Introduction

Although plants can grow without soil or organic matter, soil serves as a cheap and abundant growth and rooting medium to keep the trees upright. The requirements of a soil for optimum pecan production are therefore only of academic value. One has to cope with what is available unless the soil is completely unsuitable. Managing irrigation and fertilisation properly can overcome a considerable number of soil limiting factors. Of all the soil factors involved, soil depth, internal drainage and clay content are the most important factors. To correct these soil factors are difficult and expensive if at all possible.
Once the suitability of the climate for pecan nut production is confirmed, one has to investigate the suitability of the soil and what pre-plant and cultural practises need to be implemented.
The minimum requirements for soils are as follows;

  • Minimum effective depth of 500mm.
  • No restrictions to drainage in the top 1000mm layer. Signs of poor drainage are green, blue and/or yellow mottles especially in the sub-soil.
  • A clay+silt content of less than 50%
    Once these are confirmed, the limitations in the soil profile need to be notated. Check for the following;
  • Layering. That is the sequence of layers with different textures in the top 500mm layer. The presence of layering will dictate the mode of soil preparation. When the soil is layered, a mixing action during preparation is required. Otherwise, only a loosening action will be required.
  • The chemical composition of the soil which will determine the pre-plant applications of fertilisers during soil preparation. Take soil samples from the different layers in the top 500mm soil. Deep cultivation (deeper than 500mm) is sometimes recommended but this is an expensive operation requiring special equipment.
    The optimal chemical conditions in the soil for pecan nut production are as follows. Compare the analytical results of the soil sample and adjust accordingly.
  • pH(water); 6,50 to 7,50. Use the information in Table 1 to adjust the pH. NB the masses in Table 1 is for the top 150mm layer of soil and must be doubled for the top 300mm layer of soil etc.
  • P Bray 1; More than 25mg/kg. Add 4kg P for every mg required and mix with the top 300mm layer of soil. NB, one kg P = 10kg single super phosphate (10% P) or 5kg double super phosphate (20% P).
  • % Calcium; 65 to 70% for soil with a clay content of more than 10-15% and 60 to 65% for other soils.
  • % Magnesium; 15 to 20%
  • % Potassium; 5 to 7,5%
  • % Sodium; as low as possible but not more than 3,0%. If the ratio is higher, apply gypsum on neutral and alkaline soils and lime on acid soils.

Table 1. The mass of lime (kg/ha-15cm depth) required to raise the pH(water) of soil with various concentrations of clay to 6,5.

pH(water) Clay content of the soil %
 <5% 5-10 11-15 16-20 21-25 26-30 31-35 36-40
6,40  0  0  0  500 500 1000 1500 2000
6,30  0  250  500  500  1000  1500  2000 2500
6,20  250  500  750  1000  1500  2000  2500  3000
6,10  500 750 1250 1500 2000 2500 3000 3500
6,00  750 1000 1500 2000 2500 3000 3500 4000
5,90  1000 1500 2250 2500 3000 3500 4000 4500
5,80  1250 1750 2500 3000 3500 4000 4500 5000
5,70  1500 2000 2750 3500 4000 4500 5000 5500
5,60  1750 2250 3000 4000 4500 5000 5500 6000
5,50  2000 2500 3250 4500 5000 5500 6000 6500
5,40  2250 2750 3500 5000 5500 6000 6500 7000
5,30  2500 3000 3750 5500 6000 6500 7000 7500
5,20  2750 3250 4000 6000 6500 7000 7500 8000
5,10  3000 3500 4250 6500 7000 7500 8000 8250
5,00  3250 3750 4500 7000 7500 8000 8250 8500
4,90  3500 4000 4750 7250 7750 8250 8500 8750
4,80  3750 4250 5000 7500 8000 8500 8750  9000
4,70  4000 4500 5250 8000 8250 8750 9000 9250
4,60  4250 4750 5500 8250 8500 9000 9250 9500
4,50  4500 5000 5750 8500 8750 9250 9500 9750

Importance of Soil Preparation and Chemical Corrections:

To get a picture of the soil conditions that favour pecan production, it is instructional to look at the soils and soil conditions in native pecan areas. The soils that predominate in the original region of the pecan, Carya illinoinensis, are acidic. Native soil pH levels are generally below 6.5 and often lower than 6.0. The salinity levels of these soils are typically low, and sodic soils are rare.

