ABSORPTION 173 



decomposition of most solvents except carbon dioxide. 

 This compound is less soluble in warm water than in cold. 

 Then, too, the amount of water; the movement of water, 

 which latter removes decomposition products and exposes 

 fresh surfaces to the action of the solvents; the size of soil 

 particles; the arrangement of soil particles; all have a decided 

 effect on the rate and amount of decomposition. Some min- 

 erals like feldspar decompose fairly readily, others, like mica, 

 decompose with considerable difficulty. And again, small 

 minerals like apatite may be enclosed within other minerals, 

 like quartz, for example, and soil solvents cannot touch them. 

 All these factors, of course, are in addition to the amount 

 of organic matter, the rate of its decomposition, and the 

 number of bacteria. 



To sum up, then, the decomposition of mineral particles 

 in the soil, while it appears rather simple, is in reality depend- 

 ent on many factors which are onlv more or less controllable 

 by the farmer. But if he understands the ordinary progress 

 of favorable decomposition, he can modify his controllable 

 factors accordingly. He can cultivate, maintain the supply 

 of organic matter, and look after drainage or irrigation as 

 the case mav be. 



137. Absorption. — From what has been said it might be 

 thought that when once a plant food becomes soluble it 

 undergoes no further change, and if not taken up at once by 

 the plant is in danger of being leached from the soil. The 

 danger of leaching is by no means as great as might be 

 expected, and any given plant food element undergoes a 

 great many changes before it meets its final fate in the 

 plant or in the drainage water. Compounds are constantly 

 going into solution or being made available, and going 

 out of solution or being made unavailable. Compounds 

 are also held in the soil by physical means. In other words 

 the soil not only makes plant food available from its reserve 

 stores, but it in large measure prevents them from being 

 removed from the soil by leaching. This process of retention 

 of soluble salts or of elements in soluble compounds is called 

 absorption, or sometimes fixation. The latter is not so good 

 a name since it may be confused with nitrogen fixation which 



