ROTATION AIDS IN FERTILIZATION AND LIMING 267 



rains wash it considerably and pack it, forming a smooth, hard sur- 

 face. The other poition, on wiiich were grown clover, corn and oats 

 in rotation, is mellow, has a crmnm}'- structure, and heavy rains do 

 not affect it as they do the soil on the first portion. The difference 

 in these two plots is largely the result of the growing of clover. 



Conserving Fertilizing Elements Through Rotation. — Not all 

 crops require the same amount of the different plant-food elements. 

 Some are gross feeders, and others make heavy drafts upon one or 

 two elements' (see Table of Crop Constituents, Chapter VI). Cab- 

 bage and spinach, for example, quickly exliaust the soil of nitrogen. 

 In a rotation it can be arranged that one crop requiring a large quan- 

 tity of one element, or all of them, does not follow another of a 

 like characteristic. Through rotation, therefore, the draft upon the 

 soil reserve of nitrogen, phosphorus, and potassium may be equal- 

 ized so that the productive power of soils may be prolonged. 



The growing of such crops as tobacco and cabbage and all 

 shallow-rooted crops without rotation causes heavy losses of plant- 

 food elements through leacliing. These crops require rich soils, and 

 hence heavy fertilization becomes necessary. The soil is supplied 

 with much more available material than the crops can use, and 

 hence conditions are favorable for heavy losses through leaching. 



In Wisconsin, a study was made of the phosphorus losses on 

 sixteen tobacco fields. These fields were cropped, on an average, 

 forty-six years — thirty of which were tobacco. Durmg this time 

 they received an average of thirty apphcations of manure at the 

 rate of eighteen loads, or about twenty-seven tons, to the acre. 

 These investigations showed that during the forty-six-year crop- 

 ping period, this system of farming caused the loss (above that 

 removed by the crops) of an amount of phosphorus per acre from 

 the surface soil eight inches deep, sufficient to supply the phos- 

 phorus needs of at least seventy 75-bushel corn crops. 



When deep-rooted crops such as corn, alfalfa, clover and vetch 

 are grown in rotation with shallow-rooted plants, much of the 

 fertilizing elements which leach down into the lower strata is 

 utilized. Moreover, deep-rooted plants bring up considerable 

 plant-food material that is, in turn, used by other crops. This is 

 especially true when soils are more or less "open" in character. 



Rotation Aids in Fertilization and Liming. — Rotation offers 

 an opportunity to get the greatest returns from manure and com- 

 mercial fertilizers by applying them to the crops which can make 

 the best use of them. Much coarse, fresh manure or litter gives 