In contrast, in the pecan-producing areas in semi-arid and arid south-western United States, most soils are alkaline, with pH levels greater than 7.0. Many are calcareous, containing solid calcium carbonate deposits, and pH’s above B.O are common. Soil sodium levels may be high enough to negatively impact soil structure. The high pH of arid region soils intensifies micronutrient deficiencies in pecans. Zinc deficiency is most common, and foliar zinc application is part of routine orchard management for south-western pecans. Iron, manganese, and nickel deficiencies are also encountered. Although we know that pecans prefer slightly acid soils, acidification of alkaline soils, and particularly calcareous soils, is impractical. Therefore, growers typically manage around high soil pH by adjusting fertilization practices, rather than by making soil treatments.

Pecans require adequate moisture. They are not a particularly drought tolerant species {in Arizona and New Mexico pecans use approximately 42 inches of water per season}. Neither do they like prolonged wetness; pecans perform best in well-drained soils. In orchard surveys conducted in Arizona, pecan production was greatest in trees growing in sandy loam soils. Yields generally decreased as clay content increased above approximately 15%, and as sand content decreased below 60%.

Consideration of soil texture may be useful when locating new orchards, but there is little that can be done to alter soil texture in existing orchards. There are practices, however, that can help to ensure good soil drainage. Physical tillage operations can help when there are physical boundaries, such as hardpans. Slip-ploughing can open up a soil trench that can help interrupt soil textural boundaries that impede drainage. If soil dispersion caused by high levels of sodium reduces drainage, chemical treatments such as gypsum or sulphuric acid (in calcareous soils only) can improve drainage.

Addition and incorporation of organic amendments can also improve water infiltration and drainage, and increase soil water-holding capacity. Un-composted animal manures can be applied, but care should be taken to make sure the manure is not too saline. Composted manure generally has lower salt content than fresh manure. Poor quality irrigation water can introduce water infiltration and soil drainage problems. High sodium irrigation water may require treatment with gypsum or acid. Saline irrigation water requires application of adequate leaching water. Pecans are sensitive to salinity, and prefer soils with saturated paste electrical conductivity levels below 2.5 dS/m (1600 ppm). In all irrigated orchards, adequate leaching is needed to prevent salt accumulation. The amount of leaching water needed increases with irrigation salinity. Irrigation water with a salt level of 1 dS/m (640 ppm) requires roughly 10% excess irrigation water, whereas the water needed for leaching with 2 dS/m irrigation water is approximately 25% more than tree needs. Irrigation water with greater salinity will not support maximum pecan production levels, even with large amounts of leaching water.

More on soil preparation

The purpose of soil preparation before planting is to optimise the chemical and physical properties of the soil in order to have a homogeneous layer of 50 to 60cm depth for optimal root development.  This will enable the trees to reach maturity quicker and improve the productive life of the trees. A sound root system will be able to sustain a healthy canopy with more resistance against adverse conditions of heat and cold. A deeper root system, especially during the initial stages will also be less susceptible to adverse climatic conditions.

The preparation can therefore only follow the laboratory and site analyses. Site analyses of profile pits will determine the type of preparation required.
The two main actions are loosening with little mixing and thorough mixing. In practise loosening is obtained through a criss-cross action with a tine implement. The second action should be 60º on the first and down the slope. Mixing is done by using several implements including tines, tines fitted with wings and dish ploughs.
The importance of soil preparation is to create an uniform growth medium as to physical and chemical properties for the trees’ roots to thrive for many years. This also implies maintenance of this loose soil profile.

