly, to raise the Zn status of the leaves by 30mg Zn per kg dried leaves, requires therefore the absorption of 30×20 = 600mg Zn per tree or 600×100 = 60000mg Zn per ha.  At an efficiency rate of 25% a mass of 240g Zn per ha must be applied. At 3000 litre per ha and 200ml zinc nitrate (5,5% Zn) per 100 litre water, 330g zinc per ha will be applied.   This spray will therefore be successful.

The mass of the nutrient that can be retained depends on the volume of water that can be retained on the surface of the leaves as well as the concentration of the nutrient in the spray solution. The volume, in turn depends on the size of the droplets.

In order to retain 2000 litre water on the leaves, the correct droplet size must be selected. The smaller the size of the droplets, the less water can be retained and the more sprays are required to total the 2000 litres (Table 8).

Table 8. The number of sprays required to put 2000 litres water per ha on the leaves of mature trees as influenced by droplet size.

Droplet size in micron (or in mm)
Number of sprays
60 (0,06)
 3639
500 (0,50)
 6
1000 (1,00)
1

The ideal droplet size for nutritional foliar sprays is 500 to 1000 micron or 0,50 to 1,00 mm. For pest control droplets with an average size of 300 to 500 micron are preferred. The optimal droplet size for bait applications is 4 to 6mm average diameter. The size of the droplets is a function of the pressure applied during spray, the orifice of the nozzles and the whirller plates used (Table 9).

Table 9. The relation between orifice, whirller plate number and pressure and the size of the droplets.

Orifice
Whirller plate no
At 5 Bar
At 7 Bar
At 10 Bar
At 15 Bar
 D2 1mm
 25
 90
 78
 70
 65
 D5 2mm
 25
 145
 133
 125
 120
 D8 3,2mm
 25
 183
 174
 165
 160
D2
56
 250
 238
 225
 215
D5
56
 390
 365
 340
 320
D8
56
472
 435
 400
 370

 

Droplets with a diameter of 10 to 40 micron will float for hundreds of metres but those with a diameter of 250 micron less than 2 to 3m.

A droplet with a diameter of 800 microns has eight times the volume of one with a diameter of 400 microns and 64 times the volume of that of 200 micron diameter.  That is one reason why smaller drops will dry out quicker. The volume per surface area of small drops is so much less.  For example one millilitre can be divided into1 900 drops with a diameter of 1000 micron or in 1 900 000 drops with a diameter of 100 micron.

Formulation.
Amongst the inorganic carriers of cations like K, Ca and Mg is nitrate the most effective for foliar sprays. Many resent studies indicate that the nitrate form of most cations (K, Ca and Mg) and metals (Zn and Mn) are more effective as foliar sprays than sulphates or most organic formulations.
The importance of concentration does not relate to that of the product in its undiluted state, but that of the spray solution after the recommended dilution.  The required concentration of the nutrient element is the effective concentration. The dilution rate of a product should therefore be aimed to reach the highest possible concentration without damaging the leaves.

The effective concentrations of the various nutrient elements can be benchmarked against the concentrations of chemicals known for their success to increase the concentration in the leaf satisfactory (Table 10). This is applicable for citrus and many other crops.

Table 10. The effective concentrations of a number of nutrient elements applied successfully as foliar sprays.

Product
Concentration of active ingredient in the product
Dosage g of ml per 100 litre water
Effective concentration of the water soluble part in the spray mix in mg per litre.
Zinc nitrate
 5,5%
 200ml
110mg Zn
Zn-EDTA
10%
100g
100mg Zn
Solubor
20%
150g
 300mg B
Manganese sulphate
23%
200g
460mg Mn
Copper sulphate
25%
25g
62mg Cu
Potassium nitrate
38% K
 3000g
 11400mg K
Urea
46% N
1000g
4600mg N
MAP
 26% P 12% N
2000g
5200mg P
MKP
 28% K 23% P
2000g
5600mg K 4600mg P
Magnesium nitrate
 10% Mg 11% N
1250g
1250mg MG

Any new product can be evaluated according to these guidelines. For a product to have a potential to be effective it should have a concentration of at least 80% after the recommended dilution compared to that of the benchmark chemical.

Additives can improve absorption, but if the effective concentration is less than 80% of  the benchmark chemical, then sufficient proof of such claim, must be available.

