Soils and Leaf Sampling for Pecannuts

1. Introduction

Leaf analyses are an indicator of the nutritional status of the trees. During the research into this method a relationship was established between the concentration of the nutrient elements in the leaves and production. This relationship was developed for almost every nutrient element. For some like chloride and sodium only the maximum tolerable concentration was determined (See below)

Generalised production curve as affected by the concentration of the nutrient elements in the leaves.

When the concentration of a nutrient increases from a very low status, the responds in production is dramatic as shown by portion AB. The part BC represents the best result in terms of production and nutrient status. It is however impractical and hardly possible to keep the concentration of the nutrient within the narrow range of BC and in practise the concentration range between C and D is regarded as the optimal range. Any variation within this range will therefore be regarded as optimal and will not influence production.
A decrease in production is experienced when the concentration of the nutrient is increased more than the maximum from D to E.

These optimal ranges are not depended on external conditions like soil type, climate etc but these factors will differ between plant species and even selections within a specie (Table 1).

Table 1. Concentration range for the various essential nutrient elements

Nutrient element % Concentration range for the various macro-nutrient elements
Deficient Optimal Excess
Nitrogen N <2,00 2,50-2,90 >3,10
Phosphorus P <0,08 0,12-0,20 >0,40
Potassium K <0,60 0,75-1,50 >2,00
Calcium Ca <0,20 0,30-0,75 >1,00
Magnesium Mg <0,20 0,30-0,75 >1,00
Sulphur S <0,15 0,20-0,50 >0,75
Chloride Cl 0,01-0,25 >1,00
Nutrient element mg per kg Concentration range for the various macro-nutrient elements
Deficient Optimal Excess
Sodium Na 0-2500 >5000
Copper Cu <3 5-20 >30
Iron Fe <30
Manganese Mn <25 50-300 >500
Zinc Zn <25 50-100 >200
Boron B <15 25-100 >150
Molybdenum Mo <0,03

Leaf analysis is not a simple process. Different techniques can give different values for the same element on the same sample.  Nitrogen is present in two forms in leaves. The majority is present as amine nitrogen (NH2+) as in proteins and a small portion is present as nitrate nitrogen.   The method originally used to establish the optimal values evaluates only the amine nitrogen because the nitrate represents less than 5% of the total nitrogen content of a leaf.  It is not that important with pecan nuts but with annual crops where the nitrates may represent as much as 25% of the total N, it is important to choose the correct method.

Other important limitations that should be consider when leaf analyses that are applied are;
•    The results of a leaf analysis on the nitrogen and magnesium status of the trees can only be properly evaluated if the trees show no signs of a visual or hidden deficiency.
•    The concentration of Ca in the leaf has no relation with the physiological disorders related to a calcium deficiency. The leaf analyses give a summary of the calcium status over a 7 to 9 month period with no indication of a period of Ca stress. The physiological disorders are the result of a very short period of Ca deficiency.
•    The concentration of iron in the leaf has only any value in the deficient range of <30mg/kg.

2. Soil Analyses

Soil analyses supply useful information to decide what measure to take to correct deficiencies, imbalances and excesses in the nutritional status of the trees. With a soil analysis it is attempted to remove from the soil in a few seconds corresponding masses of the nutrients that a plant will remove in 8 to 10 months. Each method applied, therefore went through a series of evaluating steps before it can be accepted as a suitable method for soil analyses.

These methods determine only the plant available portion of the nutrient in the soil, including a part of the reserves that will be utilised over the next few months. The portion measured therefore includes the water soluble and the readily available portion of the nutrients.

These are important principles and the reason for more than one method for the same element that will give different values for the same soil. The variety of methods was developed in an endeavour to simulate the plant. That is also a reason why different methods are used for different crops and in different areas.

Seven different methods are in use in South Africa to test for available P, 2 for K and 3 for pH of the soil. Therefore it is also important to mention the method used on the analytical report. For example, the three methods for pH will give three different answers for the same soil. On average pH(KCl) + 0,50 = pH(CaCl2) + 0,50 = pH(water) or pH(KCl) + 1,00 = pH(water). The description in brackets indicates the chemical used to suspend the soil namely KCl (potassium chloride) or CaCl2 (calcium chloride) or water at a specific ratio with the soil. A pH reading of a soil of 5,00 means nothing until the designation is attached. If it was a reading from a KCl suspension the pH of that soil is almost optimal. However if it was done in a water suspension, the soil is too acid. The relation between these three methods mentioned above, is based on averages but is seldom applicable in practise. Factors like the salt content of the soil have an influence on the reading. Therefore it is not advisable to jump between methods.

Due to the non-existence of a relation between the P content of the soil and that of the leaf, and that we strive to maintain a pH(water) between 6,5 and 7,5 the Bray 1 method for P is preferred. This does not implicate that any of the other methods are inferior.
The base cations in the soil (K, Ca, Mg and Na) are extracted by 1N ammonium acetate (pH 7). These results are then be used to determine the following.
• Can K be supplied by means of soil applications or foliar sprays?
• Does the soil contain enough Ca to supply the trees and maintain the structure of the soil?
• Existence and magnitude of any imbalances.
• Can Mg be supplied by means of soil applications or foliar sprays?
• What is the magnitude of the salinity hazard?

