Manuring And Fertilizing

Cover crops may be said to be supplementary to tillage. In the

previous chapter this function has been discussed. It now remains to

point out another important function--that of a green manure crop

adding humus and plant food to the soil. Not only do some cover crops

add plant food and all humus to the soil, but they tend to conserve

these by preventing leaching, especially of nitrates, and they help to

render plant food more available by reworking it and leaving it in a

form more available for the tree. They sometimes act as a protection

against winter injury by holding snow and by their own bulk. They also

help to dry out the soil in spring, thus making the land tillable

earlier.



There are two great classes of cover or green manure crops, leguminous

and non-leguminous. A non-leguminous crop merely adds humus and

improves the physical condition of the soil. In itself it adds no

plant food, although it may take up, utilize, and leave behind plant

food in a more available form for the tree's use. But in addition to

these benefits, leguminous crops actually add to the soil plant food

in the form of nitrogen which they have the ability to assimilate from

the air by means of bacterial organisms on their roots.