Written by Simon Hodgson
Like most things to do with cover crops there is no simple answer that will suit all situations. What must be taken into account are the goals aimed for, and once those are clear, management of the crop should be done accordingly.
This account is non-scientific and is put into terms which are simple to understand.
Timing of cover crop termination has a large influence on the result achieved. Consider that a plant goes through two major stages of development – the vegetative stage where it “builds a factory” and a reserve to enter the second phase, which is the reproductive or flowering stage.
Simplified, a plants reason for growing is to form a flower and then after fertilisation a seed, which guarantees the survival of the species. The physiology of the plant varies in these stages. In the early stages of plant growth the foliage is “soft and green’ and is forming nutrients and reserves for the next stage – the plant closes down at night and exudates in the form of liquid carbon, are exchanged into the soil to feed the soil microbes.
If one were to manage a cover crop in its vegetative growth phase, there would be significant amounts of nutrients which would be recycled by the soil microbiology. These nutrients would go through the digestive stages of the microbes and be available to the plants. The bacterial community would thrive as the Carbon to Nitrogen ratios would be skewed in the favour of Nitrogen. There would not be significant building of carbon in the soil but here would be an increase in the soil bacteria populations.
Conversely if a cover crop is allowed to grow through its vegetative stage into its reproductive stage the makeup of the plant differs. The plant “knows” that it has to hold a seed load off the ground so the stem becomes harder and more rigid with the formation of lignin – lignin and other structural tissues are “hard brown” carbon compounds and have a higher carbon to nitrogen ratio. The hard carbon compounds require fungal breakdown, so letting the cover crop grow to this stage before managing it will provide food for some of the fungal species in the soil and will stimulate their populations in the same way that the “soft green” material stimulates the bacterial populations. These “hard brown” carbon compounds are seen as carbon building material in the soil.
Timing of management is thus dependent on the desired outcome.
To build soil nutrients and stimulate bacterial populations – mange the cover crop when it is soft and green.
To build carbon and stimulate fungal populations – manage the cover crop when it is hard and brown.
These are very general “rules of thumb” that work.