Re-compaction of a loosened soil is a natural process but will be enhanced by over-irrigation and orchard traffic. To delay re-compaction, schedule irrigation properly and restrict all orchard traffic to the designated orchard roads. No tractor or implement must be allowed to run close to the trees.

Planting of young trees.
Once the soil is properly prepared, planting of the orchard can begun. Keep the following in mind when planting the young trees.

  • Remove the bottom 30% of the growth medium when the trees are transplanted.
  • Use a garden fork to prepare the planting hole.  A spade will cause soil compaction on the sides.
  • Irrigate as the planting progresses and do not wait until all the trees have been planted before the irrigation starts.
  • Irrigate for about two weeks next to the stem and ensure that the ball of roots does not dry out.
  • As soon as the roots penetrate the adjacent soil, reduce the irrigation.
  • Protect the stems when applying herbicides.
  • Control aphids and other insects.
  • Ensure optimal conditions (sap flow, temperature and irrigation) when making applications to the stem.
  • Take leaf and soil samples during January of the third year to evaluate the nutritional status and convert to a program for bearing trees.

Fertalisation of young trees

Fertilisation of young non-bearing trees when irrigation is applied through microjets.
Wait for about six weeks before any fertilisers are applied.  Generally the young trees require only nitrogen and foliar sprays with micro-nutrients.

Fertilisation during the first year after planting.  
Split 250g limestone ammonium nitrate (LAN) per tree into 10 applications during August to March. Potassium at a rate of 2x25g per tree can be applied where the potassium status in the soil is low.  Do not mix the LAN and potassium chloride. Apply the fertilisers over an area from the stem to about 50cm beyond the drip line of the trees.

Mix 150g zinc oxide + 100g SoluborR + 200g manganese sulphate + 150g copper oxychloride + 1000g low biuret urea per 100 litre water and apply every 4 weeks. Wet the leaves only.

Fertilisation during the second year after planting. 
Increase the LAN to 500g per tree. Split and apply as during the first year. If required increase the potassium chloride to 2x50g per tree and apply as during the first year. Apply the foliar sprays as per the first year.

Fertilisation during the third year after planting.
Increase the LAN to 750g per tree. Split and apply as during the first year. If required increase the potassium chloride to 2x75g per tree and apply as during the first year. Apply the foliar sprays as per the first year.

Take a leaf sample during January of the coming year to adjust the fertilisation program to that of bearing trees.

Fertilisation of young non-bearing trees irrigated by drippers.   
When drip irrigation is used, the trees must be fertigated. Fertigation starts when the trees have been transplanted in well prepared soil, six weeks after transplanting.

Apply the same basic principles as with microjets. Put a dripper next to each stem and ensure that the root ball is wetted. Thereafter the drippers must be shifted gradually to be placed at a distance of about 500mm from the stem, during the sixth month.  With drippers it is essential to ensure that the water and fertilisers do not penetrate deeper than the effective root system.

Fertigation during the first year after planting.  
Split 30kg N + 10kg P + 20kg K + 10kg Ca + 10kg Mg +plus 10kg S per ha in at least weekly applications during August to March.
Mix 150g zinc oxide + 100g SoluborR + 200g manganese sulphate + 150g copper oxychloride + 1000g low biuret urea per 100 litre water and apply every 4 weeks. Wet the leaves only. If the water or soil contains enough boron, the SoluborR must be omitted.

Fertigation during the second year after planting.
Split 50kg N + 20kg P + 30kg K + 10kg Ca + 10kg Mg +plus 10kg S per ha in at least weekly applications during August to March. Apply the foliar sprays.

Fertigation during the third year after planting.    
Split 75kg N + 25kg P + 50kg K + 20kg Ca + 20kg Mg +plus 20kg S per ha into at least weekly applications during August to March. Apply the foliar sprays.

Take a leaf sample during January of the coming year to adjust the fertilisation program to that of bearing trees.

