Humus Formation

Humus formation in tropical climate is less since temperature is high and at high temperature mulching materials oxidise and lost to atmosphere. When we grow cover crop in rainy season and by the time mulch is formed and by the time it is useful for next cropping cycle, much of it will be lost in the summer coming after the rains. 

I grow cover crop like sunhemp, pureria javanica and also leguminous trees like glyrecedia and subabul. Initially I had a confusion when to cut and mulch these covercrops. I could see that when I cut the covercrop or leguminous trees after rain stops, then the cover will be lost and it becomes very dry. So I stopped cutting any grass or covercrop or tree cover after the rain. So basically to form good humus in your farm, grow as many different types of covercrops and after cutting and mulching, make sure there is good shade for the mulch. Another option is to make trenches in the farm and then mulch the trenches, since sunlight and wind does not reach trench directly, good humus will be formed here.

Recently while reading the book 'Restoring the Soil' by Roland Bunch came across the following text.

A combination of two or three gm/ccs may provide the best results. For example, using trees and another type of low growing gm/cc could be a practical answer to this problem. By reducing the ambient temperature at least 10°C, dispersed trees can cool the fields enough so that the gm/ccs’ organic matter will not be burned off, and the soil’s fertility can be maintained. The temperature can be lowered even more if the trees are not pruned until the months right before the next rainy season, as is normally the case. Thus, dispersed shade can largely eliminate the problem of dry season burn-off of the gm/ccs’ organic matter and nitrogen.

Some more informations about green manuring...

https://eap.mcgill.ca/publications/EAP51.htm

"Agronomists have argued that green manuring will increase either the humus content or the supply of available nitrogen in the soil, but rarely both at the same time. The humus content is only increased appreciably if material fairly resistant to decomposition is added to the soil (high Carbon:Nitrogen ratio), and this type of plant material is typically low in nitrogen (less than 1.5 per cent on a dry-weight basis). The available nitrogen supply is only increased if readily decomposable material high in nitrogen, such as immature green plants, is incorporated into the soil. The amount of organic matter that may accumulate will vary with the soil, climatic conditions, and the age and type of crop.

The accompanying diagram shows that mature residues, whether legume or non-legume, have a much higher C:N ratio than do the younger succulent materials. Also, the legume crops, which have two to three times the nitrogen content of the non-legumes, have lower C:N ratios throughout their life cycles, and even if turned under when mature (e.g. 3rd year alfalfa, 2nd year clover) will yield their nitrogen more quickly than if oats or buckwheat straw were ploughed under. "