Live Stock Breeders' Association. 251 



tion of farm manure, there is a positive addition to the soil of the 

 materials of which crops are made, so that with sufficient manure 

 the soil may be made richer and richer for an indefinite length of 

 time. 



The only element of plant food that can be added to the soil by 

 crop rotations is nitrogen, which can be secured from the air by 

 clover and other legume crops. 



It should, be understood, however, that the most marked and 

 most common effect produced by clover, by which increased yields 

 of succeeding crops are secured, is not due largely or primarily to 

 the addition of nitrogen, but rather to the power of clover to liberate 

 mineral plant food from the soil for the use of the following crops. 



This process may be continued successfully until the supply of 

 phosphorus (or of potassium, in some cases) becomes so reduced 

 that even the strong-feeding clover plant cannot secure enough 

 phosphorus for its own growth. When this condition arrives the 

 clover crop begins to fail, and the only recourse is to begin to re- 

 turn the exhausted plant food. It may be returned in bone meal, 

 in raw rock phosphate, or in sufficient amounts of farm manure. 

 Indeed, the most marked and beneficial effect of farm manure is 

 often seen when it is applied for the clover crop. This fact okne, 

 which is a common observation, is sufficient to show that farm 

 manure has a value not possessed by clover or by crop rotations. 



We should not discourage the rotation of crops, because crop 

 rotation helps us to grow larger crops, and to be successful in 

 farming requires that large crops shall be grown, even though 

 correspondingly large amounts of plant food are removed from the 

 soil. 



On the other hand, we should plan always to return in some 

 form the kind or kinds of p-lant food that are becoming deficient in 

 the soil. 



Let us consider in further detail the effect of crop rotation upon 

 soil fertility. Suppose we are practicing a four-year rotation, in- 

 cluding corn for two years, oats with clover seeding the third year, 

 and clover for hay and seed crops the fourth year. Let us assume 

 such crop yields as have been produced, and as can be produced, in 

 normal seasons on the richest, best-treated land with good seed and 

 good farming; namely, 100 bushels of corn per acre, 100 bushels of 

 oats and 4 tons per acre of clover, including perhaps 3 tons in the 

 hay crop and 1 ton in the seed crop. If we do not succeed in se- 

 curing these yields, we should at least try to make such yields possi- 

 ble, and we should approach as near to them as we c^n. 



