148 



SOIL PHYSICS AND MANAGEMENT 



soils the content runs more uniform and to greater depth, due to 

 deeper root development. In swamp soils there is frequently a 

 great accumulation in the surface and upper subsurface, with rather 

 a sudden decrease at a distinct line. Timber soils show a greater 

 decrease in the subsurface than prairie soils. The organic matter 

 is usually deeper in alluvial soils than in others. The distribution 

 depends to a large extent upon the depth of root development, the 

 effect of burrowing animals, the accumulations that are taking place 

 as in bottom and swamp lands and the cracks produced by shrink- 

 age, which is especially characteristic of clay and clay loam soils. 



Organic Matter in Soil Strata, * 



Value of Organic Matter to Soils. It is next to impossible 

 to assign a definite money value to organic matter as in the 

 case of nitrogen, phosphorus, and potassium. The difficulty arises 

 from the fact that vhen incorporated with the soil it has sev- 

 eral different effects, physical, chemical, and biological, any one of 

 which is of sufficient importance to justify its use. The value of 

 organic matter must in the end be determined from the value of the 

 increase in crops produced. This has been worked out for manure 

 and is being determined for other forms of organic matter, such as 

 crop residues and legumes. The things for which it is of value are 

 as follows : 



1 . Granulation is one of the most important properties of heavy 

 and medium soils. This gives permeability for both air and water, 

 and very desirable working qualities that heavy, non-granular soils 

 do not possess. In fact, some of the most intractable soils are clays 

 that are quite low in organic matter. The granular structure lessens 

 the tenacity. This latter is especially noticeable in heavy soils. 

 There is no one constituent so beneficial to such a large class of soils 

 as organic matter. Its removal from a soil destroys its power to 

 granulate almost entirely. When the humus is taken from brown silt 



