Essentials for successful green manuring
Firstly, acrop must fit in well with your cropping plan: you should regard it as a catch crop, to be grown and dug in during the period between the harvesting and sowing of food crops. Secondly, the green manure crop must be easily sown and must germinate readily, and there should be no need for elaborate preparation of the seedbed. Thirdly, the seed should be reasonably cheap; and finally, it must do most of its growing before the beginning or after the end of the crop growing season, so that the is clear for your crops during spring and summer—this is especially important in small, intensively cultivated gardens.
In most cases, it is essential to dress the soil with a nitrogenous fertilizer when the green manure crop is dug in, because the soil bacteria that carry out the early stages of the breakdown need nitrogen to do their work. If this is not readily available, they will rob the soil of any nitrogen that it contains and the soil will become ‘starved’ of nitrogen, and unable to produce good crops.
The situation is different, however, if the green manure crop is a legume (a member of the pea and bean family, including clover and lupin). These plants have swellings, or ‘nodules’, on their roots, which contain bacteria that are able to ‘fix’ nitrogen gas from the air so that it is available to plants. With leguminous green manure crops, therefore, no fertilizer need be added. For the same reason, it is not necessary to add fertilizer if the crop previous to the green manure crop was a legume, such as a row ofor beans, provided that its remains were dug into the soil.
The right soil conditions
It is essential to ensure that the soil is well-drained so that it contains plenty of air, to supply both the roots of the green manure plants and the bacteria which effect their breakdown when they are dug in.
On the other hand, the soil should never be allowed to dry out; keep it well watered, just as you would for any other crop. If your soil is too acid, you will need to apply lime (at a rate given by the result of a soil test) to make it more alkaline. The soil bacteria that are involved in the breakdown processes cannot thrive in acid soil.
Also, if the bacteria are to do their work properly, they need warmth; a soil temperature of at least 10°C (50°F) should be aimed for. As with aeration, this can be achieved by good drainage: well-drained soil warms up quicker and retains its heat better than poorly-drained, waterlogged soil. Maintaining a high enough temperature should present’ no problem with green manure crops that are dug in during summer and early autumn, though it may prove a problem during the winter.
Light sandy soils and light loams warm up earlier in the year than heavy clays; chalk soils aie intermediate between the two extremes in this respect. Peat soils, though relatively easy to drain, are usually far too acid for the soil bacteria, unless plenty of lime is added.
Apart from removing or digging in any remains of crops and lightly forking the soil, no elaborate preparation of the plot is necessary. It is a good idea to dress the soil before sowing with a granular nitrogenous fertilizer, such as sulphate of ammonia, at the rate of about 30 g per sq m (1 oz per sq yd), to encourage lush growth of the green manure crop.
Next, sow the seeds of the green manure crop, either broadcast or in rows or drills, depending on the plants chosen, and then cover lightly with soil by gently raking it over the seeds. Sowing broadcast means scattering the seeds, as early as possible, over the site. It is more difficult to cover the seeds with soil (to assist germination and prevent birds from eating them) when sowing broadcast, and it is also less economical compared with sowing in rows or drills.
As thegrow rapidly, they provide an effective form of weed control. The crop should be allowed to grow until the first frosts arrive, or until the flower buds begin to form; do not leave it beyond this stage, as the stems become tough and woody and decay too slowly to be of value as a manure. Also the bacteria that break them down will have to ‘rob’ the soil of large quantities of nitrogen to deal with the woody tissues. If, for any reason, you have left your plants to grow to this stage, remove and them.
It is also unwise to dig in a green manure crop while it is still young, as the plants will then decompose very quickly in the soil, making for only a slight improvement in soil structure but releasing nitrogen rapidly, with the danger that it might be lost unless the following crop is sown or planted directly afterwards. More mature green manure crops will release nitrogen at a slower rate and produce more humus, thus having a more beneficial effect on soil structure.
It is difficult to say exactly how long it takes for a green manure crop to be converted into humus after it has been dug in, because so much depends on the particular plant used and upon the soil conditions, especially soil temperature.
As a rough guide, the first stages of the breakdown, when the soil bacteria have a heavy demand for nitrogen, are likely to be completed within about 6-8 weeks during warm weather, although in winter this may take several months. It is wise to postpone growing crops on the plot until this breakdown period has finished. If a crop has to be sown soon after digging in the green manure plants, give a dressing of sulphate of ammonia or another granular nitrogenous fertilizer at the rate of 60 g per sq m (2 oz per sq yd).