My interest in the subject of no-till farming came about by default. Having started off with virgin soil in a poor physical condition, I noticed that the soil’s physical properties were improving as economic restraints forced me to cut back on tillage – I’d intended to revert to normal tillage later, and as a vegetable producer, no-till was the last thing on my mind.
The search for answers
After a few years of this, and using only chemical fertilisers, noticed that the soil’s organic content was steadily improving and crops were growing better. had to understand how this happened. started to reflect on statements made by lecturers when attended Cedara College 45 years earlier. was told that in a mixed farming system, it was good practice to plant Eragrostis curvula on unproductive lands denuded of organic content. Hence its common Afrikaans name, oulandsgras (old lands grass). We were told that after about six years, provided enough nitrogen was applied, when the soil was ploughed it would be so transformed due to the improved organic content that we were advised to plant a cash crop like cabbages or potatoes first time round.
On the other hand, if nitrogen was insufficient, the soil would remain as poor as it was when we started. No explanation was provided and we all accepted this, as these results were from trials conducted at the college. Even the amount of nitrogen to apply was given to us. also remembered that humus had a carbon:nitrogen ratio of 10:1. ’d assumed that perhaps the organic content improved so quickly because had fertilised well. also knew that tillage would stimulate the microbial life to tackle the soil’s organic content. A 50-year-old flashback took me right back to Weston Agricultural College, where was told that when land was first cultivated in South Africa, ploughing out veld stimulated the soil organisms. With the application of lime and superphosphate, mineralisation would break down humus and release nitrogen, making maize farming very profitable, but at the expense of organic content.
Asking the experts
I started to phone local agronomists and ask them whether there was a formula which could be used to determine how much nitrogen is required to maximise the formation of crop residue into humus. No luck – they told me that with a narrow carbon:nitrogen ratio, crop residues would break down faster, but the nitrogen content of the residue wouldn’t affect the amount of humus formed. felt there had to be a formula.
Bringing up the E. curvula story, suggested that well-fertilised grass would yield about 8t/year of dry matter and they agreed. added that with insufficient nitrogen, we may expect about 3t/year. Agreement again. then asked why after six years, the grass which would yield 3t/year would be in as bad a state as before planting and not improved to 30% of the lands getting 8t/year? After a moment’s silence, the reply was, “You have a point.” then started to contact agronomy professors at US universities. The first two came back with vague answers making statements like “the nitrogen content does have a stabilising effect on humus,” etc. then got hold of Prof Steve Thien at Kansas State University, who not only confirmed there’s a formula determining how nitrogen affects humus formation, but said all his students do a project to prove it!
The nitrogen formula I can’t believe that knowledge of such economic importance has been so confined. It’s so profound it affects virtually every farmer, and it’s especially important in understanding no-till farming. Put simply, the formula indicates that humus will contain about 50% carbon and 5% nitrogen, with a carbon:nitrogen ratio of 10:1. It is determined as follows: Most crop residue contains 40% carbon on a dry matter basis. The carbon:nitrogen ratio differs according to nitrogen content, not carbon content. If the carbon:nitrogen ratio is 30:1 or less there’s enough nitrogen for maximum humus formation. If the ratio is greater than 30, the amount of humus which can form is limited by lack of nitrogen. A maximum of 35% of the carbon in undecomposed organic matter will be available for humus formation, provided there’s enough nitrogen.
A minimum of 65% of the carbon in undecomposed organic matter will be oxidised to carbon dioxide during the microbial respiration associated with humus formation. A higher percentage will be lost when nitrogen is lacking. The humus formed will contain about 50% carbon and 5% nitrogen, with a carbon:nitrogen ratio of 10:1. 140kg of carbon can be used for humus formation in all crops – if enough nitrogen is available. This is 280kg of humus as humus is 50% carbon.
Turning residues into humus
Residues are essential to humus production. A practical way to determine the amount needed is to weigh the residue on 100m2 of land. Pull out plants by the roots, as these also count, and make allowance for roots remaining in the soil. With maize it’s not always possible to relate residue to grain yield.
The same yield can sometimes be obtained with half the population. Let’s say you use wheat straw as a residue. Wheat straw has a carbon:nitrogen ratio of 80:1. 1t of wheat straw has 400kg of carbon and, with a ratio of 80:1, it also has 5kg nitrogen. 140kg/t carbon is available for humus formation. Humus has a carbon:nitrogen ratio of 10:1, so 14kg nitrogen is required to maximise humus formation. As only 5kg is present here there’s a shortfall of 9kg/t. That 5kg of nitrogen at the 10:1 ratio (carbon: nitrogen) = 100kg of humus if no nitrogen is added (as humus contains 50% carbon). If 9kg nitrogen is added (14kg total), 280kg humus can be formed – almost three times more than if no nitrogen is added. Natural microbial respiration will break down as much humus as is formed with insufficient nitrogen, and the soil won’t benefit. This is what happened to the E. curvula mentioned above. Here no-till may save costs, but won’t improve the soil.
Residue from maize
Maize residue has a carbon:nitrogen ratio of 60:1. 1t of residue contains 400kg of carbon, which is divided by 60 to indicate 6,7kg nitrogen. 14kg nitrogen is needed for maximum humus formation, a shortfall of 7,3kg/t. 6,7 x 10 = 67kg carbon = 134kg humus per ton, with no nitrogen added. However, 280kg/t humus residue formed when nitrogen is added to make up shortfall. 4t/ha residue produces 1 120kg humus with sufficient nitrogen, but only 536kg when none is added.
Ensuring no-till success
When applying nitrogen, take into account its importance for humus formation, and how humus formation happens. Nitrogen is required gradually, commensurate with the decay of organic residue. Adapt the manner and frequency of application to the requirements of both the crop and residue. It won’t help to place all our nitrogen too far from the residue, as it may benefit the crop but not humus formation. We should now be feeding the crop as well as the residue and eventually the soil. We are in fact buying back our lost humus with extra nitrogen.
Legumes all have a carbon:nitrogen ratio narrower than 30:1, and leave a surplus of nitrogen after maximum humus formation which the following crop can use. This is why soil with legumes included in rotation improves much faster with no-till. Using the formula, you’ll be able to maximise humus formation with all crop residues. In theory, no-till has to work.
Occasionally plough pans and impervious layers need to be broken up with a ripper, but the build up in humus will make soil much more farming-friendly and lower input costs. There may well be instance where no-till can’t be practiced, but I suspect the nitrogen aspect has played a major role in many failures. Grazing harvested lands causes compaction and makes tillage necessary, so withhold livestock when soil is wet. The build-up of organic matter will speed up recovery from this compaction. Study groups are necessary to iron out difficulties and share knowledge and experience. Contact Bill Kerr on (016) 366 0616 or e-mail [email protected]. |fw
