116 SOILS AND MANURES 



more fully considered later on. The oxidation of com- 

 pounds of iron and other minerals also takes place more 

 rapidly when the soil is well supplied with lime. 



Not only does lime unite with free acids, but it also reacts 

 with various salts in the soil, combining with the acid 

 radicles and liberating the bases. This appears to be a 

 condition precedent to the absorption of phosphoric acid 

 and of bases by the soil. If it does not occur, various salts 

 used as manures become comparatively ineffective, or even 

 positively injurious. The calcium salts so formed are 

 mostly soluble in water and pass into the drainage, and 

 considerable loss of lime results. 



In all these ways lime adds greatly to the effectiveness 

 of manures. In cases of reduced fertility the soils are 

 sometimes singularly unresponsive to all kinds of manuria] 

 treatment, and, though the addition of lime itself may pro- 

 duce but little improvement, it often makes the soil respond 

 generously to the manures subsequently applied. Farmers 

 are sometimes deceived in this way. In order to test 

 whether or not a soil requires lime, a small dressing is 

 applied to a corner of a field, and if it produces no visible 

 result they not unnaturally conclude that lime is not 

 required. 



Calcium is an essential constituent of plants, and the 

 fact that the lime in the soil is, therefore, a possible source 

 of plant food has been, perhaps, unduly emphasised. 

 Plants may obtain this element from lime, but it is pro- 

 bable that, if that ingredient were absent altogether, they 

 could obtain all the calcium they require from calcium 

 sulphate and other soluble salts. In fact, there is some 

 reason to believe that most of the calcium found in plants 

 is derived from that source and not directly from the car- 

 bonate. The great improvement in the quality of the 

 herbage, which often follows an application of lime to 

 pastures, is probably due mainly to its effects on the soil, 



