114 ELEMENTS OF AGRICULTURE 



what are called double silicates, which are much less 

 soluble in water than the ordinary silicates. In this 

 form they are preserved in the soil, and are said to be 

 fixed. A good clay loam contains about .5 per cent of 

 potash and sandy soils about .1 per cent. This would 

 give for the clay soil 19,602 pounds of potash per acre 

 of surface soil, and for the sandy soil 3,920 pounds of 

 potash. Such a clay soil could furnish enough potash 

 for nearly 500 crops of tobacco of 1,000 pounds per 

 acre, or 390 crops of hay of two tons per acre. The 

 sandy soil could supply potash for about 100 such crops 

 of tobacco and nearly eighty such crops of hay. But 

 this store of potash is not all available for crops; a 

 large part of it is locked up just as the phosphates are, 

 and the soil parts with it very slowly. These unavail- 

 able compounds of potash become available for plants 

 in much the same way as the phosphates, that is, by 

 the action of the weak acids of humus. These acids 

 form compounds with the potash which are called 

 humates, and which are readily taken up by plants. 

 Here again, through manufacture of humus, the soil 

 bacteria are of great importance. 



110. Lime in the Soil. — Some soils may be deficient 

 in their supply of lime, which is an essential plant food. 

 Besides being an essential plant food, lime is of great 

 value in soils because of its action upon humus. 

 Humus, as you have already been told, contains a num- 

 ber of weak acids which act on the phosphates and 

 potash compounds in the soil. These humus acids also 

 combine with the lime in the soil, and by their action 

 on the lime, the phosphates arid the potash com- 



