36 Effective Farming 



16. Classification of soil according to rainfall. The terms 

 humid, arid, semi-arid, and semi-humid are employed to indi- 

 cate the relative quantities of rainfall in localities, and these 

 same terms are applied to the soils. A humid soil is one that 

 receives on an average thirty inches or more of rainfall in a 

 year ; an arid soil is one that receives less than ten inches ; a 

 semi-arid soil is one that receives from ten to twenty inches; 

 and a semi-humid soil is one that receives from twenty to thirty 

 inches. The amount of rainfall naturally influences the crop 

 production of a region, but as some soils do not have the prop- 

 erty of retaining much water, it is also necessary that the rain- 

 fall be distributed over the growing season, if crops are to 

 prosper. 



17. Heavy and light soils. The terms heavy and light, 

 when applied to soils, refer to the ease with which they can be 

 tilled, not to their weight. A soil that is hard to work and is 

 sticky when wet is known as a heavy soil ; one that is easy to 

 work and not sticky when wet is a light soil. Clays, clay loams, 

 and muck soils are heavy ; sands, sandy loams, and loams are 

 light soils. 



18. Structure of soil. The term structure refers to the 

 arrangement of soil particles, that is, the way they fit together. 

 It is influenced not only by the texture, but also by the tillage 

 the soil receives, by the quantity of moisture and humus in 

 the soil, by the use of lime, and by the freezing of water in the 

 soil. Three kinds of soil structure separate-grained, puddled, 

 and granular are recognized. 



Separate-grained structure. In a soil having a separate- 

 grained structure, each particle exists free and does not cling to 

 adjacent particles ; this condition is found in coarse, sandy soil. 



Puddled structure. A puddled condition exists when a heavy 

 soil becomes so packed when wet that the fine particles nearly 

 fill the pore spaces between the coarse particles. The work- 

 ing of a heavy soil or the tramping of it by live-stock when it is 

 wet causes it to become puddled. A puddled soil after it dries 



