494 ANNUAL REPORT OF THE Off. Doc. 



that the plant have at its disposal a sufficient amount of available 

 phosphoric acid; the addition of this for the benefit of the catch crop 

 will not be lost, but be recovered again by the crop of wheat, and 

 by making conditions favorable for the growth of the cow pea the 

 plant is in a position to fully exert its function in the way of appro- 

 priating nitrogen. 



Danger of Injury to Soil by Green Manuring. 



A word of caution should be given concerning the turning under of 

 green crops as manure. If the conditions of season are not favor- 

 able, i. e., too warm and moist, the land may be injured rather than 

 benefited by the turning under of so large a mass of material. It 

 may ferment or change too rapidly, the rapid decay developing an 

 acid, which may be so great as to cause injury to the soil. If, on 

 the other hand, it is too dry, the crop does not decay for some time; 

 a mass of vegetable matter lies between the surface and subsoil, 

 which prevents capillary attraction, and results in a greater drying 

 of the surface soil than would be the case if no crop had been grovTii. 

 In either of these cases the possible disadvantages should be recog- 

 nized. Hence, where a crop of some size is turned under in hot 

 weather it is desirable to add lime, which will neutralize the acid 

 and assist in the decomposition of the vegetable matter. If the 

 crop is too heavy, a part at least should be removed and great care 

 taken to thoroughly "firm" the soil after it has been turned down. 

 It is also well to remember that a too frequent turning under of 

 heavy green manure crops will rapidly increase the soil's content of 

 nitrogen, and unless large applications of minerals are made, the 

 soil may become so rich in this element as to cause too great a 

 leaf growth of the cereal plants. When this stage is reached the 

 practice should be changed, the crop of cow peas should be used as 

 before, but preferably used as forage or made into hay, and only the 

 roots and stubble turned under for the wheat. The chances are 

 that in most cases this would be the better practice from the stand- 

 point of soil improvement — it certainly would be a more economical 

 method of practice where the forage can be used to advantage. 



The Application of the Principles in Other Rotations. 



The principles that have been pointed out in this discussion rela- 

 tive to the addition of crops that may be used in this rotation, in 

 order to improve the soil, hold true for other crop rotations, when 

 the land is left bare for any considerable period, and also in refer- 

 ence to the improvement that may come from the introduction into 

 the soil of considerable vegetable matter containing nitrogen, which 