Additives that can improve the penetration of nutrients into the leaf like acidification (best at pH5,0 to 6,0) and reduction of the surface tension (wetters), will also improve absorption.   When wetters are added, the water tends to form a thin layer in stead of droplets on the surface. Much less water is retained and the film dries out much faster. Therefore the mass of the chemical and the contact period is reduced. Urea and fulvates, as mentioned above, are additives that can increase the absorption of potassium from 30 to 33% (urea) and 30 to 40% (fulvates) (Chamel, 1969 and Table 18).

Contact time.
Nutrients can only be absorbed when in solution. Therefore the spray solution must be kept on the leaves as long as possible before it dries out.  About 80% of the potential absorptions happen during the period just after spraying. If this period is too short, the efficiency will be reduced. . When the spray solution dries out and left a residue on the leaves, the “laws of cuticular penetration” will determine how much of the residue will penetrate the cuticle (Schönherr, 1999). This “law” is based on the relative humidity (RH) required to re-dissolve salts. According to this,   calcium and magnesium chloride will require a RH of 33%, potassium carbonate 44%, and calcium and magnesium nitrate requires a RH of 56% to dissolve. Salts like di-potassium phosphate, mono-potassium phosphate, potassium nitrate and chelates from acetate, lactate and propionate require a RH close to 100% and will not easily dissolve. This theory does not consider condensation and the formation of free water to dissolve the residues.

The duration of the contact period is a function of RH, temperature and droplet size (Table 11).

Table 11. The effect of relative humidity (RH), temperature and droplet size on time (in seconds) required to dry the drops.

RH %
Temperature °C
Droplet size in microns
Drying time in seconds
 70
 20
 100
 20
 70
20
 50
 5
 40
 20
 100
9

In practise a contact period of 15 to 20 minutes is possible and should be the aim. That is one reason why foliar sprays during the night are more effective than during the day. During the night the temperature is lower and RH is higher. This is especially important for applications of magnesium, potassium and also urea. Therefore apply the sprays during the night, early morning or late afternoon when temperatures are lower and RH higher.

Mechanisms of foliar absorption of nutrients. 
Currently three theories endeavour to explain the mechanisms of absorption of nutrients and other chemicals by the leaves. These three mechanisms need not exclude each other and are possibly together responsible for the absorption of water, nutrients and chemicals.

The three mechanisms are the following;
•    Inter fibril pores
The outermost layer of cells of the leaf is called the cuticle and consists of cutin, which is water repelling. Water and other chemical compounds are secreted through the cuticle and it is also possible that water and nutrients can enter the leaf through this mechanism.
The next layer of cells consists of cellulose, pectin, hemicelluloses and wax. The structure is formed by interconnecting fibrils which leaves openings (pores) for water movement. The number of inter fibril pores is enormous. Numbers like 108 per mm2 (100 000 000) are mentioned. Around the guard cells of the stomata, the concentration of these pores is higher which partly explains the better absorption by the underside of the leaf compared to the upper side.
The inter fibril pores are very small, about 1,0 nano metre in diameter. To put this small openings into perspective, compare the pore size with the diameter of the urea molecule, which is even smaller at 0,44 nm. Below the epidermal cells is the plasma-lemma consisting of lipo-proteins, which offers no barrier to water and nutrients passing through the inter fibril pores.
These fibrils are negatively charged and will attract cations and repel anions but let uncharged molecules like urea pass with ease.
•    Stomata
Pecan nut leaves contain about 20 times more stomata cells per mm2 on the underside than the upper side. The average diameter is 8 micron but these openings are filled with gas which cannot easily being displaced by water.  Nevertheless enough proof exists to support the stomata as an entry point of water and nutrients through the leaf.   The fact that potassium is better absorbed during the night when the stomata are closed indicates that this is not the only mechanism of absorption.
•    Modification of the waxy layer.
Usually the waxy layer will repel water but some chemicals have the ability to change this to a point where water and nutrients can penetrate.   Such modifications are possible by adding urea and fulvates to the spray solutions. The modification lasts only for a few minutes after the application. Increased absorption of potassium was reported by adding urea and fulvates (Chamel, 1969).

Compatibility of spray materials.

