Soil Testing: Benefits and Limitations
Without a doubt, soil testing has come to the forefront of UK agricultural policy over the past few years. With the roll out of mandatory testing in England and Northern Ireland and the recent release of the Scottish National Test Programme, farm soil analysis is increasingly commonplace.
Benefits of Soil Testing
Soil testing is a valuable farm management practice which has the potential to improve production efficiency, reduce costs, and improve water quality. Understanding the nutrient status of our soils enables targeted nutrient and lime application.
Soil testing can also be used beyond its traditional role in nutrient budgeting for more holistic management of soil health and resilience.
Soil testing includes any analysis which provides information about the physical, chemical, and biological properties of soil. This analysis can include tests for pH, various nutrient levels, cation exchange capacity, soil organic matter, microbial mass, and soil texture.
The soil parameters analysed should be chosen based on the context and goals of each individual farm. pH analysis is particularly relevant in Scotland, where almost half of all farms in the FAS Soil & Nutrient Network are adversely affected by low pH levels.
Most of the cost and labour involved in soil testing is related to sample collection and transport. Generally, soil samples are collected by walking in a “W” pattern through a field, stopping every few paces to take a sample with a corer or auger.
Sampling should be done at least six months after fertiliser or lime application and six weeks after manure application. Each field should be tested every 3-5 years, so sampling a few fields each year is a good strategy to spread costs.
Levels of Testing
Basic tests of pH, P, K, and Mg are sent back with fertiliser and lime recommendations which support nutrient budgeting. When interpreted correctly, soil analysis can help reduce bought in fertiliser requirements and optimise soil conditions for crop yields.
While basic soil tests are useful for nutrient management plans, they provide limited information about soil health and function. For example, crop micronutrients are not considered in the legal minimum soil test. Furthermore, common chemical tests ignore physical and biological aspects of soil health, which also affect crop growth and nutrition.
Laboratory soil organic matter tests are good indicators of more holistic soil health. Decomposing organic matter is the backbone of a healthy and diverse soil ecosystem.
Organic matter stores carbon, improves soil structure and fertility, and increases water holding capacity. Soil organic matter is measured in the lab by burning dry soil and recording the percentage of mass lost.
Organic soil carbon is one component of organic matter which has gained special attention over the past few years. Soil carbon measurement for carbon accounting requires deeper soil samples and an additional measure of bulk density. Importantly, farm soil carbon increases over decadal timescales, so management-related sequestration will likely not be detectable until at least 5 years from the initial test.
Other key physical and biological indicators of soil health, such as earthworm count, decomposition rate, and infiltration rate, can be assessed on-farm with just some wellies and a spade! For example, the visual evaluation of soil structure (VESS) test developed by SRUC can be completed on farm in less than 20 minutes.
Effective Soil Management
Gathering information through soil testing is just the first step in effective soil management. Next steps include:
- expert advice,
- manure analysis,
- nutrient budgeting,
- soil aeration, and
- well-timed nutrient and lime application.
Importantly, one soil test is just a snapshot in time. To get the most out of soil testing, analysis should be repeated every few years to readjust management and monitor trends in soil health.
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