When fields have a limited yield, farmers usually think it’s being caused by a lack of nitrogen. If the leaves turn yellow, they automatically apply more nitrogen. Consequently, this nutrient is often over-supplied. I n John Fair’s Guide to Profitable Pastures he warns that this practice could reduce the soil’s inherent ability to supply usable nitrogen. “Excessive nitrogen causes a change in the composition of the microorganisms from nitrogen fixers to nitrogen feeders. Nitrogen, in effect, becomes like a drug – the more you give, the more you’ll have to give to maintain yields,” he says. In contrast, a lack of nitrogen is usually the most limiting factor on organic farms and determining the optimal levels is difficult. “Soil samples provide reliable guidelines for determining all other macro-nutrients except nitrogen.
Most farmers simply estimate how much nitrogen is needed, and as a result, tend to over-apply it. Very few actually test the soil to determine the nitrogen levels,” says Fair. S oil expert Neal Kinsey explains the reason it’s so difficult to make application recommendations is because nitrogen can either be used up by the crop, or become part of the humus complex, or it can leach into the soil, erode away, or escape as gas. He points out that all the nitrogen of any commercial source may be lost if the crop is waterlogged for more than four days.
Figuring out nitrogen applications H e suggests these guidelines to determine nitrogen applications – first determine the amount of nitrogen needed to produce a specific yield in a certain crop. For example, around 225kg/ha of nitrogen would be needed to produce 9,6t/ha of maize and 135kg/ha to produce 2,7t/ha of wheat. An additional 10% should be added to this amount to ensure sufficient supply. Next, the amount of nitrogen already available to the plant needs to be determined. Organic nitrogen is part of the humus or organic matter in the soil and constitutes 97% to 98% of the total amount.
Ammonium, nitrate and other soluble compounds are readily available, but ammonium is held by soil colloids and is less available to the plant than nitrate. It accounts for less than 1% of the total nitrogen level in the soil. According to the Albrecht system, soil with a 5,1% humus content would have 112kg/ha of nitrogen available over a 12-month period, while soil with a 2,3% content would have 74kg/ha of nitrogen.
Legumes and nitrogen
When planting legumes such as soya beans and lupins, the nitrogen that will become available through the production of these crops should also be taken into account. This is because legumes fix nitrogen from the air through their symbiotic relationship with the Rhizobium bacteria. Kinsey cautions that the amount of atmospheric nitrogen would depend on growing conditions. Legumes only fix around 70% of their needs from the air under excellent production conditions, 50% under average conditions and less than 25% under poor conditions. However, these crops are unable to fix nitrogen during the early stages of growth. Next, the amount of nitrogen to be added to the soil must be calculated.
So if, for instance, 314kg/ha is needed to produce 3,4t/ha of soya beans, add 10% to ensure there’s sufficient nitrogen. This takes us to a crop requirement of 345kg/ha. A soil humus content of 2,3% should provide 74kg/ha of nitrogen towards production in the soil. An average year is expected, meaning that soya beans would be able to fix around 50% (157kg) of their nitrogen needs from the air. The farmer would only need to apply an additional 114kg/ha of nitrogen. If the plant requirements are 314kg + 10% = 345kg, the farmer needs to apply 345 – (74 + 157) = 114kg/ha. Eliminating the guesswork Legumes can leave nitrogen in the soil for the next crop and Kinsey advises that instead of guessing how much is left, use an ammonium or nitrate measurement. A soil nitrogen test or a tissue analysis could also be used during the growth stages to determine whether there’ sufficient nitrogen in the soil and whether the crop’s nitrogen needs are being met during production.
Manure – more than meets the eye
Brown manure can help to increase humus levels and nitrogen levels. Green manure, however, doesn’t have the same effect, as the soil needs nitrogen for the decomposition process. If there isn’t sufficient nitrogen for the crop to break down, it will compete with the new crop for nitrogen. Manure is an excellent source of nitrogen, but farmers must regularly analyse it to determine the nitrogen levels that would become available from this resource and also during every season. Once farmers know the nitrogen content of the manure, they can divide it by two to determine the nitrogen that will become available to the next crop. It is important to note that only half of the nitrogen contained in manure will become available during the first year after application. For more information contact SA Biofarm on (012) 333 4222 or e-mail [email protected]. |fw
