TEXTURE AND STRUCTURE OF SOILS 43 



soils are not flocculated to the same extent by lime, but are thus 

 affected in proportion to the quantity of clay they possess. 



Of the different forms of lime, quick-lime and water- 

 slacked lime are more active in producing a granulated struc- 

 ture of soil than is ground limestone, marl or air-slacked 

 lime. This is one reason why the burned lime is superior 

 to ground limestone for use on heavy clay soils, on which 

 there may be a pronounced difference in the effect of the two 

 kinds of lime on crop production. Warington reports a 

 statement of an English farmer to the effect that by the 

 use of large quantities of lime on heavy clay soil he was 

 enabled to plow with two horses, while three were necessary 

 before applying lime. 



47. The soil survey. — The purpose of a soil survey is to 

 classify and map the soils in a given area according to their 

 crop relations and their physical properties, and to correlate 

 these soils with those in other areas. The soil unit, or what 

 may be termed the soil individual, is the type, and on a soil 

 map each type is given a different color. Every soil type 

 has a certain peculiar and characteristic appearance and 

 certain inherent properties that distinguish it from every 

 other type. When the type is known some practical infor- 

 mation regarding its texture and its amenability to tillage 

 and to drainage may be predicted, and something in regard 

 to its productiveness and the crops to which it is adapted 

 may also often be inferred. 



48. Classification of soils. — In order to distinguish 

 between soils, and to give a basis on which to separate them 

 into the types to which reference has been made, a form of 

 classification has been adopted in this country that takes 

 into consideration much of what is known of their history 

 and their properties. Thus the first large division into 

 which a soil falls is known as the soil province, which is 

 based, in a general way, on the process of formation. A 