When compatibility is discussed, three aspects are involved;
•    Chemical compatibility.
Chemicals that will react with each other to cause scorching of the fruit or leaves, or rendering either one or both less effective, should not be mixed. Examples of scorching due to mixing are copper suspensions like copper oxychloride and zinc or magnesium nitrates. The last two are usually in an acidic medium which will dissolve more copper from the copper in suspension and increase the concentration of soluble copper to phytotoxic levels.  Some potassium nitrate has an alkaline reaction and with zinc nitrate will precipitate the zinc leaving the solution ineffective as far as Zn is concerned.
•    Application times and volumes.
Application of bait requires big droplets and a low volume that are not compatible with the requirements of foliar feeding.
The application time of certain sprays is very important. If the times are not compatible the two chemicals cannot be mixed.
•    Reduced efficiency.
In general the efficiency of an element will be reduced by the addition of a second one to the spray solution. Usually the decrease in efficiency is sometimes so small that mixing justifies the savings in spraying cost.  However, some elements will reduce the efficiency of another, without any chemical reaction, to such a degree that they are not compatible. A well known example is potassium and magnesium. When potassium is sprayed the uptake of Mg from the soil is suppressed and visa versa. Therefore these two should not be mixed.

12. Liming and Liming materials.

Natural soil forming processes, cultivation and fertilisation are some of the factors that cause the pH of the soil to decrease. Low pH conditions are not conducive to plant production and need to be corrected.

The low pH conditions can be corrected by applying liming materials to neutralise the acids in the soil. Chemicals used must produced OH—ions to neutralise the H+-ions in the soil.  Liming materials like dolomitic and calcitic lime supply these ions when dissolved in water. The OH- then reacts with the H+ in the soil as follows;

CaCO3 (or MgCO3) + H2O ↔ Ca++ + HCO2- + OH-
OH- + H+ in the soil = water

Liming (increasing the pH) or acidification (reduction of the pH) develops according to the same buffer systems in the soil. In sandy soils the buffer capacity is low and the reaction curve is a straight line. Clayey soils like “Black Turf” have very strong buffer capacities and a stepwise change in the pH is experienced when buffered soils are limed. Natural soils may have many stages of buffering (dissociation constants) where the pH remains the same whether lime or acid is applied.

For the maximum effect, liming materials should be mixed with the soil. Therefore it is important to apply lime (where required) prior to planting and mix it properly with the soil in the root zone. After planting, lime cannot be incorporated properly with the soil. Such damage to the roots is unwanted.

In existing orchards the lime is applied to the surface where it will dissolve slowly and eventually reaches the acidified soil. Therefore is it very important to start applying when the pH(water) of the top 30cm approaches 6,00

Increasing the pH of acid soils will also improve the availability of elements like N, P, K, Ca, Mg and Mo and increase the resistance to root diseases. That on its own is worth more than the few sporadic negative effects of liming.

Liming materials.
All materials that have the potential to deliver OH—ion are potential liming materials.  This includes hydroxides, carbonates and bicarbonates of potassium, calcium, magnesium and sodium.  The traditional liming materials are magnesium- and calcium carbonates better known as dolomitic and calcitic lime.  The oxides and hydroxides of Ca and Mg are also applied on a limited scale. Calcium silicate is used on sugar cane (Table 12).

Table 12. Liming materials available in Southern Africa.

Material
CCE*
Dolomitic lime
 109
Calcitic lime
93
Hydrated lime
136
Slaked lime
179
Calcium silicate
40-60

*Calcium Carbonate Equivalent = With pure calcium carbonate’s taken as 100

The quality of liming materials is determined by several factors.
•    Fineness
The finer the insoluble material, the larger the exposed surface area that can be eroded.
•    Hardness
The softer the lime particles, the better.
•    Neutralising capacity.
The neutralising capacity is determined by the mass of acid that can be removed by the OH- generated by a unit of the lime.  This is called the Calcium Carbonate Equivalent or CCE. The higher the neutralising capacity the less lime needs to be transported to do the job. Usually the values are indicated as CCE(HCl) or CCE(Resin) when determined by the hydrochloric or resin methods.
•    Chemical composition
Calcitic and dolomitic lime neutralise acid through the formation of OH–ions. They also apply calcium and magnesium to the soil.  Calcium silicate will also neutralise the acid but applies Ca and silicon. The value of the “extras” needs to be considered.

13. Organic material and nutrition.

Although plants can grow and produce optimally in the complete absence of organic material, it seems that a certain level of organic material in the soil can benefit productions. The benefits are not easily quantified and their value must be regarded as relative. The value of organic material is more readily quantifiable in soils with a coarse texture (sandy soils) than in soils with a fine texture (clayey).
Unfortunately the accumulation of organic matter in pecannut orchards also threatens the production of quality pecannut fruit. Before the hazards can be qualified and managed, accumulation of organic material in citrus orchards cannot be promoted.

Organic material is applied for five major reasons.
•    To increase the organic fraction of the soil.
•    To reduce leaching of nutrient elements, mainly N from sandy soils.
•    To establish a heterogeneous population of microbes.
•    To mobilise accumulated and fixed plant nutrients from the soil.
•    To improve the general wellbeing of the soil in terms of the physical, chemical and biological aspects.

