SOII, DEFICIENCIES 245 



deeper every year or by subsoiling. Both porous and stiff soils 

 are benefited by organic matter, which makes the former less 

 porous and the latter less stiff. Lime may improve stiff soils by 

 making them more pulverulent. Wet soils are improved by under- 

 drainage. Dry soils may be aided by frequent stirring to prevent 

 evaporation of water from them, or by irrigation. Hard pan may 

 sometimes be removed or prevented from forming, by proper 

 plowing, or subsoiling. In some cases it is profitable to use ex- 

 plosives to remove it. 



Chemical Deficiencies. The chief chemical deficiencies of soils 

 are acidity, alkali (excess of soluble salts), deficiency in organic 

 matter, deficiency in lime, and in available plant foods, phosphoric 

 acid, potash, or nitrogen. These deficiencies have been dis- 

 covered by means of field experiments, aided by other methods of 

 experiment. 



Recognition of Chemical Deficiencies. There are three general 

 methods of ascertaining the probable chemical deficiencies of 

 soils : 



1 i ) Field experiments. 



(2) Pot experiments. 



(3) Chemical analysis. 



Field Experiments. All soil theories must eventually be tested 

 in the field. Field experiments are, therefore, fundamental, and 

 information secured by other methods must eventually be brought 

 back to the field, and stand the test of actual crop production 

 upon the soil. The more removed from field conditions the work 

 of the investigator, the more careful he should be to bring his 

 results into relation with actual crop production. Unless tested in 

 this manner, theories may be developed which are entirely too far 

 removed from agricultural conditions. 



Methods of making field experiments with fertilizers are out- 

 lined in Chapter XVI. Similar methods are used in other experi- 

 ments, arranged according to the information desired. 



Field experiments are subject to vicissitudes of weather, insect 

 pests, variations in soil, etc., and hence must usually be carried on 

 for several years, or else en an extensive scale. Unexpected 



