Photo: Bill Kerr
For embarking on this journey, we need to understand the importance of increasing the organic and microbe content of our soil. This is the key to opening the door to increased yields, healthier crops and greater profits.
The first step is understanding how to increase the organic matter in your soil.
A new formula
When I started to see the benefits of my new approach, I wanted to understand the mechanics so as to maximise the rate of increase of humus in the soil. I was looking for a formula. This I could not find in South Africa, and I approached the agronomy professors of three agricultural universities in the US for help.
Two came back with vague, unhelpful answers, and the third answer was from Prof Steve Thien from Kansas State University. He gave me the formula and stated that all his students had to do a trial to prove the formula. That is how much importance he places on his students having a full understanding of its importance.
Thien maintains that in virtually any crop on a dry matter basis, there is about 400kg of carbon per ton. Of this, 140kg can make humus and the remainder is lost to the atmosphere as CO2 due to the activity of the soil micro-organisms.
As humus is 50% carbon, the maximum amount of humus that can be formed from 1t of dry material is 280kg of humus. Humus has a carbon nitrogen ratio of about 10:1, so there has to be sufficient nitrogen present as well.
Wheat crop residue has a C:N ratio of 80:1, so 1t of this residue only contains 5kg of nitrogen. Therefore, only 100kg of humus can be made from the residue. However, a crop like soya bean, having a high nitrogen content, would make the full 280kg of humus.
I have never come across a farmer who had this important knowledge. This is why legumes are so important in cover crop mixtures and why one needs a leguminous crop in rotation for soil improvement.
With this system, we always have to have material on the soil not only to protect it but to ensure the soil organisms – referred to as the soil food web – have sufficient decaying matter for them to keep functioning. This material, whether it be crop residue, manure or compost, is left on the soil where it will slowly be digested by the soil food web.
If this is worked into the soil, it will result in a rapid increase of soil bacteria that will take up nitrogen from the soil for their bodies, which then becomes unavailable for the plants until they have digested all the organic matter and die out from lack of further food. Only then the nitrogen is released back into the soil from their dead bodies.
This period of insufficiency of nitrogen for the plants is referred to as a negative period. This does not happen with no-till as the decomposition is slow, so there is no great spike in the bacterial population.
By harvesting the crop and not having to till the soil or having a negative period, we can plant the next crop straight away, which is a tremendous advantage.
Bill Kerr is a vegetable specialist and breeder.