The results of the cation content are also expressed as ratios. A typical report is illustrated in Table 2.

Table 2 An example of a report on the base cations. Optimal ranges are given in brackets below.

K mg/kg
Ca mg/kg
Mg mg/kg
Na mg/kg
K%
Ca%
Mg%
Na%
Mg:K
Ca+Mg+Na:K
 235
 763
198
 32
 9,7
 61
 27
 2,2
 2,73
 6,33
*
 *
 *
 *
(5-7,5)
(70-75)
(20-25)
(<3)
(<5)
(<18)

(* The clay content of a soil determines the optimal concentration of K, Ca, Mg and Na and therefore cannot be listed. The higher the clay content the higher the optimal concentration.

These ratios are less important in soils containing less than 10% clay. The ratios are of importance because they have effects on the absorption of the cations and the structure of the soil.  These ratios are not applicable to plants cultivated with hydroponic systems. In nutrient solutions the concentration of K is about five times that of Ca and in the soil exactly the opposite.

3. Sampling Procedures

Soil analyses supply useful information to decide what measure to take to correct deficiencies, imbalances and excesses in the nutritional status of the trees. With a soil analysis it is attempted to remove from the soil in a few seconds corresponding masses of the nutrients that a plant will remove in 8 to 10 months. Each method applied, therefore went through a series of evaluating steps before it can be accepted as a suitable method for soil analyses.

These methods determine only the plant available portion of the nutrient in the soil, including a part of the reserves that will be utilised over the next few months. The portion measured therefore includes the water soluble and the readily available portion of the nutrients.

These are important principles and the reason for more than one method for the same element that will give different values for the same soil. The variety of methods was developed in an endeavour to simulate the plant. That is also a reason why different methods are used for different crops and in different areas.

Seven different methods are in use in South Africa to test for available P, 2 for K and 3 for pH of the soil. Therefore it is also important to mention the method used on the analytical report. For example, the three methods for pH will give three different answers for the same soil. On average pH(KCl) + 0,50 = pH(CaCl2) + 0,50 = pH(water) or pH(KCl) + 1,00 = pH(water). The description in brackets indicates the chemical used to suspend the soil namely KCl (potassium chloride) or CaCl2 (calcium chloride) or water at a specific ratio with the soil. A pH reading of a soil of 5,00 means nothing until the designation is attached. If it was a reading from a KCl suspension the pH of that soil is almost optimal. However if it was done in a water suspension, the soil is too acid. The relation between these three methods mentioned above, is based on averages but is seldom applicable in practise. Factors like the salt content of the soil have an influence on the reading. Therefore it is not advisable to jump between methods.

Soil sampling when micro-jets are used.
1.    Take a soil sample at each or every second index tree.
2.    Use an auger or spade.
3.    Remove any plant material on the surface below the drip line of the tree. Ensure that the sample is taken in the area that is also fertilised.
4.    Dig or drill to a depth of 30cm and put the slice or core in a clean plastic bucket. Collect at least 20 slices or cores in the bucket. Mix the soil in the bucket and take about 400g as a sample.
5.    Put the sample in a clean plastic bag, tie and add a label. The label must contain orchard name/number and farm name and some contact details.
6.    Dispatch to the laboratory.

 Figure 2. Leaflets 6 and 7 are suitable for the sample.

Soil sampling when drippers are used.
1.    Take a soil sample at each or every second index tree.
2.    Use an auger or pipe (25-50mm diameter).
3.    Remove any plant material on the surface as well as the top 10cm layer of soil. Ensure that the sample is taken in the area between the dripper and the perimeter of the wetted ball (Figure 2).
4.    Drill to a total depth of 30cm (from the surface) in the hole and remove the core. Put the core (10-30cm) in a clean plastic bucket. Collect at least 20 cores in the bucket. Mix the soil in the bucket and take about 400g as a sample
5.    Put the sample in a clean plastic bag, tie and add a label. The label must contain orchard name/number and farm name and some contact details.
6.    Dispatch to the laboratory.

Laboratories.
The following laboratories are able to perform the leaf and soil analyses. Please contact them for details regarding cost and other arrangements.
1.    NviroTek Labs ; Leoni 082-889 1039 or Francois 082-889 0133
2.    SGS; 021-852 7899 or Stuart 082-600 7332

Fertilisation programmes.
Although the soil and leaf analyses data form a very important part to formulate a fertilisation program there value can be increased by adding orchard information. The most important information required is;
1.    Cultivar
2.    Age
3.    Type of irrigation
4.    Number of trees per ha.
5.    Yield
6.    Quality aspects.
7.    Previous fertilisation (What, when and how much).
For fertilisation programmes based on the leaf and soil analyses results and the orchard information, contact jgk@telkomsa.net 082-785 7595

Figure 3. The sampling position in relation to the dripper, surface and perimeter of the wetted zone. 

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