Humus – learn to appreciate it

Chemical fertilisers came into their own after the Second World War. They were convenient to use and food production became much easier, relatively speaking.

Soil with a low humus content can look like this when cultivated: full of hard, compact clods.
Photo: Bill Kerr

The downside was that the organic matter of soil became neglected. Initially, chemical fertilisers worked well with the organic content of the soil, but then that content was depleted, and many areas became unproductive dust bowls.Now, I am not knocking chemical fertilisers. I am merely saying we need to take better care of the soil’s organic content.

This is what happened: farmers were drawing on the soil’s ‘bank balance’, which is the organic content, or humus in this case, to subsidise their production, until they became overdrawn.

Not just a reserve
Humus contains nitrogen (N) and other minerals which are released as the humus breaks down. However, humus is not only a reserve of nutrients. As mentioned in a previous article, it also has various other functions which make production easier and safer. It binds to soil particles, creating aggregates which allow for adequate aeration and better water penetration. It also acts as a sponge to retain more water and holds nutrients to prevent leaching.

Another important function of humus is to provide food for an environment where the right kind of soil organisms can thrive.
To ascribe a value to the nutritional content, we can make the following calculation. We know 1ha of soil to a depth of 15cm weighs about 2 000 000kg, and organic matter of 4% equates to 80 000kg. Humus contains 50% carbon (C) and has a C:N ratio of 10:1, so we have 40 000kg of C and 4 000kg of N/ha.

In commercial terms, it is equivalent to 14t LAN (limestone ammonium nitrate) as a reserve.In addition, we have about 400kg each of phosphorous, potassium and sulphur locked into it. We need to have the organic content listed in our soil analysis and we need to try and raise it as much as is economically viable for our cropping system.

According to Prof Steve Thien from Kansas State University in the US, most crop residues on a dry matter (DM) basis have 400kg of C/t. Of this, a maximum of 140kg can be developed into humus. The remainder is used by the soil organisms during respiration and released from the soil as carbon dioxide. Humus is 50% C and so a maximum of 280kg of humus is produced from 1t of DM.

Magic ratio
If we take the extreme example of wheat straw, we have a C:N ratio of 80:1. This translates into 5kg of N in the residue. As 140kg of C is available, we would require 14kg of N to produce the maximum humus possible. This gives us a shortfall of 9kg N. If the 9kg N is not applied during breakdown, only 50kg of C will be used for humus production of 140kg – a shortfall of 90kg of C, or 180kg of humus/t of DM.

This explains why some crops deplete the humus content of the soil so much when not rotated with legumes, which have more than enough N. The magic C:N ratio is 30:1. Wider than this and we will have to add N to prevent the balance of C being lost to the air. You can reach an equilibrium in soil where the organic matter – either added or derived from crop residue – maintains the organic content at a constant level. This is sustainability.

Having active organisms in the soil means that C is lost by respiration as they feed on the soil’s organic content. To build up soil humus, you need to add more than is lost until you reach a level you deem acceptable. Humus is lost more easily in warm soils where organisms are stimulated by the temperature. Sandy soils that are well-aerated add to this loss and in some cases
it may become too costly to try and build up the humus level to what is considered an acceptable level.

If this is the case, a different strategy must be adopted.