SOIL SAMPLING

Soil sampling refers to collecting a small quantity of soil referred to as a soil sample, that is a representative of the soil in an area or garden. Soil testing is the laboratory analysis of a soil sample to determine its physical and chemical properties.

Objectives/Reasons for sampling

The objectives of soil sampling and testing are:

• It enables the farmer to know the nutrients that are either missing, or are inadequate for plant growth.

• To know the pH of the soil. This is important because crops perform well within a specific range of pH.

•It shows the physical properties of soil which enable a farmer to know his or her soil type.

•To know the organic matter content of soil.

•It enables farmers to estimate accurately the expected increase in crop yield after application of fertilizer/manure.

•It enables a farmer to apply correct type of crops to grow basing on pH.

•It enables a farmer to apply correct type and amount of fertilizer needed in the soil.

Methods of soil sampling
Traverse method
It is at times known as grid method. Soil samples are picked along the diagonals of the garden.
Traverse method of sampling showing the sites where soil samples are collected

Zigzag method
Soil samples are picked from randomly chosen sites in the garden following a zigzag pattern.
Zigzag method of sampling showing the sites where soil samples are collected

Tools used to carry out soil sampling
These include the soil auger, hand hoe, slasher, spade, hand trowel, bags for holding collected samples and labels for marking the samples.
Procedure for soil sampling
The following procedures are followed:
1. The area from which samples are to be collected is divided into plots of convenient size.
2. Clear the vegetation from each spot where the samples are to be collected. The spots in each plot of land should be as many as possible.
3. Using the auger, scoop out soil from the first spot and place it in a clean bag and label the sample.
4.In the absence of the auger, use a hand hoe to dig a hole on the spot up to the desired depth. Then using a spade or a trowel, take a slice of soil from the side of the hole and place it in a clean bag and label the sample.
5.Make sure that two samples are collected from the spot, one representing topsoil (0-15 cm depth) and the other representing subsoil (15-30 cm depth).
6.Following the same procedure, collect samples from the rest of the spots in the plot of land.
7.Add together the samples of the topsoil layer from all the spots so as to form a composite sample.
8. Mix the soil thoroughly and then divide it into four equal portions. This is called quartering.
9. Discard three portions and retain one as a sub-sample.
10. Mix the sub-sample thoroughly, divide it again into four equal portions, discard three and retain one portion.
11. Keep on mixing, dividing and discarding three portions until you remain with a sub-sample of about 500 grams. This sub-sample is taken as a representative of soil in the plot of land from where the samples were collected.
12. The final sub-sample is labelled and taken to the lab for analysis. Information on the label include, name of collector, place of collection, date of sampling, depth of sampling, tools and method used, and purpose of sampling.
13. Similarly, all the samples of subsoil layer from all the spots should be added together to form a composite sample, and then treated the same way as in the case of topsoil samples.
Precautions to take when collecting soil samples.
•Avoid collecting samples from unusual places, such as anthills, ditches, kraals, footpaths, areas near manure heaps, water logged spots and recently fertilized sites. Samples from such sites give results that do not represent the soil in the sampled area.
• Always use clean equipment to avoid contamination.
•Care should be taken when handling the samples to avoid mixing them up.
• Collect as many samples as possible so as to make a good representative sample. Collect 10-20 samples in every 2-5 hectare plot of land.
Note: The following areas should be avoided during sampling.
• Water logged/poorly drained areas.
• Under vegetation cover.
• manure heaps
• Old house sites.
• Vegetation burning sites.
• Pathways
• Fence lines
• Anti hill areas
• Heavy vegetation heaps