84 ELEMENTS OF AGRICULTURE 



of combination is found in all soils, and comes from the 

 decay of rocks which contain phosphorus compounds. 

 It is in some instances found combined with lime, in 

 which form it is called bone phosphate, making up, 

 as it does, a large part of the bones of all animals. 

 Often it is found combined with iron and aluminum, 

 when it is called iron and aluminum phosphate. 

 Most soils contain comparatively small quantities of 

 phosphates, .1 per cent, being a fair supply; some rich 

 soils may contain as much as .5 per cent, but this is 

 unusual. Poor soils contain not more than .05 per 

 cent. Besides the phosphates mixed with the finer 

 soil particles, many of the coarse fragments in the 

 soil contain a small quantity of phosphates, which, as 

 the fragments decay, is added to the fine soil. 



85. Potassium in Soils. — Compounds of this element 

 are found in all soils, and come from the decay of rocks 

 containing potash. The rocks which supply most of 

 the potash of soils are called feldspars, and are 

 found all over the earth. The potash in the soil is 

 usually combined with silica to form compounds 

 known as potassium silicates. 



Clay comes largely from the decay of the same rocks 

 that supply potash to the soil, and as clay is also a 

 silicate it is often united with potash to form double 

 silicates. The amount of potash in different soils, 

 of course, varies greatly; in some soils it reaches as 

 high as 2 per cent; in poor soils it often falls below .1 

 per cent. 



86. Calcium in Soils. — Compounds of this element 

 make up a small part of all fertile soils, and are the 



