14 THE SOIL OF THE FAEM. 



The soil, however, is not a mere sieve through which 

 any matter in solution can pass freely. It has a power of 

 retaming, as in a filter, many saline and otiier substances 

 that may be present in the water permeating it. The 

 experiments of Way, Voelcker, and others have shown 

 that when surface waters charged with the products of 

 vegetable decay are brought mto contact with argillaceous 

 sediment, they part to some extent wdth their potash, 

 ammonia, silica, phosphoric acid, and organic matter, 

 which remain in combination with the soil; while, under 

 ordinary conditions at least, neither nitrates, soda, lime, 

 magnesia, sulphuric acid, nor chlorine are retained. The 

 phosphates are probably retained m combination with 

 alumina or peroxide of iron, and the silica and organic 

 matters also enter into more or less insoluble combina- 

 tions. It follows from these reactions that drainage- 

 waters, especially from clay soils in a good state of jduI- 

 verization, are found to carry off nitrates, -sulphates, 

 chlorides, or carbonates of soda, lime, and magnesia. In 

 light and sandy soils the power of retaining nutritive 

 substances is less than in the case of heavier soils, or than 

 soils having much vegetable matter. Were it not for this 

 power, the soluble substances present in the soil, whether 

 naturally or applied in manures, would often be speedily 

 washed out of it; and tillage and draining would much 

 more rapidly impoverish the land than they do, by allow- 

 ing its soluble constituents to be carried off by water. 



The power of soils to absorb and retain moisture is in 

 direct ratio^ot only to the quantity of organic matter in 

 the soil, but also to the fineness of the particles of the soil. 

 Hence it becomes important, in a practical point of view, 

 to secure a proper degree of fineness in tlie particles of 

 a soil if it is to withstand drouth. During dry weather 

 plants require a soil which is both absorptive and re- 

 tentive; and that soil which is capable of seizing atmos- 

 pheric moisture, and holding it when the atmosphere is 



