94 SOILS AND FERTILIZERS 



of available plant food, but if considered separately each 

 would have weak points. 



Since plant food is obtained mainly from the silt and 

 clay, the amount present in these grades of particles 

 determines largely the reserve fertility of a soil. A 

 soil in which 70 per cent of the total potash is present 

 in the silt and clay is in better condition for crop pro- 

 duction than a similar soil with a like amount of potash 

 which is present mainly in the sand. Because a soil 

 has a given composition, it does not follow that all of 

 the different grades of particles have the same composi- 

 tion. In fact, the different grades of soil particles in 

 one soil may have as varied a composition as is met 

 with among different soils. 26 



The figures under I in the table give the composition 

 of the particles, while under 2 are the results calculated 

 on the basis of the total amount of each element in the 

 soil. For example, the clay contains 1.47 per cent of 

 potash, while 50.8 per cent of the total potash of the 

 soil is in the clay particles. 



A soil may contain a low per cent of an element, 

 mainly in the finer particles and evenly distributed so 

 the crop is better supplied with food than if more were 

 present in the larger particles, unevenly distributed. 

 The distribution of plant food in the soil has not been 

 so extensively studied as the question of total plant 

 food. The distribution of plant food in both surface 

 soil and subsoil, as well as in the various grades of 

 soil particles, is an important factor of fertility. 



