73 ELEMENTS OF AGlUCULTUIiE 



CHAPTER XV.— Composition of Soils 



78. Water. — All fertile soils contaki water, though 

 the amount varies greatly under varying conditions. 

 The desert soil contains no water; the soil of the swamp 

 contains an excess of water; and between these two ex- 

 tremes are the great areas of cultivated soils. Water is 

 a necessary part of all fertile soils. 



79. Organic Matter. — Besides water there are two 

 kinds, of matter making up soils — organic and inorganic. 

 If a sample of soil be gently heated in an oven it loses 

 in weight, and the loss is due to the evaporation of 

 moisture. If the sample be burned at a dull, red heat 

 it loses more in weight and changes color; this loss is 

 due to the burning of organic matter, which passes off 

 as gas and smoke just as it does when a plant is burned. 

 See page 46. 



The organic matter of soils results from the decay of 

 plants and animals. In cultivated fields the organic 

 matter comes mainly from the stubble of harvested 

 crops, the decay of dead weeds, and from the addition 

 of stable manure. In wood land it results from the 

 decay of dead leaves and the fallen branches and trunks 

 of trees. As the organic matter of plants is made up of 

 C, H, 0, N, S, so :he organic matter in the soil must 

 be made up of the same elements; and as organic mat- 

 ter decays these elements form compounds which serve 

 in time to build up other plants. Thus the organic 



