Soil sampling is the most important procedure used to understand the nutrient status of the land. The primary objective when collecting a soil sample for laboratory analysis is that its composition be representative of the conditions that exist in the field. The general procedure involves the random collection of several individual soil cores over the designated area and combining them to form a composite sample for analysis. If soil samples are carefully collected and processed, the test result will be very useful.
Soil Sampling Methods
When sampling the soil, a small sample will represent a large amount of soil in the field. Because a poorly collected sample can lead to poor fertilizer management over an extensive area, no amount of care in preparation and analysis can overcome the problems of careless or inappropriate sampling in the field. A critical step in obtaining accurate soil tests is collecting representative samples in the field. Typically, uniform fields should be sampled in a simple random pattern across the field collecting at least 15-20 equal size soil cores (Figure 1).
Figure 1. Simple random soil sampling for uniform land.
Fields with significant landscape or soil type variation should be divided into separate sampling areas. From a field that has been divided into subunits, random subsamples are obtained (Figure 2). Differences may include soil colour, slope, degree of erosion, drainage, soil texture, or other factors that may influence soil nutrient levels.
Figure 2. Stratified random sampling for non-uniform fields.
The other method of sampling is called systematic sampling where subsamples are taken at regular spaced intervals in all directions (Figure 3). However, this should only be used in conjunction with a good soil map of the area, so that the soil conditions at each point are known.
Figure 3. Systematic soil sampling.
Soil Sampling Steps and Laboratory analysis
- Collect detailed information of previous crop(s) grown, when the field was last limed and fertilized and rates of application so that realistic yield targets can be set.
- Remove foreign objects from where the soil samples are to be taken (stones, sticks etc).
- In a 50 ha uniform land, take about 20 samples to make a representative sample from each depth e.g. 0-30 cm or 30-60 cm. See Figure 4
- Mark the samples (Land name, land number, soil depth, date). See Figure 5
- Add the 20 samples in clean bag (not a fertilizer bag). Break the clods and remove roots.
- Mix the sub-samples well. Approximately 1 kg of moist soil must be placed in a bag to comprise a representative sample
- Mark the bag and tie it properly.
- Soil samples are then sent to the laboratory for analysis of macro- and micro- elements essential for crop growth, as well as any other tests that may be required.
- Results of soils analysis are then used to assist in fertilizer recommendations for specific crops.
Soil sampling from fields used for crop production in most cases should be done after harvest and before planting the subsequent crop.
Figure 4. Soil sampling using a soil auger
Proper soil sampling and soil testing is essential to determine soil fertility levels and take good nutrient management decisions. Appropriate nutrient application can increase yields, reduce production costs, and prevent unnecessary surface and groundwater pollution.”
By Dr Nicky Mushia