What no-till farmers need to know about nitrogen

There are farmers who have had wonderful success with no-till and others who claim it doesn’t work for them, or it’s too slow to kick-in. Understanding the role that nitrogen plays in humus formation is the key that unlocks the puzzle, writes vegetable producer Bill Kerr.
Issue date : 26 September 2008

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My interest in no-till came about purely by default. I had started farming on virgin soil in a poor condition, but noticed that the soil improved proportionate to my economic restraints which had caused me to cut back on tillage operations. M y intention was that would go back to normal tillage when the economic constraints passed. As a vegetable producer, no-till was the furthest thing from my mind. After a few years of minimal till and using only chemical fertilisers, noticed the organic content, and so the amount of humus in the soil, had steadily increased, improving the condition of the crops.

This led me to want to understand how that had happened. started to reflect on statements had heard at lectures at Cedara College 45 years earlier. n one of my flashbacks recalled being told that in a mixed farming system, it’s good practice to plant Eragrostis curvula on lands that have been over-farmed. The common Afrikaans name for this grass is oulandsgras (directly translated as old land’s grass). It’s a pioneer species that can survive on badly degraded soil.

We were told that after about six years, provided sufficient nitrogen was applied, the higher levels of organic material would transform the soil. We were advised to plant a cash crop like cabbages or potatoes after that treatment because the soil would then be sufficiently improved to produce a rewarding crop. But if too little nitrogen was applied, the soil would remain as poor as when we started. No explanation was provided and we all accepted the college’s advice because they had conducted trials. We were even given the amount of nitrogen to apply.

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Unveiling the obvious
Another flashback told me humus had a carbon:nitrogen (C:N) ratio of 10:1. These thoughts troubled me and led me to assume that because had fertilised well, perhaps that was why the organic content on my virgin lands had improved so quickly. knew tilling the soil stimulated microorganisms to tackle the organic content and this knowledge was reinforced by a 50-year-old flashback to Weston Agricultural College.

On that occasion was told that when lands were initially prepared in South Africa, after ploughing out veld, the organic material ploughed into the soil provided drought relief through moisture retention, and in the process stimulated microorganisms. Furthermore, by applying lime and superphosphate, mineralisation (breaking down of humus and resultant release of beneficial micronutrients), would release nitrogen and make maize farming very profitable, but at the expense of organic content or humus.

Investigating a theory
Armed with these thoughts, started to phone agronomists and asked them whether there was a formula to determine how much nitrogen is required to maximise the formation of crop residue into humus. N o luck. They told me a narrow C:ratio crop residue would break down faster, but that the nitrogen content of the residue had no influence on the amount of humus formed. I replied that there had to be a formula and on one occasion I described the E. curvula scenario to them. Well-fertilised grass would yield about 10t of dry matter per annum, and there was agreement. But with insufficient nitrogen we could expect only about 3t. Agreement again. So why was the yield improved? After a moment’s silence, the reply was “you have a point”. I then started to contact agronomy professors at US universities.

The first two came back with vague statements like that nitrogen content does have a stabilising effect on humus. And then I had a breakthrough. I got hold of Prof Steve Thien at Kansas State University who not only confirmed my belief that there is a formula, but he mentioned that all his students have to do a project to prove the formula. Wow! I couldn’t believe knowledge of such economic importance has been so confined. It’s so profound it affects virtually every farmer and is especially important to understanding the mechanisms of no-till farming.

A theory in practice
To determine how much humus can be formed, we need to measure the amount of plant material or residue left in the soil after harvesting. So, pull out all the plants on 100m2 of land, including the roots because obviously they also count. Allowance must be made for other root structures left in the soil. As the crop grows, it uses the available nitrogen produced by microorganisms as they break down the previous crop’s residue after no-till .

The manner and frequency of application of nitrogen must be adapted to the requirements of both the crop and how much residue was left on the soil. It won’t help to band place all the nitrogen too far from the residue as it may benefit the crop and not humus formation later on. Legumes all have a C:N ratio narrower than 30:1 and therefore a surplus of nitrogen is left in the soil at maximum humus formation. This is why farmers who include legumes in rotations have faster soil improvement with no-till.

At the heart of the matter
My alarm bells rang some months ago when I read about trials being done in KwaZulu-Natal where various treatments were being tried to reduce a soil dwelling pathogen affecting no-till maize. A treatment with anhydrous ammonia as a nitrogen source produced good results. This product has an 82% nitrogen content and has a sterilising effect on the soil, effectively fumigating the area around the point of injection. This could account for suppression of the pathogen, but it also has another profound effect.

Once the area again becomes habitable to microorganisms, it will be re-colonised first by bacteria prompted to frenzied activity by all the nitrogen, but few other microorganisms to keep them in balance. They also use the soil carbon as a food source and in the process cause rapid mineralisation releasing more nitrogen into the soil. So, plants growing during this treatment get an extra dollop of nitrogen resulting in more vigour compared to the control. The extra nitrogen will be at the expense of soil organic content and the opposite is achieved of what we want from no-till.

Don’t underestimate nitrogen
This was illustrated to us by Graeme Sait, the CEO of NTS at a course last year. In tropical areas during the Second World War landing strips were treated with anhydrous ammonia to rapidly deplete humus and make hard surfaces. On the positive side, there are products available that can be sprayed onto crop residue to accelerate decomposition.

They usually consist of a range of microorganisms and the nutrients to increase their activity. Clearly, we can stimulate nitrogen-producing microorganism without nitrogen, even sugar feeds them, but a formula should be considered and it’s worth discussing this with your fertiliser representative. In theory, no-till just has to work. I suspect nitrogen has a major role to play in failures. Avoid compaction by only grazing livestock when the soil is dry. Once the organic matter builds up, the recovery from this compaction is much more rapid.

There are occasions when plough pans need to be broken up with a ripper, but once the build-up of humus changes the soil, input costs are lower. Study groups are necessary to share knowledge and experience. And there is something we should always keep in mind. If Israel is forced to attack Iran, the current oil price may seem a bargain. |fw