No. 7. DEPARTMENT OF AGRICUI-.TURE. 451 



nitrogen, 27 pounds of phosphorus and 84 pounds of potassium per 

 acre per annum. 



If Ave disregard potassium (which is not very important because 

 of the richness of Rothamsted soil in that element) the amounts 

 of plant food applied and the average yields produced during half 

 a century are not markedly different. The yield of the fertilized 

 plot averages 1.3 bushels higher during the fifty years, but .7 bushel 

 lower during the last ten years than the manured plot. 



It is a very common and very erroneous belief that crop rotation 

 possesses about the same value as the application of farm manure. 



The great difference between these tw^o processes is that crop 

 rotation is a stimulant and ultimately reduces the fertility of the 

 soil to such a degree that the crops fail, especially the crops that 

 have the most stimulating effect, as clover; whereas, in application 

 of farm manure there is a positive addition to the soil of the mate- 

 rials 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 rotation is nitrogen, which can be secured from the air by clover 

 and other legume crops. 



It should be understood, however, that on many soils the marked 

 effect produced by clover, by which increased yields of succeeding 

 crops are secured, is not due largely or primaril}- to the addition of 

 nitrogen, but rather to the power of clover to liberate mineial plant 

 food from the soil for the use of 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 phos- 

 phorus for its own growth. When this condition arrives the clover 

 crop begins to fail, and the only resource is to begin to return the 

 exhausted plant food. Phosphorus may be returned in bone meal, 

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

 the most beneficial effect of farm manure is often seen when it is 

 applied for the clover crop. This fact alone, which is a common ob- 

 servation, 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 in almosr 

 any system rotation helps us to grow large crops, and to be suc- 

 cessful in farming requires that large crops shall be grown even 

 tho!]gh correspondingly large amounts of plant food are removed 

 from the soil. 



It is certainly good farm practice, and usually the best farm prac- 

 tice, to remove the largest quantities of plant food from the soil, 

 for the simple reason that large crops require large quantities of 

 plant food; but it is no less important to restore to the soil, when 

 needed, as large or larger quantities of plant food than are removed 

 — by turning under legume catch crops and residues not removed 

 from the field, by returning manures produced on the farm, and so 

 by the purchase of commercial plant food, such as phosphorus in 

 bone meal or rock phosphate, or potassium in concentrated potas 

 slum salts. Thus the most important process in all farming o]iera- 

 tions is the circulation of plant food, without which the fertility of 

 most cropped soils cannot be permanently maintained. 



Let us consider in further detail the effect of crop rotation on soil 



