DECOMPOSITION IN THE SOIL 81 



iin intimate relation to the aeration of tho soil, and the op- 

 portunity for the growth of the aerobic organisms that are 

 the most prominent agents in the final decomposit ion of 

 organic matter. In marsh soils the accumulation of humus 

 is -tvat, clue to the water-logged condition. The opposite 

 extreme is noted in the coarse, sandy soils in which the air 

 supply is at a maximum. Neither of these soils represents 

 the highest type of fertility. Marsh soils are low in fertil- 

 ity, because the kind of compounds demanded by the most 

 important cultivated crops have not been formed therein. 

 Sands are relatively infertile, beeanse no store of new plant 

 food is accumulated. The maximum of fertility is attained 

 in the loam soils, which contain enough oxygen to enable a 

 portion of the organic matter to be completely decomposed, 

 and yet permit of the gradual accumulation of humus. 



All types of soils are much better aerated under cultiva- 

 tion than under natural conditions. The result is that the 

 store of humus is gradually reduced and the fertility of the 

 soil is diminished. The humus affects the fertility not only 

 by forming a store of plant food, but by its effect on the 

 water-holding capacity of the soil and on its texture. If the 

 farmer does not return to the soil most of the organic mat- 

 ter it has produced, soil exhaustion is inevitable. 



The maintenance of the humus content of the soil is one 

 of the important problems of the farmer in the older 

 legions of the world. The type of farming has much influ- 

 ence on this. If grass-growing is the chief industry, no or- 

 ganic matter is returned to the soil and the humus 

 content will be constantly depleted. The same is true of 

 grain-farming when no effort is made to return the straw to 

 the soil, as is the case in the grain-growing sections of this 

 country where the straw is burned. If the farm crops are 

 fed to animals, and what they produce is sold in the form 



