During the mid-1970s, I worked as an extension officer in the eastern Free State. In those years, soil classification was the
‘in thing’ – soils were rated in terms of cash crop yield potential and optimal fertiliser levels, and plant densities were calculated accordingly. But none of the standard soil tests of the time measured the humus (organic carbon) level, and its importance in determining yield was thus ignored.
It was only later, after spending time with farmers, that I came to learn about the crucial role of humus. I recall, for example, being on a farm where the soil had recently been classified. In a section of land, maize had been planted in very shallow, low-potential virgin soil In a second section, maize had been planted in deep, high-potential soil, which had been under cultivation for more than 50 years.
Remarkably, the maize did better on the shallow soil than on the deep soil, yielding nearly 2t/ha more grain than the deep soil. It was humus that had made the difference! Regular readers of this column will recall a piece about shale being turned into soil after being planted to crownvetch (Coronila varia) on a steep road cutting on the N3 about 7km from the Mooi River toll plaza in KwaZulu-Natal. Nothing had grown on this section of blue shale until, in the mid-1980s, one visionary pasture scientist planted several crownvetch plants at the bottom of the cutting. In less than 30 years, most of the cutting was covered.
Increase in humus
When I took samples of of blue shale (zero vegetation) and similar shale populated by crownvetch, the latter revealed a dramatic increase in humus and soil minerals. The percentage of humus had increased fourfold, from 1,3% to 5,2%. Phosphorus and sulphur, both macro-nutrients, had increased nearly two-thirds and more than threefold respectively.
Two important micro-nutrients, zinc (Zn) and manganese (Mn), had also increased sharply; levels of Z had risen nearly ninefold and levels of Mn were 14 times higher!
All this had been brought about by plant/soil microbe association. The crownvetch supplied the soil micro-organisms with the energy to solubilise the minerals necessary for this perennial legume. Clearly, the microbes had solubilised more than the crownvetch could use. I then conducted tests on red shale on a slope in the Ficksburg district of the Free State, where crownvetch has been growing for close to 30 years. Sampling the bare red shale areas and areas populated with crown revealed that the percentage of humus had more than doubled (1,5% to 3,5%), while that of phosphorous had quadrupled.
Humus is comprised, among other substances, of humic acids, which solubilise minerals in the soil, making them available to plants. They bind to and stabilise chemical fertiliser, making it more effective because less is leached or lost to the atmosphere. For the past three years I have been conducting trials for a company that markets a humic acid derived from the wood pulp industry. Just 20kg of their product – without any other fertiliser – greatly improves the yield of maize, wheat and beans. The cost of applying the product is about R100/ha, transport costs excluded. I have also measured additional yield increases as a result of adding the product to fertiliser.
We’ve certainly come a long way in recognising the true value of humus!