ORGANIC CONSTITUENTS OF SOILS 143 



or quickly decaying organic matter, such as clover or manure, may 

 give better results than a soil full of old, slowly decomposing organic 

 matter unless the conditions are most favorable. There should be 

 sullicient organic matter to keep the soil in good physical condition 

 and also furnish nitrogen for maximum crops. The organic con- 

 tent depends upon several factors, as follows: 



(a) Moisture exerts a double influence in aiding the accumula- 

 tion of organic matter in soils. In the first place, it is favorable to 

 the growth of plants. It makes very little difference how little or 

 how much moisture is present in the soil, some plants have adapted 

 themselves to growing under those conditions. Even where water 

 stands nearly all the year, cat-tails, flags, sedges, and some grasses 



Fio. 74. FIQ. 75. 



Fio. 74. Fragments of plants found in soils. (Bulletin !>0, Bureau of Soils.) 

 Fio. 75. Fragments of insects found in soils. 



grow luxuriantly. In the second place, the presence of excessive 

 moisture tends to preserve the plants, which ultimately form soil 

 themselves or become mixed with the mineral matter and aid in 

 forming soil, such as peats, peaty loams, and mucks. M ven soil with 

 an ordinary amount of moisture prevents complete oxidation of the 

 roots and other fresh vegetable material that becomes incorporated 

 with it. Soils containing small amounts of water, such as sands 

 provide very favorable conditions for oxidation, and hence the 

 organic-matter content of such soil is low. 



Overflow land generally contains more than the adjacent upland 

 because of the greater growth due to a richer soil, the better facilities 

 for its preservation because of greater moisture content, and the 

 deposition of some organic matter along with the sediment during 

 periods of overflow. This deposit may cover leaves and grasses, thus 

 preserving them from complete decay. 



