Everything you need to know about soil testing

From my interactions with farmers, one critical step I have noticed they always ignore is soil testing.

Soil testing is the process of analysing the contents of your soil to find out the composition of nutrients and other properties. It provides farmers with the information critical for profitable farming.

Soil testing simply put, is measuring the soil’s health and fertility. How often the tests should be done depends on a variety of factors.

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However, farmers are advised to do soil tests annually or before a huge crop planting activity.

A soil test is done in an agricultural laboratory that gives very precise results and there are several of these. The tests cost from Sh1,000 to Sh2,500.

A simple soil test will give you information about the macronutrients content composition in soil, it will describe the Nitrogen, Phosphorus and Potassium (NPK) content, the pH level and magnesium content that is necessary for plant growth. Detailed soil tests go further than this and describe micronutrients or trace elements as well.

Why is it critical to test soils?

Soil testing is a farm management tool that equips farmers with the proper knowledge that:

Lowers production costs through reduction on crop nutrition inputs like fertiliser and associate labour costs.

Helps in knowing proper understanding of the soil condition and aids in proper soil improvement planning.

Helps in avoiding over-fertilisation through the use of correct and precise amounts.

Determines the expected growth potential of the soil.

Helps in preventing soil degradation through better farming practices like no over-fertilisation.

To carry out a soil test, one has to send a kilo of soil to the relevant laboratory. The kilo is enough for all the necessary tests.

The process of collecting the soil is referred to as soil sampling and can be done in two ways. Before sampling soil in your farm ensure that:

No recent input (fertiliser or green manure) has been added to the soil.

Do not collect soil samples from old homesteads, cow bomas, termite hills, fences, lime piles, spillage areas or areas where compost pits were. They tend to give false information about the farm and are not a true representation of the farm.

Avoid collecting wet soils

One can collect soil in two main ways: grid sampling or zone sampling. Grid is used when you do not have a history of the farm and zone is used when you can identify zones in the farm that have been used for specific activities.

For instance, if you have a maize plot and an avocado orchard on the same farm then samples will be taken from the two zones. The depth of soil collection is influenced by the crop to be grown, cultural practices adopted (zero tillage, mulching) tillage depth and nutrients to be analysed.

For most annual shallow rooted crops, a depth of 0-25 cm would be okay as the roots go up to 25cm deep but for tree crops like tea, coffee, avocadoes, oranges etc. one must collect soil beyond the 25cm mark to 50cm (within their natural root zone.

When collecting soil samples scrape away any surface litter before getting your soil as it gives off the wrong indicator of soil fertility. One can randomly select sites to collect from or use a more organised approach. Always get soil samples from all farm areas, combine them well in a bucket then pick a kilo per soil analysis test. It is important that farmers sample soils correctly as the test is 100 per cent dependent on the sampling. Farmers should consult their county extension officers if they have a challenge with soil sampling. Soil labs also offer sampling guidance.

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