Whatever you’re growing, testing the soil to make sure it’s up to the job is a sound strategy. In addition, a soil test means saving money by only applying fertiliser that is genuinely needed. But what exactly should we be testing for?
Soil tests should measure soil health including available nutrients, the presence of earthworms and microbes, and the physical status of the soil.
Early soil tests
Soil scientists have been able to test soil nutrient levels in laboratories since the 1840s. Early work in NZ focused on testing plant nutrients like phosphorus, potassium, and sulphur, and on achieving the correct balance of elements in the soil.
Later, experiments and field trials showed that plants weren’t always able to take up the elements in the forms that were being tested, and that different soils around NZ behaved unexpectedly when fertilisers were applied.
In sum, early soil tests were all about measuring soil chemistry.
Soil tests today
Today, chemistry is still the mainstay of standard soil tests. Such tests generally measure soil acidity (pH), the ability of the soil to hold on to certain elements (CEC), and the levels of plant-available nutrients such as phosphates and sulphates. Sometimes, tests for micronutrients like copper, cobalt, and selenium are added if a deficiency is suspected.
These tests tell us how much nutrient is stored in the soil, and how much is immediately available to the plant. The goal is usually to determine the types and quantities of fertiliser needed to achieve the growth we want.
However, the soil is more than just chemistry. It has physical and biological properties that affect its capacity to grow healthy pasture and crops. What is more, these same properties are being touted for their ability to mitigate environmental issues such as nutrient losses to waterways and the atmosphere.
So, measuring the physical status and biology of our soils along with the chemistry is good practice. Fortunately, we have tests underpinned by extensive field and laboratory measurements which allow us to do just that.
Tests for soil biology
Tests for soil biology generally include measuring the abundance and diversity of microorganisms in the soil sampled. Identifying particular species of microbes is now possible thanks to DNA-based techniques.
These measurements indicate soil health in a number of ways. For example, a well functioning soil will have a balance of bacteria, actinomycetes, fungi, and yeast. Such a balance ensures organic matter is effectively transformed into plant-available nutrients, avoiding release of harmful gases to the atmosphere from inefficient breakdown of residues and remains.
The tests can also guide management practices. For instance, a high number of anaerobic bacteria in soil samples will indicate poor drainage and soil compaction are likely problems. And measuring the number of N-fixing bacteria in soil samples can indicate the need (or not) for nitrogen fertiliser.
Tests for soil physical properties
The physical properties relevant to a healthy soil can actually be measured in a lab. But these tests are often expensive. An alternative is to get a Visual Soil Assessment (VSA) conducted by your local Nutrient Advisor when taking soil samples for nutrient testing.
The VSA is a method developed by Graham Shepherd for NZ conditions. Soil properties such as texture (types of mineral particles), structure (clods and soil aggregates), and porosity (spaces between soil aggregates) are measured to assess soil aeration and drainage and such like. The results can inform us of microbial activity and plant health since both require well aerated and drained soils to thrive.
Likewise, assessing the soil for colour and smell helps indicate soil and plant condition. For example, a dark brown soil with a sweet earthy smell indicates organic matter is being efficiently broken down, so plant nutrients are available and environmental losses are less likely.
Going beyond chemistry
So, given that soil tests are the best way to ensure your soil will perform, don’t just test the chemistry. Get your local Nutrient Advisor to request soil biology tests. And have them conduct a Visual Soil Assessment when they visit.
This article was published in the Coast & Country News.
