This consists in growing a crop of some quick-growing plant, which when near maturity is to be ploughed in or dug into the soil, with the object of enriching it in humus or organic material and nitrogen. Sometimes the crop is fed to cattle, and the manure from the sheds is afterwards returned to the land. As one of the chief objects of green manuring is to supply nitrates to the soil, such leguminous plants as the Red, White, and Crimson Clovers, Peas, Vetches, Beans, Lupins, etc, are favoured for the purpose, because the bacterial nodules on their roots possess the power of fixing the free nitrogen from the air (see p. 127). Such non-leguminous crops as Mustard, Rape, Buckwheat, Borage, etc, are also grown as green manures because of their bulkiness and rapidity of growth, and the large amount of humus, etc, they return to the soil.

Whichever of these crops is grown the effect upon the soil is beneficial. The roots penetrate the soil and divide it into finer particles. Mineral and metallic foods are dissolved by the secretions from the roots, and being rendered soluble in water can be absorbed into the system of the crop. The soil becomes drier by the absorption and transpiration of moisture if it is inclined to be too wet; and eventually when the crop is ploughed in, or dug in completely, large quantities of humus become incorporated with the soil. As the green stems and leaves and roots decay in the dark a certain amount of heat is generated, carbonic acid gas is liberated and proceeds to dissolve the inorganic materials in the soil, and all the wonderful chemical changes due to the presence of humus take place in proper order to make the soil richer than it was before.

Many experiments have been carried out to prove the value of "green manuring", and an interesting paper on the subject will be found in the Journal of the Board of Agriculture, for June, 1897. The following figures show how a soil may be enriched in nitrogen and other foods when a green crop is incorporated with it: -

Name of Crop.

Dry Substance per Acre.

Fixed Nitrogen per Acre.

Equal to Nitrate of Soda.

lb.

lb.

lb.

Lathyrus Clymenum ...

5100

154

1000

Peas

7140

198

1267

Mixed leguminous plants

5998

165

1028

Lupins, white ...

6273

162

1039

„ blue

7020

171

1081

„ yellow ...

5090

130

847

From this experiment it would appear that Peas are the best crop to use as a green manure. Not only is there a larger supply of dry substance (over 3 tons per acre), but nearly 200 lb. of nitrogen is fixed in the soil. This is equal to over 1/2 ton of nitrate of soda. Reckoning the value of nitrate of soda at 10 per ton, the pea crop yielded up nitrogen to the value of 5 per acre, in addition to the other foods supplied by the decaying stems, leaves, and roots. The Yellow Lupins supplied over 5000 lb. of dry matter, and 130 lb. of nitrogen to the acre, and is thus the poorest of the leguminous fertilizers. Notwithstanding this fact it appears that any of the green manures mentioned are capable of supplying more nitrogen to the soil than is needed by most crops. Hops require about 200 lb. of nitrogen per acre, and this quantity can be supplied in advance by a crop of Peas. But Potatoes require from 50 lb. to 120 lb. of nitrogen per acre, and leguminous crops can supply far larger quantities as shown. The value of leguminous crops as manure was well known to the ancients, and Virgil in his "Georgics" refers to them thus: -

"At least where Vetches, Pulse, and Tares have stood, And stalks of Lupines grew (a stubborn wood), The ensuing season, in return, may bear The bearded product of the golden year".