130 Fertilizers 



with the specific bacteria present in the soil, their use 

 results not only in the addition of nitrogen to the soil, 

 which may be useful for other plants, but by the accumu- 

 lation of vegetable matter, which improves the physical 

 character, usually imperfect in this class of soils. The 

 nitrogen thus introduced into the soil is also in a very 

 good form; that is, it has a tendency to decay rapidly 

 and thus supply the needs of other plants, but the helpful 

 additions to the soil are limited to organic matter and 

 nitrogen. The mineral constituents absorbed by the 

 crop may be more available for other crops, but they 

 formerly existed there. No additions of these are made 

 by the growing of the crop; hence no system of green- 

 manuring can be made successful unless there is a previous 

 abundance in the soil of the mineral elements, or unless 

 these have been directly applied. (See Figs. 8 and 9, 

 Plate V.) 



The most useful crops. 



The crops most useful for green-manures are red clover, 

 crimson clover, alfalfa, sweet clover, winter vetch, soy 

 beans and cowpeas, because of their capacity to gather 

 nitrogen, and because of their period and time of growth. 

 Whether these plants will gather all of the nitrogen of 

 their growth from the air, other conditions being good, 

 depends upon whether the soil is rich or poor in nitrogen, 

 since it has been shown that these plants will gather at 

 least a part of the nitrogen from the soil in preference to 

 that from the air, unless they are starved in respect to soil 

 nitrogen. The amounts that may be gathered from the 

 air, therefore, are not measured by the total content of 

 nitrogen contained in the plant grown (which may, in the 

 case of good crops, amount to as much as 200 pounds to 



