By Harry Brook
It’s become very trendy lately to talk about soil and in terms of “health”. This naturally leads to the question of “What’s a healthy soil?”. It’s a good question and you probably won’t get many answers the same. For sake of argument, I would suggest it is strongly related to your soil organic matter.
This interest in soil health has evolved from some producers playing around with mixes of crop rather than a monoculture of just one variety or plant. This is called “polyculture”. They have found some interesting results, but it seems to work best when used in conjunction with cattle or other grazing livestock. Usually the mix has annual and perennial plants, legumes, tap-rooted plants and annual grasses. The intent and philosophy of it is to stimulate the soil biota (bacteria, fungi and nematodes) to increase soil activity. The ultimate goal is to then free up some of the nutrients locked into the mineral component of the soil.
There are huge reserves of nutrients that are complexed in the clay particles in the soil. As these clay particles weather, they slowly release nutrients for crop use. The idea with polyculture is that a very active soil may release more nutrients than just a crop of wheat or canola. Having a large variety of crops grown on a field does increase the diversity and activity of soil organisms. Using pulses does add to the soil nitrogen reserves. However, remember that every bushel of crop or pound of meat raised and sold from that field is a net nutrient loss. Livestock do extract less nutrient than an annual crop, but it is still a net loss. Chemical fertilizer or manure are both good ways to ensure the nutrients are there for the crop.
Can you test for soil health? Cornell University has come up with a test on the biological activity in your soil and they’ll even give you a breakdown of some nematode pest pressure. It tests for the traditional nutrients in the soil as well as assessing physical issues in the soil. This can include the available water-holding capacity, soil compaction, and soil aggregate stability. The greater your populations of microbes, the more they glue soil particles together. It also tests soil respiration, an indirect measure of soil microbe populations through their CO2 production.
Is this an answer to soil health? Perhaps it's part of the story. A lot of soil health is also related to the organic matter present in the soil. If you have high organic matter (>6%) the soil is more forgiving of poor management. High organic matter can compensate for short-changing a crop with fertilizer as it will supply the missing nutrients to the growing crop. However, you can’t keep taking more out of the crop than you put in. This eventually leads to reduced organic matter and, if it continues, ends up with soil with poor structure, puddling and crusting of the soil surface and the associated crop emergence problems.
Recent Agriculture Canada research has proven that the more diverse your crop rotation, the more diverse your soil microbe populations. Each different crop will favour a different spectrum of soil organisms. This can help in prevent serious disease issues in the crop and may even help the nutrient sourcing of the crop. Part of the soil microbes is the AM fungi, also know as the arbuscular mycorrhizal fungi. They can act as an auxiliary root system, helping the crop access nutrients its own roots cannot get. I’ll talk about them next week.
Along with a little bit of science you get an explosion of “biological” products that are supposed to boost soil health, and, by the way, crop profits. Beware of the snake oil. Test, measure and prove it to yourself if it has value.
Soil health is a very complex and diverse question. Certainly crop diversity can improve soil health but soil health is more field specific than just a simple number. I still think you can’t go too far wrong using organic matter as a metric.
See below for a couple of interesting videos on the topic of soil health.