Soil sampling

The reliability of a soil test is only as good as the sample you submit. The soil sample that is being analyzed should represent the whole field that needs to be tested in order to get a good recommendation. 

Choosing sample location

For each field (e.g. maize field, cabbage field, beans field etc.) separate soil tests should be done. The same applies for areas that appear very different in comparison with the rest of the field, for example an area where a specific crop is doing worse than the rest of the field. For each crop and significantly different area within the field there might be a different fertilizer needed.

You start by defining how to separate a farm into sub-fields so that representative samples can be taken. The areas to be sampled should be as uniform as possible in terms of soil type and crop. This means separate samples for every plot with a different crop and also for areas which are very different from each other (e.g. on a hill and down at the valley or one sample to represent a particularly good and the other to represent a poor area).

Therefore unusual areas such as; close to ditches, roads, manure heaps, water bodies (at least 20 m away), visibly contaminated sites should be avoided. Additionally, take samples at least in 200 m distance from roads, where cars are passing.

Figure 1: How to devide your field and select sub-sample locations

How to take a sample

Take 20-25 soil samples (subsamples) with a soil auger to get a representative sample of the whole field. If no soil auger is available then a machete can be used as well. For each area that needs to be tested one sample bag needs to be filled with soil. For larger fields it is suggested to collect 1 sample per 5 acres (2 hectares).

To get a good idea of the whole field the samples should be collected while walking in a W or zigzag pattern across the area of the whole field. Every few steps (x points see Fig.1 below) the auger is inserted straight into the soil with and approximate depth of 20 cm to reach all the important soil layers, do not go beyond 30cm. Continue with turning the auger (Fig. 2) a few times, to compact the soil and then pull it out carefully. You can remove the soil from the auger by using a stick or knife. If the field has been ploughed recently, don’t sample on top or at the bottom of the ridges, but in the middle of the ridge. In case the field is located on a hill or slope then two samples should be collected, one from the top part of the slope and one from the lower part of the slope.

Figure 2: Take the sub-sample with soil augur and collect the sub-samples in a bucket

All the subsamples from one field can be collected in a bucket (Fig.2). Once all the subsamples of one field are mixed in the bucket the soil can be filled in the sample bag. The sample bag should contain at least ½ kg of soil to provide enough material for the analysis, but not more than 1 kg of soil (1 bag of flour). Anything that is not soil should be removed such as grass, woody roots, worms, rocks etc. The sample should be kept dry until it has been delivered to the lab. If the sample is not immediately brought to the lab but stored for a few days, it is best to store the sample in a dark, dry and slightly colder place, if possible refrigerated at -4 degrees.