9O PRINCIPLES OF AGRICULTURAL CHEMISTRY 



good tilth, even though worked when wet. Non-calcareous clays 

 under such conditions are likely to contract into stony masses. The 

 exact proportions of carbonate of lime necessary to produce this 

 condition of the soil remains to be determined. 



The black prairie soil of Texas locally called "black waxy" and 

 termed the "Houston black clay" by the Bureau of Soils, is an 

 example of this kind of soil. When wet it is gummy and waxy, 

 but when dry and well cultivated it is friable and easily worked. 

 This soil contains one per cent or more of lime. Clay, unlike silt, 

 chalk, and humus, increases greatly in coherence when it dries, 

 and finally becomes a hard, solid substance. 



Nature of Tilth. 1 If the particles of a soil are each inde- 

 pendent of the other, so that their relative positions are easily 

 shifted by gravity or water, they are said to possess the single- 

 grain structure. Such soils, if sandy, are loose, and become as 

 compact as their particles allow. Clay soils of this character are 

 very sticky when wet, and if worked while wet form clods on 

 drying. 



A soil which, when plowed, breaks up into a mass of compound 

 particles of various sizes, loosely piled upon one another and 

 separated by comparatively large interspaces, is said to possess the 

 crumb structure and to be in good tilth. The crumbs may be 

 held together by moisture, clay, humates, carbonate of lime, and 

 sometimes silica and oxide of iron. 



Crumbs of sand held together by water collapse on drying. 

 Clay is frequently the substance which holds the crumbs together ; 

 in such case, the crumb structure remains even after the land 

 dries out. Beating rains and cultivation while too wet destroy 

 these crumbs. 



Soil particles united by carbonate of lime, humates, silica, or 

 oxide of iron are more or less permanently cemented into com- 

 pound particles. In some calcareous soils we find sandy and silt 

 concretions varying from several inches in size to microscopic 

 proportions. Humus is a great aid in securing good tilth. 



Compound particles are also formed by natural processes, due 

 1 Warington, Physical Properties of Soil, p. 36. 



