PHYSICAL, CHEMICAL, BIOLOGICAL 57 



soil, call for a loose seed-bed in which granular structure is accentuated. 

 Wheat, rye, clover and the grasses, an the other hand, demand a rather 

 compact, fine-grained seed-bed, and, therefore, do not demand an equal 

 degree of granulation. 



Solubility of Soil Minerals. — Plants take their mineral food only 

 when it is in solution. This necessitates a degree of solubility of the 

 essential plant food minerals that will meet the maximum needs of the 

 plants. The solubility of the soil particles depends upon a number of 

 factors, and is a rather complex process. In pure water the solubility is 

 very slight, but as the water of the soil becomes impregnated with car- 

 bonic acid gas, organic compounds and mineral compounds, these all 

 exert an influence on the degree of solubility of other mineral constituents. 

 Solubility is also markedly influenced by temperature. This fact is well 

 recognized by the housewife, who by heating dissolves sugar in water 

 until it becomes a syrup; so the solubility of the soil minerals is increased 

 by a rise in soil temperature. 



Rate of Solubility Depends on Texture and Kind of Minerals. — The 

 rate of solubility is approximately in proportion to the surface of the 

 particles on which the solvent acts. Consequently, we find as a rule 

 larger amounts of plant food in solution in soils of fine texture than we do 

 in soils that are coarse in texture. This doubtless accounts for the practice 

 of the more extensive use of fertilizers on sandy soils. It is also true that 

 the different minerals have varying degrees of solubility, some being far 

 more soluble than others. The limestone particles in a soil mass are 

 much more readily soluble than the quartz, and, consequently, lime 

 disappears from the soil. Plant roots also have an influence upon solu- 

 bility by means of certain excreta given off by the roots. Since, therefore, 

 carbon dioxide, organic compounds and plant roots increase the solu- 

 bility of the soil particles, it is plain to be seen that the incorporation of 

 organic manures with the soil and the production of good crops tend 

 always towards a more productive soil, except in so far as the minerals of 

 the soil are exhausted through plant removal. 



Soil Bacteria Increase Solubility. — The bacteria of the soil are also 

 instrumental in increasing the solubility of the soil minerals. Since, for 

 their greatest activity, bacteria require proper sanitary conditions, such 

 as aeration, a neutral soil medium and organic matter as their food, it 

 will be seen that fertile soils encourage increased numbers of bacteria, 

 which in turn make for increased fertility. It is, therefore, essential for 

 the tiller of the soil to understand the various factors which enter into 

 soil productivity, and to perform his part in encouraging the development 

 of those which are beneficial and discouraging those which may be de- 

 structive. 



Rapid Solubility Results in Loss of Fertility.— The rate of the solution 

 of soil minerals should not far exceed the needs of the crops grown, lest 

 there be an unnecessary loss of plant food through leaching and the con- 



