358 HOW THE NEEDS OF SOILS MAY BE DETERMINED 



but little progress has been made. Some of these methods, 

 however, have come nearer than the total analysis in ascertain- 

 ing the cropping possibilities of a soil, or in determining its 

 fertilizer needs. 



Uses for Chemical Analysis. — Soil fertility investigators and 

 field men usually prefer to know the chemical composition of 

 the soils on which they work. In this respect, chemical analyses 

 are of great value, especially in continued experiments, in studying 

 the effect that certain methods of fertilization and cropping have 

 upon the chemical properties of a soil, or upon the supply of the 

 plant-food elements. 



In soil survey, chemical analyses are especially valuable in 

 comparing soil types and in tracing any correlation between the 

 chemical and physical properties of certain types. 



Frequently, farmers desire chemical analyses of soils in order 

 to determine their quaUty as compared with other soils. 



Four things are commonly determined by chemical analyses, 

 namely: (a) the need of lime; (6) the supply of nitrogen and 

 organic matter; (c) the phosphorus supply, and (d) the potassium 

 content.^ 



Testing Soil for Its Need of Lime. — Since an acid soil is detri- 

 mental to economic crop production, testing a soil for acidity 

 or for its need of lime is of primary importance. Some farmers 

 have limed certain soils because they thought they were acid, 

 when, later on, tests showed that no lime was needed. This 

 represents a typical case of wasted energy and money because of 

 guesswork. Many farmers have failed again and again trying to 

 grow alfalfa on acid soils, all because they did not consider the 

 necessity of determining beforehand whether or not the soils were 

 adequately supplied with lime. Again, many farmers do not know 

 whether it would be best to lime their soils or not. Simple acidity 

 tests can answer such questions definitely (p. 234), 



2 Trustworthy samples for full chemical analyses should be collected by 

 trained men. A soil auger is commonly used m getting the samples. At least 

 ten different borings from 'different parts of the field exammed should consti- 

 tute the sample to be taken mto the laboratory. Samples of the surface soil 

 are usually taken to depths of six and two-thirds to eight inches. The subsoil 

 should be examined to a depth of at least three to four feet for texture, struc- 

 ture, permeabiHty to water, etc. For practical purposes it is not necessary 

 to make chemical analyses of subsoils 



When the soil sample reaches the chemical laboratory, it is air- 

 dried, screened of its stones, coarse gravel, etc., thoroughly mixed and 

 finely pulverized. 



