ROTATIONS DO NOT KEEP UP MINERAL SUPPLY 35 



tion or " burning out " of humus. If the same land is sown to grass, 

 the humus supply will be maintained or increased. With manure on 

 grass, the humus may be increased. 



(2) Experience indicates that where clover is grown once in four 

 years, the nitrogen supply can often be maintained, even when the 

 clover is not plowed under. 



(3) Plants having quite different root systems, as legumes, root 

 crops which have deep tap roots, and grains having fibrous roots, 

 may live on the same land with less competition than when all have 

 similar root systems. The same is probably true of plants having 

 different habits of growth, as wheat, which grows principally in early 

 summer, and corn, which grows in late summer.' 



(4) Most weeds that trouble grain crops are destroyed when 

 the land is put to grass. Most weeds are easily controlled under rota- 

 tion. Certain injurious insects may become abundant when the 

 land is in the same or similar crops. For example, corn root worms 

 live only on corn roots, and may accumulate in great numbers when 

 the land has been in corn for several years, but all are destroyed when 

 the land is put in some other crop a few years. In the same way, 

 some plant diseases are controlled by changing crops. 



Aside from maintaining productivity, rotations help in other 

 ways by distributing the labor, and decreasing chance of total loss of 

 crop in bad crop years. 



Rotations Do Not Keep Up Mineral Supply. While a well 

 managed rotation, with manure, may keep up the humus and 

 nitrogen, it will not keep up the minerals. In fact, it may exhaust 

 minerals faster than single cropping, since a 30 bushel crop of wheat 

 removes more minerals than a 15 bushel crop, and nothing is done in 

 a mere rotation to return minerals. 



In most cases, the soil was well exhausted of at least some mineral 

 element before rotation was adopted. Ultimately on most soils, 

 minerals must be supplied in some form. In the oldest farmed sec- 

 tions of the country, namely, the New England States, rotations are 

 no longer sufficient and farmers find it necessary to add some min- 

 erals in the form of commercial fertilizers. The lime is especially 

 low, due to leaching, and must be added to most of these soils for best 

 results. 



