Green manuring is another relatively inexpensive way of adding organic matter to the soil, but the system is usually confined to vegetable gardens when empty beds are lying idle in winter. In our climate, because there are many vegetables which can be grown in winter, there is often little space left over for a green manure crop. However, if you do have an empty bed there are several green manure crops you can use. Seeds of wheat, barley or oats can be broadcast at 30–60 g per square metre in autumn to provide a large bulk of material to dig into the soil in spring. Legume crops which add nitrogen through nodule bacteria in their roots are usually preferred. Suitable crops for autumn planting are field peas and vetches. The recommended seed rate is 15–30 g per square metre. Dig the crops in when they begin to flower in spring. Good summer-growing legume crops for warm northern climates include dolichos (lab lab bean) and cow peas. These are sown in spring and dug in by mid-summer when they flower. A complete fertiliser must be broadcast at one-third of a cup per square metre when sowing all green manure crops.
Water the crops a day or two before digging them in. If the crop is very tall, flatten it and chop up with a sharp spade. After digging, keep the soil damp but not wet, then after three weeks dig the soil over again. It will take another three or four weeks for the organic matter to decompose. If there is any sign of yellowing in the following crop, give side dressings of a nitrogen fertiliser.