ORGANIC CONSTITUENTS OF SOILS 145 



black walnut, hackberry, elm, ash, and bur-oak. The shade of the 

 trees and the undergrowth that slowly crept in killed the grasses, 

 and the plants that replaced them supplied very little organic mat- 

 ter to the soil. The leaves and twigs accumulated upon the surface 

 and decayed completely or were burned by forest fires. The organic 

 matter that had accumulated was slowly being removed by oxidation 

 of nitrification, with the result that the soils were gradually changed 

 until a light-colored soil resulted. When this change had taken 

 place the trees mentioned above were gradually replaced by white 

 oak, hickory, and others adapted to light-colored soils or soils low in 

 organic matter. Several generations of trees were required to effect 

 this change. So great was the reduction of organic matter that the 

 timber soils contain less than half as much as the prairie. The anal- 

 yses of 1(54 samples of timber soil show 1.93 per cent in the surface 

 and 0.77 per cent in the subsurface. 



(c) Limestone. Soils rich in limestone are usually well sup- 

 plied with organic matter, due to the fact that limestone encourages 

 a larger growth of vegetation, especially of legumes, and is very 

 effective in retaining humus in the soil against leaching. 



(d) Latitude and Altitude. As a general rule soils of north- 

 ern latitudes have more organic matter than those of southern. 

 While the conditions for a luxuriant growth of vegetation are not so 

 favorable in the north, yet the conditions for its preservation are so 

 much better that the result is a larger organic content. This is well 

 shown in Illinois. The deposit of loess in the State along the Mis- 

 sissippi liiver is the same throughout the length of the State. The 

 analyses of eight samples of deep loess from the south end of the 

 State show 1.11 per cent of organic matter, while four samples from 

 the north end show 3.K(! per cent. Eighteen samples of timber soil 

 from the south end of the State show 1.5 per cent of organic matter 

 in the surface and 0.58 |HT cent in the subsurface, while the same 

 general character of soils in the north shows 2.4 and 0.0(5 per cent 

 respectively. The same is true of the prairie soils. The brown 

 prairie soils from the lattiudo of the southern part of the early Wis- 

 consin glaciation show 4.5 per cent, while the samo type 150 miles 

 to the north contains (5.1 per cent. 



Changes of Organic Matter. When vegetable matter becomes 

 mixed with soil it undergoes a physical change produced by bacterial 

 action in which the plant tissues are destroyed, and it becomes A 

 black or dark brown homogeneous mass. At the same time a chem- 

 ical change takes place. The process at first is quite rapid, but later 

 10 



