190 SOILS AND FERTILIZERS 



The effect of the lime was not only to increase the yield of 

 alfalfa hay, but also its protein content, as well as that of the 

 weeds growing with it. The rate of nitrate formation in the 

 soil was also greater when limed. 



246. Liberation of plant-food materials. — It has gen- 

 erally been held that the application of lime to soils renders 

 some of the other plant nutrients more soluble by reason 

 of the exchange of lime for these substances in the insoluble 

 combinations found in soils. This has been discussed in 

 section 115. There is little doubt that magnesia is thus 

 rendered more available, but magnesia is rarely lacking. 

 Potash is often said to be made soluble, but although such may 

 be the case with some soils it is probably not true of all, and 

 there is really little evidence to substantiate the claim in any 

 case. The use of lime, under some soil conditions, may render 

 phosphoric acid more available, probably by supplying a base 

 more soluble than iron or alumina, with which, in soils defi- 

 cient in lime, the phosphoric acid might otherwise be combined. 



247. Effect on plant diseases. — The presence of abun- 

 dance of lime retards the development of certain plant diseases, 

 such as the " finger-and-toe " disease to which cabbages 

 and some root crops are subject. On the other hand, it 

 may promote some diseases, as, for example, potato scab. 



248. The use of inagnesian limes. — Some limestone 

 contains a considerable proportion of magnesia. When 

 grown in water cultures, many agricultural plants are injured 

 when the proportion of magnesia is greater than that of 

 lime. In soil, however, magnesia is not nearly as soluble 

 as lime and consequently there may be many times more 

 magnesia than lime present without as much actually being 

 in solution. Hence it is seldom that magnesia is injurious, 

 and magnesian lime may be used to overcome soil acidity 

 except possibly in the few soils in which the ratio of magnesia 

 to lime is already very high. 



