Rules of Green Manuring
To give land a rest is a good thing. Even as far back as Old Testament times the practice was to rest theevery seven years.
In a large garden a good plan is to rest one-seventh of it every year, and to sow this seventh with awhich can be dug or ploughed in at the end of the season. This method, known in America as Sheet Composting, consists of spreading green manure over the land like a big sheet, applying an activator over the top, and then digging or ploughing in the material so that the rotting process takes place in the soil.
But green manure methods, if wrongly applied, can have dire results. For when crops are sown and are dug into the land in a fresh condition the soil organisms have to get to work on the green plants and break them down. In doing so they starve the land of nitrogen, for they have to borrow as much of this plant food as possible in order to do their work. Fresh green manure, therefore, normally causes a reduction, at any rate for the time being, in the available nitrogen content of the soil.
One notable exception is the annual lupin, which is used extensively in Ger-many for green manuring. This plant has nitrogenous nodules on its roots, so that when it is dug in the soil suffers no nitro-gen starvation. But even with lupins there are problems of undigested organic substances remaining which may make the roots of subsequent crops quite unhappy.
Because green manure does not give as quick results as good composted vegetable refuse it is not often used in ordinary gardens. It is slow in action — it usually takes six months before the fresh green matter is properly rotted down and is ready for plant roots to use. The best results with green manuring are achieved under the following conditions:
1. The land is properly drained so that sufficient air is present.
2. The soil is adequately limed so that it is not acid.
3. An activator is applied in order to provide the energy with which the organisms can start work.
4. The soil is warm.
There are three main systems of green manuring, (a) the temporary ley, (b) the crop to suit the soil and (c) double green manuring.
THE TEMPORARY LET
This method is to sow a grass mixture over the soil in the spring. The grass that comes up is allowed to establish itself, and is then cut every 10 days, the mowings being allowed to fall back into the soil. In the autumn, before the land gets cold, a fish manure is applied all over the surface of the grass at 3 oz. per sq. yd., or, if poultry are kept, the bird manure can be used at 5 oz. per sq. yd.
A ley mixture which will give good results, both on heavy and lighter soils, is — 4 parts by weight perennialS.23; 4 parts timothy S.51; 1 part clover S.100; 2 parts coltsfoot S.143; 2 parts rough stalked meadow grass; mix them together well and use at the rate of l oz. per sq. yd. Sow the seed in March. In very dry years it may be necessary to use some overhead irrigation in order to get the grass growing well. In October, turn in by digging or with a rotary cultivator.
THE CROP TO SUIT THE SOIL
Mustard is a very suitable crop for sandy, gravelly soils. It has the advantage of being a very quick grower, and can be dug in after 7 weeks. Before digging in, knock it about well with a spade and sprinkle it with a fish manure at 7 to 8 oz. per sq. yd.
If poultry manure is used instead, quite heavy quantities will be needed to prevent thefrom denitrifying the soil. One-eighth of an acre under mustard may need as much as 8 cwt. Poultry manure.
For medium loams a number of crops do well. These include rye, vetches, field, clovers and lupins. For heavier clay soils red clovers and vetches are good. Cut all these green crops down and smash them up well before digging them into the soil.
Sow mustard at 1/16 oz. per sq. yd., rye, vetches and lupins at 1/8 oz. per sq. yd., field peas at l oz. per sq. yd. And clovers at 1/16 oz. per sq. yd.
DOUBLE GREEN MANURING
It is possible to work out a system of double green manuring with the idea of obviating the difficulties arising from denitrification and of eliminating docks, thistles, nettles, couch grass and other perennialwhich as a rule last much longer in the soil than normally-grown green manure. If weeds are got rid of in this way, they make a valuable form of humus.
The double system is simply the sowing of two green manure crops in one season instead of one temporary ley. Prepare the land in either the autumn or early spring. Rake it over and sow tares at the rate of ½ oz. per sq. yd. When the tares come into flower in late June or July, smash them with a spade or other implement and then apply dried blood at 3 oz. per sq. yd., or poultry manure at 8 or 9 oz. per sq. yd. Leave the green manure with this activator on top for eight days before digging in. Allow an-other three or four days for the soil to settle, and then rake it level. Then sow rye at l oz. per sq. yd. And rake it in lightly.
Allow the rye to grow until, say, the end of October. Then cut it with a pair of shears, treat it with dried blood or poultry manure at the same rate as for the tares, and then dig in. In this way two green manures are incorporated into the soil in the one year, and the ground is thoroughly cleaned as a result. This double green manuring method has given first-class results in new gardens, by ensuring that the ground is clean and by adding humus to the soil to give the new garden a good start. It is difficult to buy and transport dung, but double green manuring does the same work as the dung with the minimum of expense.