Soil health is easy to say but hard to define. Let’s start with an observation. Healthy soil allows plant roots to grow deep down. Unhealthy soils are compacted and crop roots cannot penetrate this hard layer.
Soil health depends on good aggregation and stable pore space structure.
SOIL HEALTH = Aggregation = pore space structure
What does that mean? Look at the picture from a government soil management handbook. Which soil is healthy? The dark soil with crumbly structure looks much better then the light coloured soil in big chunks.
Pore spaces can explain the differences between the two pictured hands-full of soil. Research can help us see into the invisible world of microscopic pore space structure.
A BC government soil management handbook describes a healthy Okanagan Loam soil as having 50% pore space by volume with 25% large pore spaces (AIR) and 25% small pore spaces (WATER), with the rest being 5% organic matter and 45% Mineral (sand-silt-clay).
The large pore spaces allows air throughout the soil profile and water to drain through after heavy rains. The small pore spaces store water for future crop use. The water holding capacity of any soil depends on the small pore space structure.
Air flowing down deep into the soil is important for the beneficial soil life to breath, like in the roots of Apple trees (and most other plants), where the Mycorrhizae fungus live. When the good microbes are well fed and actively growing, they release nutrients and make them available to crops. A stable pore space structure supports the beneficial soil life to release nutrients to crops.
Chemical soil tests do not measure soil tilth and are of little use in assessing the microbe’s health. What tests do? More on that in another blog, next month.
Right now we are trying to understand what soil health is and how you can define it.
The Canadian organic certification standards define soil health as good soil structure or tilth (see section 5.4.3 in the Production Guide). This physical characteristic of the soil is close to the farmer experience as they work their fields with machines.
Here is a list of key functions that researchers attribute to healthy soil :
Source: Soil Health 101; usda/ncrs
Physical …support –
Nutrient cycling –
Filtering and Buffering –
HABITAT FOR BIODIVERSITY –
All of these key functions, depend on the delicate pore space structure held together by the fungus, Actinomycete bacteria and other soil life.
How can you assess if your soil is healthy, besides the slow changes of % organic matter? That is what we shall explore next month in our Tilth Indicator blog post.