No. 6. DEFARTMiiNT OF AGRICULTURfC. 486 



to insure maximum giowtli, as it would be ia order to grow any 

 otlier crop, the purpose of which is the crop itself, rather than for 

 its effect in soil improvement. This is entirely reasonable, as the 

 mineral constituents cannot be attained from any other source than 

 the soil, and they are quite as essential in the complete growth and 

 development of a crop as is the nitrogen. 



The point is, that green manuring does not result in building up 

 and making better a soil that is naturally poor in the mineral con- 

 stituents. It can only contribute the nitrogen, and that in itself 

 is not a complete food, it is only one of the essential, though very 

 important elements. On heavier soils, overlying strong subsoils, 

 as, for example, clay loams, and which are usually rich in dormant 

 mineral constituents, the practice of growing the legumes may re- 

 sult in building up the soil, both in available nitrogen and in availa- 

 ble minerals, as these plants have the power of acquiring the min- 

 erals from such soils more readily than certain other classes of 

 plants, and thus by their growth there is an accumulation of nitro- 

 gen, as well as an accumulation of the mineral elements in a more 

 available state, and the soil is improved in all directions without 

 the direct addition of any fertilizing substance. The roots running 

 deeply in the soil and subsoil gather the mineral food and store 

 it in their larger roots near the surface, where it is not only avail- 

 able from the standpoint of other plants reaching it, but available 

 from the standpoint of its being more readily acquired by them. 

 Such soils may be improved and kept in a high state of fertility 

 by the frequent growth of a renovating crop, whereas, on light soils 

 which are poor in all constituents, the improvement will be in pro- 

 portion to the amount of minerals supplied, as they will measure 

 in a degree, at least, the rate of appropriation of the nitrogen. 

 These considerations are practically true for all of the plants in 

 the group of legumes that has been considered, though the different 

 plants may apparently differ materially in their rapidity or degree 

 of growth, even on the same soil and without the addition of other 

 materials. The fact remains, however, that genuine improvement 

 of poor soils cannot be expected from the growth and use of green 

 manure crops alone. 



The Plowing in of Green Manures. 



Another point that should be clearly understood in the matter 

 of soil improvement by means of green manures is the time for plow- 

 ing down the crop. In the first place, the character of soil exerts 

 an influence on this point. As a broad, general rule, there is danger 

 of injury to light soils in plowing down a heavy burden of green 

 material, particularly in warm weather, whereas upon heavy soils 



