SOILS AND FERTILIZERS 499 



summer, often brings about the formation of considerable acid, or, 

 in other words, makes the soil sour. This condition may be cor- 

 rected by the use of lime, which should be applied at the rate of a 

 ton to a ton and a half to the acre. Some farmers claim to get much 

 better results by applying the lime before plowing under the crop. 

 In any case, it is advisable to let the green manure after being 

 plowed under decay a month or more before planting the succeeding 

 crop. 



The full results of a green manure are obtained only after it is 

 completely decayed and changed into a soluble form. This requires 

 considerable time, so that the beneficial results are often shown for 

 two or more seasons. Generally speaking, cultivated crops are the 

 best to follow green manures. The tillage of such crops hastens the 

 decay of the vegetable matter and, by aerating the soil, favors addi- 

 tional nitrogen fixation by the soil bacteria. Corn, cotton, potatoes, 

 and tobacco, on most soils, derive great benefit when following green 

 manures. Rye and oats are likewise good crops to use in this way. 

 Wheat and barley give varying results, often very favorable, but not 

 infrequently there is no increase or even a loss. This is especially 

 true the first season following a green-manure crop. 



The Choice of a Green-Manure Crop. Whenever a green-ma- 

 nure crop is to be used, preference should be given to a legume if it 

 is available, because such a crop will add materially to the nitrogen 

 content of the soil. Where there is a choice between two or more 

 legumes, the question arises as to which is to be preferred. Among 

 the important points that determine the value of a leguminous crop 

 are (1) its value as forage, both in quantity and quality, either as 

 hay or pasture; (2) its ability to supply additional nitrogen. Other 

 points that deserve consideration are the cost of the seed, the ease of 

 plowing under the crop, the deep-rootedness of the plants, and their 

 ability to choke out weeds. On lands of fair quality the largest profit 

 is usually obtained by using the crop as fodder. This may be done 

 by harvesting it for hay, leaving only the stubble to be added to the 

 soil, or by pasturing the crop, by which means the most of it is re- 

 turned to enrich the land. 



Where the whole crop is plowed under, the effect on the sub- 

 sequent crops is determined mainly by three factors, namely, the 

 amount of humus formed, which is directly measured by the ton- 

 nage of the crop; the physical changes brought about in the soil, 

 and the quantity of nitrogen taken from the air and added to the 

 soil. It is generally believed that the addition of nitrogen is the most 

 important of these factors. While different legumes are known to 

 vary in their respective abilities to absorb atmospheric nitrogen, 

 there is much yet to be learned on the subject. If the benefit of 

 different legumes is proportional to the amount of nitrogen each 

 adds to the soil, then their relative values can be determined in ad- 

 vance. Thus far the value of different green manures has been de- 

 termined wholly or mainly toy the influence they have on the suc- 

 ceeding crop or crops. 



