153 



LEGUMINOUS PLANTS FOR GREEN MANURING 

 AND FOR FEEDING. 



By E. W. Allen, Ph. D., Assistant Director of the Office of Experi- 

 ment Stations, U. S. Department of Agriculture. 



Reprinted from Farmers' Bulletin, No. 16. 



Green Manuring. 



Green manuring, or ploughing under green crops raised for that pur- 

 pose, is one of the oldest means of improving the fertility of the soil. 

 It was advocated by Roman writers more than two thousand years ago, 

 and from that time until now it has formed a most important resource 

 of the farmer, especially where the supply of barnyard manure is insuffi- 

 cient. Its advantages are many. The more striking are that it fur- 

 nishes the surface soil with a supply of the fertilising materials needed 

 by crops, increases the humus, and improves the physical qualities and 

 the tilth of the soil. As a humus-former green manuring stands next 

 to barnyard manure. 



By means of green manuring, land which is practically barren may 

 be brought up to a sta'e of fertility where it will produce pro6table 

 crops. Asa single instance of this may be mentioned the experiments 

 carried on by the Michigan Experiment Staiion on the "Jack-pine 

 plains" of that State. In 1888 experiments were undertaken on the 

 light sandy, almost barren, soils of these plains. Green manures were 

 used mainly, supplemented by cheap fertilisers. In three years marked 

 improvement was evident, not only in the physical character of the soil 

 but also in the increased yields of various crops. 



Again, green manuring may be used to take the place of more expen- 

 sive fertilisers and manures on soils already under cultivation. It is 

 in this latter use that it finds its widest application. 



There has been much speculation as to the manner in which the crops 

 commonly used for green manuring could gather such large quantities 

 of fertilising materials. It will be remembered that the principal fer- 

 tilising ingredients required by plants are r.itrogen, phosphoric acid, 

 and potash. These are each and all more or less essential to the 

 healthy growth of crops. Consequently they are applied to the soil in 

 the form of commercial fertilisers and other manures. In attempting 

 to explain how the fertility of the soil is maintained by green manuring 

 it has been said that plants with long roots, like clovers, feed deep 

 down in the soil or subsoil on materials beyond the reach of surface- 

 feeding plants ; and that when the tops of these plants die down and 

 are mixed with the surface soil they enrich it much the same as an appli- 

 cation of barnyard manure. This is undoubedly true, but it fails to 

 explain how such large quantities of materials can be obtained, espe- 

 cially when clover is grown continuously for a number of years. The 

 question has finally been solved by one of the most interesting and 

 important discoveries yet made in agricultural science. It has been 

 found that certain plants can feed upon the nitrogen in the atmosphere 

 and store it up in their tissues 8 8 they grow. They take their phos- 

 phoric acid and potash from the soil, but they obtain their nitrogen very 

 largely from the air. Hence they draw from the air a material neces- 

 sary to the growth of crops which in the form of commercial fertilisers, 



L 



