The purpose of soil samples and analysis is:
- To classify soils in order to suggest practices for fertiliser and lime application;
- To predict the probability of a profitable reaction to application of fertiliser;
- To evaluate soil fertility
- To determine which specific soil conditions could be improved by applying soil amendments; and
- To detect and correct imbalances in nutrient concentrations.
The most important advantage of soil analysis is probably the ability to monitor changes in soil fertility in order to apply corrective action before nutritional stress occurs.
A wide range of extraction agents and procedures are available that can be used to evaluate the nutritional status of soils. Some extraction agents are recommended for specific soil conditions, e.g. the use of Olsen extraction (0.5 M NaHCO3) for the extraction of phosphorus in alkaline soils. Specific norms exist for each of these agents and procedures which are used in the interpretation of results.
Soil analysis is essential before soil preparation to ensure that physical and chemical defects can be corrected during preparation. Regular analysis is required in existing plantings to ensure that optimal conditions for growth are maintained.
If the soil is very stoney and the stones are so large that it can not be sampled, an estimate of the incidence of the stones must be made i.e. indicate the volume occupied by the stones. This information must accompany the soil samples to the laboratory as it can have an appreciable effect on the fertiliser recommendation. A stone correction is used in the estimation of gypsum, lime, potassium and phosphate fertiliser requirements.
Profile pits are necessary to evaluate the physical/morphological characteristics and to determine the borders of management units. Pits should be dug on at least a 50m x 50m grid. If the initial soil examination reveals that large soil differences occur between pits, more pits should be dug to determine where the soil transitions are. Before commencement of sampling, the site should be divided into its various cultivation or management units. These units are areas which will require similar management based on soil form, depth and the incidence of course fragments.
Separate samples are collected from each of these cultivation/management units and samples from the different profile pits within the same unit may be mixed in order to obtain representative samples for that management unit. For soil preparation for trees and vineyards it is desirable to collect samples from the topsoil and subsoil separately. The depth of sampling must be indicated.
After preparation, samples are collected per management unit. To ensure that the samples are representative of the unit, it is once again constituted from sub-samples which are taken from different places in the unit. If the soil has been ridged, the samples should be taken from the ridge only. The first sampling ought to be done shortly after preparation to determine whether optimal conditions were created during the preparation.
For maintenance fertilisation, soil samples must be collected at least every 3 years, except in cases of very sandy or stoney soils where leaching can reach serious proportions. Samples should be collected from such soils every two years. These samples are taken in the tree or vineyard row. Because soil composition can vary drastically over short distances, long term trends can best be determined if samples are collected more or less at the same places at trees/vines that are representative of the orchard. Trees should be marked for this purpose to ensure that samples are collected more or less at the same place. Weak patches in the planting should be sampled separately.
Under irrigation it is seldom necessary to sample deeper than 600mm. Because the cultivation action often disturbs the transition between soil layers, samples can be collected on fixed depths. In order to determine the effect of especially lime and phosphate fertiliser which have not yet reacted completely, it is preferable to collect specimens at depths of 0-100mm and 100-400mm. Considering that few roots are found deeper, deeper samples are not necessary. Samples should always be collected at the same depth to build a history of results which can be related to the fertiliser that has been applied. If the soil is shallower than 450-500mm, sampling should be limited to above the restricting layer. If underlying clay is included in the sample, it can affect the results to such an extent to render them totally useless.
Soil specimens that contain no stones must weigh at least 500g, and if stoney, 1kg.
Interpretation of soil analysis results
The figures given below can be used as a general guide to the interpretation of soil analysis results. The results are also used for the estimation of lime, gypsum, phosphorus and potassium fertilisation requirements, as well as for determining if trace element supplementation will be necessary.
5,5 – 6,0
Sand > 700 ohm
Loam > 500 ohm
Clay > 300 ohm
30 mg/kg (Bray II)
23 mg/kg (Bray I)
12-15 mg/kg (Olsen)
5 – 10 mg/kg
Sand: 45 – 50 mg/kg
Loam: 60 – 70 mg/kg
Clay: 80 – 90 mg/kg
Sodium (Na): < 7%
Potassium (K): 3 – 5%
Calcium (Ca): 70 – 75%
Magnesium (Mg): 12 – 15%
Boron (B): 0,2 – 0,5 mg/kg (Warm water)
Manganese (Mn): 15 – 20 mg/kg (0,02 M EDTA)
Zink (Zn): > 0,7 mg/kg (0,02 M EDTA)
Copper (Cu): > 0,5 mg/kg (0,02 M EDTA)
< 0,5% Low
0,5 1,5% Medium
1,5 3,0% High
>3,0% Very high
For more information, please contact:
Bemlab: 021 853 1490