SOIL-FORMING PROCESSES 



37 



tliese old residual soils are poorly drained a well defined 

 mottling develops, especially in the subsoil, due to tlie ir- 

 regularities of aeration. 



The compositions of hematite and of the limonite group 

 indicate the possibility of a progressive change from red to 

 yellow by hydration : 



Hematite Fe^O. 



Red 



Limonite 

 Group . . 



fTurgite 2Fe203- H^O 



Goethite Ee^Og 



HgO 



Limonite 2Fe203 . SHgO 



Xanthosiderite . Pe203.2H20 

 Limnite Pe.,0o.3H^0 Yellow 



24. Practical relationships of weathering.— Soil-form- 

 ing processes fortunately remain intensely active after the 

 soil has been produced. The physical agencies especially 

 tend to loosen and fine the soil, contributing largely to its 

 tilth. The farmer encourages such influences by plowing his 

 land and by other tillage operations. The addition of organic 

 matter is another means whereby these physical changes may 

 be influenced. Granulation in a clay soil is due almost en- 

 tirely to natural agencies. Were it not for such activities 

 the soil would soon become physically unfit as a foothold for 

 plants. The continual chemical changes, culminating in solu- 

 tion, provide the soil-water with plant nutrients not only 

 in suitable concentration but in correct proportion. By 

 slow processes, over geologic periods, Nature has provided 

 us with soil and by the same slow processes Nature is at- 

 tempting to maintain the fertility of her creation. The en- 

 couragement and control of such agencies is of no small 

 moment in practical soil management. 



