6 CHEMICAL FERTILIZERS 



texture of the soil be too close, all these processes fail. On the 

 other hand, if the texture be too open, water will flow away 

 too easily, and the growing plants be liable to suffer from 

 drought. The porosity of a soil is dependent upon a large 

 number of factors. The size of the soil particles, the condition 

 of aggregation in which these particles lie, and the form of 

 packing of the soil par tides and soil aggregates are all matters 

 which markedly affect the porosity of the soil. The addition 

 of lime to the soil collects the particles into large aggregates, 

 which behave as if they were loosely packed and possessed 

 a more open structure, thereby assisting the flow of air and 

 water. These results will be produced by fertilizers contain- 

 ing lime like basic slag or by compound fertilizers containing 

 carbonate of lime. Compounds containing soda, such as 

 nitrate of soda, for example, break down these aggregates, and 

 ultimately produce a closer packing of the soil particles, thus 

 checking the movement of roots, air and water. Wood 

 ashes, which contain potassium carbonate, act in a similar 

 manner. Turning the soil over gives a loose packing, 

 trampling and rolling give a tight packing. Deep cultivation 

 gives a great depth of soil, which, with certain limitations, 

 is almost equivalent to an increased area of surface. 



The Soil Needs Plant Food. Nitrogen, phosphorus 

 and potassium take the first place in the list of plant foods. 

 There are other elements, such as calcium, sulphur, chlorine 

 and sodium, which maybe needed in special cases. Calcium, 

 in the form of calcium carbonate, plays an important part 

 in regulating the physical character of the soil, and it is not 

 necessary to account for it twice, both under its physical and 

 chemical aspects. Sulphur is supplied by the smoke in the 

 vicinity of towns, but in districts remote from industrial 

 factories it may be necessary to apply sulphur to certain 

 crops. When this is the case, an application of super-phos- 

 phate or sulphate of ammonia will generally be found the 

 most suitable method of supplying sulphur, since the sulphur 

 is obtained in these articles without any cost, the price of such 

 articles being dependent upon their phosphorus or nitrogen 

 contents (see p. 188). Sodium and chlorine are needed for 



