How to build up humus in your vegetable crop

To build up humus in the soil, we have to combine various strategies. As mentioned in a previous article on this series, we have to adopt a no-till philosophy. Tillage ‘burns up’ the organic content of the soil relatively quickly.

The next step is to apply some form of organic fertilisation that can be compost or manure. It is important to leave this on the soil surface or work it in partially and shallowly into the surface in the process of hoeing out weeds.

A further step that can speed up the process of humus accumulation is to plant a leguminous cover crop. Legumes have bacteria that take nitrogen from the air, which contains nearly 80% nitrogen, and share it with the plant. They form nodules on the roots that are easily seen and indicate that the plant is inoculated.

When starting off, one must apply the relevant inoculum to the seeds to start off the process. For this system there are limited options to select from. In cold areas, we often find a gap in production over the winter period that coincides with the period when grazing vetch grows well.

In subtropical areas, the slack period for producing vegetables is in summer, which suits the growing of sun hemp.

For practical purposes, we plant two rows on a bed in the position where the next vegetable crop will be planted. In the cold areas, the optimum period to plant the grazing vetch is April. It will be cut in September before the seed matures or it will become a weed.

Usually, one would plant a section to the vetch crop and it is convenient for me to plant half my land to vetch.

These legumes have a high amount of nitrogen in their foliage, more than enough for the maximum amount to ensure that there is sufficient for the maximum amount of humus to be formed from the crop residue as per the formula provided in a previous article.

Apart from the organic matter in the above-ground foliage, there is the root system also contributing to the organic content as well as the nitrogen-rich nodules. In fact, there will be sufficient nitrogen for maximum humus formation of 280kg of humus per ton of plant residue, plus extra for the crop that is planted after the legume.

Calculating the mass of humus

The volume of roots will be roughly the same as the above-ground foliage. This enables us to calculate how much humus we can produce from this crop. I do this by weighing the dry grazing vetch on 10m of bed after cutting and doubling this weight to accommodate the roots and calculate the humus as 280kg/t.

I cut the grazing vetch with a spade after having pulled the vetch over the bed with a garden rake from each side. This leaves a 50cm of mulch in the middle of the bed, and planting can immediately commence on the edges of this mulch.

The residue will slowly break down, feeding the soil food web and providing a wonderful environment for plant growth.