If the water application is monitored the concentration of nutrients can be kept constant during the first three years. The water requirement and therefore the nutrient application will increase with increase in tree size.  If for instance the water requirement doubles from year one to year two, the mass of nutrients applied will also double.  The masses applied will therefore be close to the recommended rate of 50kg N, 20kg P etc.

Fertilisation of Bearing Trees

The most important requirement of a good fertilisation program is that it must only supply those elements that the trees cannot obtain in sufficient quantities from the soil and water to maintain optimal levels for all 14 nutrient elements.  A good fertilisation program will eventually optimise the concentration of all the nutrients. Once this is obtained, fertilisation will not be the reason for suboptimal yields. Certain factors cannot be controlled, but with a balanced nutritional status, at least one variable is under control. With an optimal nutrient status, the trees can survive adverse cold, heat or drought conditions much better. Once the optimal status is obtained, fine tuning can be used to manipulate the trees.

The second aim is to obtain an optimal nutrient status at the lowest cost. By monitoring the inputs (fertilisers) and results (leaf analyses and production) only those elements that are in short supply need to be supplemented. With microjets it can mean that only N and Zn need to be applied.

That does not implicate that the cheapest fertiliser must be applied. A balanced fertiliser mix does not exist, except perhaps with fertigation and drippers where the water and soil contribute nothing to the nutrient program.  But even with single line drippers, the composition of the water needs to be considered and can supply the complete requirement for calcium, magnesium and sulphur.  The possibility that a mix like 3:1:5 will satisfy the requirements of all orchards on a farm are very slim.  A balanced nutritional status is the results of the ability of the roots to absorb the required nutrients, in the required quantities.

The third aim is to produce quality fruit that can withstand packing and handling the best in order to give the consumer value for money.

The concept of a “balanced” fertilisation program or nutritional status is used often out of context and is sometimes misused to press a certain sales point. Guano was at a stage regarded as being a “balanced” fertiliser although the sea birds have no knowledge of pecan nuts and they only want to get rid of the guano.  A certain fertiliser may be balanced for a specific condition, but can be totally off target for the 90% of other applications.  The only balanced fertilisation program is the one that is prepared for a specific orchard. Others are based on averages and usually applies too much (usually phosphorus) of one or more of the nutrients.

The following information is required to formulate an effective fertilisation program. 

  • The current nutritional status (leaf analyses).  Leaf analyses summarise the ability of the trees to set an optimal crop and grow it to maturity. The success of all the inputs is measured by means of the yield, fruit size and fruit quality.  If the accumulated inputs result in an optimal crop the fertilisation program must be repeated.  If any looming imbalance is detected it need to be corrected during the coming season.   Unfortunately many other factors than fertilisation will affect yields and quality. It is therefore possible to maintain an optimal nutritional status but still not produce an optimal crop.
  • The historical nutritional status, especially that of the previous season (data base). This will indicate whether the fertilisation program applied resulted in any changes and to indicate trends.  When the supply is too low, it has to be increased. The supply can also be increased by improving the efficiency of the fertilisers.  For instance when the supply of nitrogen is too low, the efficiency can perhaps being improved by split applications that will supply N to the plants over a longer period. The supply of N will then be improved without increasing the mass of N.
  • Current composition of the soil.   Soil analyses supply useful information to decide what measure to take to correct deficiencies, imbalances and excesses in the nutritional status of the trees.
  • Current composition of the irrigation water especially when hydroponics is practised.
    •    Information on the crop for the last two seasons and an estimate on the current crop.  By evaluating more than one year’s data, information is gathered aiding in formulating a more purposeful fertilisation program. Information includes alternate bearing, too small fruit, yield and general tree condition.  The more information supplied, the better the fertilisation program
  • Fertilisers and foliar sprays applied.   As far as application of fertilisers is concerned, the mass, timing and method are the most important aspects.
  • Practises to improve fruit set, fruit thinning, fruit size etc.   These include girdling, applications of hormones like gibberillic acid and other sprays like silicon to improve yield.
  • This information needs to be evaluated together with all the other information discussed in the previous chapters to formulate a fertilisation program for the coming season.

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