152 Plant Physiology 



or coarse sand may be chiefly three: (1) comminution 

 and greater water-holding capacity, previously discussed; 



(2) the addition of accumulated organic matter, and 



(3) the presence of a variety of microorganisms, gradually 

 transforming the organic matter. The fine state of divi- 

 sion of the soil particles also permits great freedom to 

 the further weathering influences of water and other 

 factors concerned in the rock disintegration which is con- 

 stantly in progress. 



82. Soil fertility. Fertile soils will generally contain 

 an abundance of the soil nutrients, sufficient to produce 

 crops for many successive years. This does not necessarily 

 imply that the nutrients are available in proper ratio. 

 Intelligent growers, moreover, consider not merely the pres- 

 ent production of crops, but also the maintenance of high 

 fertility in the case of fertile soils, and the development 

 of fertility in unproductive soils. It is necessary, then, 

 to have in mind the supply and the source of supply of the 

 important elements and their relative abundance. 



Sulfur and iron may be dismissed from further considera- 

 tion, since they are naturally abundant in soils, and are 

 used by plants in such limited quantities that a dearth 

 of these nutrients is not common. As would be expected 

 these two elements are only incidentally constituents of 

 commercial fertilizers. Magnesium is also ordinarily 

 present in sufficient quantities, and it may be present in 

 such excess as to be harmful, as noted later. The plant 

 producer is now certain that more attention must be paid 

 to lime, and especially to the relative abundance of lime 

 .and magnesia. Furthermore, when liming is required 

 every few years, it is a good custom to determine for any 



