Nitrogen, humus & legumes

Why it’s more profitable to fertilise a small area of grass well, rather than a large area poorly.

Nitrogen, humus & legumes
Photo: John Fair

In my previous column, I covered some aspects of Prof JJ Theron’s chapter ‘The Recuperation of Soils Under Grass Pastures’, in The Grasses and Pastures of South Africa, by Lucy Chippindall. Here he discusses rebuilding organic carbon (humus) in arable soil that has been burnt out by conventional tillage practices. What I didn’t point out is that humus is the end result of organic matter digestion by soil micro-organisms. It creates a healthy crumb structure that ensures adequate space for soil moisture and air.

It also helps the soil to breathe – oxygen moves in and carbon is ‘exhaled’. Aaerobic micro-organisms need oxygen to turn organic matter into humus. Readers who want to build up the biological fertility of their soils also need to know that humus is a vital source of nutrients and is in a constant state of flux – either being broken down or built up. Another important fact is that humus has a carbon/nitrogen ratio of about 12:1, and both are required to build humus.

Now, carbon is normally in abundance on a land that’s been planted to a grass pasture. The problem lies with nitrogen. This expensive component has to be added. With these basic facts in place we can discuss the ‘economic practicalities’ of fertilising grass with nitrogen. I can still see Prof Pine Pienaar, my pasture lecturer at university, drawing a graph of the response of grass pasture to nitrogen fertiliser.

Contrary to what I expected, the pasture yield response wasn’t linear – it resembled a forward-sloping S-curve. As the response to each unit of nitrogen applied was greater than the previous one, the graph became progressively steeper until the point was reached when it flattened off and started declining – that is, when excessive nitrogen is applied.

Building humus
Prof Theron provides a credible explanation for this phenomenon. It all comes down to damaged nature’s urge to rehabilitate itself when given the chance to do so. The destruction of humus by aggressive tillage is arrested by establishing grass pasture. The system is stabilised and nature sets about ‘rebuilding’ humus. Microbes use applied nitrogen to build humus and ‘rob’ the pasture of the nitrogen it requires. But the humus builders can only process a limited amount of nitrogen per time unit. The economic implication of this fact is that, as the amount of nitrogen applied to the pasture is increased, the percentage of nitrogen ‘stolen’ by the microbes is reduced.

Growing nitrogen
I’ve said it before and I’ll say it again: farmers lose more money by under-fertilising than by over-fertilising grass pastures with nitrogen. Said in another way, it’s more profitable to fertilise a small area of grass well than a large area poorly. Large quantities of nitrogen are required to build humus, and chemical nitrogen isn’t a solution. It’s horrendously expensive, toxic to humus builders, acidifies the soil and drives out essential plant nutrients, including calcium.There’s only one solution – plant legumes with grass, and ‘grow’ nitrogen instead of buying it.

John Fair is a leading expert on pastures. He heads up Fair’s Biofarm Assist, and can be contacted on 058 622 3585 or [email protected].