However, one cannot ignore the fact that there are many fruit and nut orchards that produce well above the industry average without a single application of organic material. It is also true that the soil of many orchards is today in a better condition than before cultivation commences. The organic matter content is not the only measurement of soil health. It is therefore necessary to be objective and evaluate the reasons for the organic drive. At this stage little information is available to conclude for or against the application of organic material to orchards. Nevertheless, by looking at the pros and cons will already be a step in the right direction. It is further important to evaluate the organic, inorganic and integrated approaches scientifically without all the emotions.

The quantity of humus that will accumulate under conventional orchard practises is very low. It is estimated that less than 1% of the material applied will eventually be converted to humus or humus-like compounds. It is however unknown how much humus is required to make a significant impact on the stability of soil structure or the water holding capacity.   An increase of only 0,2% in the carbon content of dry land maize, is reckon to have an impact on production. To increase the carbon content by 0,2% an application of 4000kg organic carbon or 7 tonnes (±10-15 m3) is required per ha 15cm deep. This equals about 10tonnes good quality compost.

The quality of organic material varies a lot. Care must be taken not to add too much salt like sodium, chloride and boron with the material. Nitrogen is required during the decay of organic material. If the material contains too little N, the microbes will source for that in the environment. The plant cannot compete with the microbes for N and may suffer a temporary nitrogen deficiency. Therefore the material, compost excluded, should contain more than 1,8% N.

Organic nitrogen is not directly available to the trees. The mineralization of organic matter is a biological process and effected by the temperature and moisture content of the soil. The grower has no control over this process and it will progress as the temperature rises during spring. In commercial pecan nut production, it is important that the nitrogen release from the organic matter follows the requirement of the trees.  In practise this is quite the opposite. Repeated application of organic material will result in a gradual accumulation of organic nitrogen in the soil. This accumulation happens unnoticed until the release rate will be high enough to have a detrimental effect on production (Over application of nitrogen). It is almost impossible to reverse the accumulation and could take years to deplete the reserves, especially in fine textured soils.

The magnitude of the accumulated N is much higher than expected. Even in a sand where no organic material other than leaves and weeds, were applied, the total nitrogen amounts to >300mg N per kg soil. If only 1% of this total is mineralised during January to May, the equivalent of >100g LAN is supplied per tree at the wrong time. This release of nitrogen usually occurs in summer when it is undesirable for pecan nut.

The other advantage of accumulating organic material in the soil is to establish a natural heterogeneous population of microbes in the soil. In turn the heterogeneous population will restrict the development of a dominating and pathogenic population. The accumulation of phytotoxins will also be limited, because such a variety of microbes can deal with almost any compound.  A fungal rich population will also help to keep the soil loose, aggregated and aerated. However, fungi need a complex source of energy like wood (Lignin). It is important to supply energy to maintain the microbe populations. Without the application of additional energy sources, the microbes will deplete the existing organic material.

Natural microbial populations from soils rich in humus or organic material contain microbes that can dissolve the P from the unavailable pool and left it available to the plants.  Others can fix nitrogen from the atmosphere while another mix can release potassium from unavailable sources. Due to the activities of these microbes a variety of chemicals are released in the root zone. This includes compounds which will stimulate root growth similar to hormones. Another relative advantage of organic material in the soil is the concentration and variety of natural chelates like that of iron, manganese, zinc and copper increase. Chelates are metal-organic complexes that will not be fixed by the conditions in the soil, which render the metals available to the plant over a longer period.

It is important to distinguish between organic material, chicken and kraal manure and compost.   Organic material is plant and animal waste products in various degrees of decomposition and has little value in establishing a fungal population. Kraal and chicken manure are sources of organic nitrogen and some other elements and are not mediums to establish a fungal population. Compost is not rotten material but a specific mix of organic materials treated under a specific set of conditions to form compost.  Important components of the composting process are the mix of organic materials, the inoculums and the conditions during the composting process which will determine the value of the compost.

 

Another part of the organic material is present in a fairly stable state and is called humus. Humus is an amorphous (has not crystal structure) material with no resemblance to the material it originates from. Humus accumulated in the soil and is flocculated on the clay with the interaction of Ca.  The flocculated humus is stable and cannot easily be digest by microbes. Through a process of condensation the humus is transformed over many years into very large molecules.  Humus is therefore equal to humins + humic acid + fulvic acids.

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