Soil samples can be taken with a drill. Photo: Airi Kulmala.
Soil tests are mainly used to determine soil nutrient content and pH in order to provide information about the need for fertiliser application. Proper soil sampling is a prerequisite for reliable test results. It is important to ensure that there is an optimum supply of all nutrients. If there is a deficiency of one element, other elements might not be utilised well and e.g. the risk of N losses increases.
Availability of nutrients present in the soil largely depends on soil pH. In general, nutrients are most readily available for plant use in the 6-7 pH range. Maintaining suitable pH levels also benefits plant nutrition through better root growth, enhanced microbial activity and better soil physical properties.
The results of soil fertility analysis are used to determine the soil pH status and the need for many macronutrients such as P, K, Mg, Ca, S, as well as micronutrients (Fe, Cu, Mn, Zn, B, Mo, Cl). A deficiency of any one of these elements can limit growth. The analysis is done in a laboratory, based on soil samples.
When to take soil samples?
The best time to take soil samples is in early spring before fertilisation or in autumn after harvest. If the samples are taken during the growing season, it is important to make sure that fertiliser granules do not end up in the samples.
How to take soil samples?
There may be regulations in each country about the number of samples to be taken per plot and subsamples per bulk sample. The sample should be composed of subsamples scattered or taken along a transect of the plot. It is good to mark the sites of the samples on e.g. a map of the field or use GPS to determine the coordinates.
The samples can be taken with a drill, a spade or other appropriate tool. The sampling depth should be the depth of the plough layer. Stones, litter or subsoil should not be included in the sample. When all subsamples are taken they must be mixed well and pooled into one sample per box. The sample boxes are labelled, sealed tightly and sent to the laboratory for analysis.
Soil sample taken from the depth 0 – 20 cm soil layer. Photo: Airi Kulmala.
Subsamples are mixed well e.g. in a bucket before one sample is taken for analysis. Photo: Airi Kulmala.
Determining the soil nitrogen content
Soil mineral nitrogen content (ammonium and nitrate) is more difficult to measure accurately than the content of the other nutrients because the soil nitrogen content is constantly changing during mineralisation/immobilisation and nitrification/denitrification processes. However, it is possible to make a laboratory analysis of the soil nitrate and ammonium nitrogen content before sowing or during the growing season in order to determine the need for initial N fertilisers and for additional applications. However, nitrogen mineralisation later during the growing season will strongly affect the amounts of soil nitrogen available for the crop.
The soil samples are taken as described above. However in the case of nitrogen analysis, the samples must be analysed immediately (within 3 hours is recommended) as otherwise the nitrogen content of the samples might change. If immediate analysis is not possible, the samples can be stored for some hours in a cooler or they can be frozen.
Different kinds of quick analytical procedures can also be used if laboratory tests are not suitable. See the example below.