52 Mineral Nutrition of Plants 



DETERMINING A SOIL's FERTILIZER NEEDS AND 

 FITNESS FOR PLANT GROWTH 



If a soil has been properly classified much will be known with re- 

 spect to its management needs and crop adaptability. If it is a podzol, 

 then we know it will be acid and low in available supplies of most 

 of the plant nutrients. Soon after being brought under cultivation, it 

 will need lime and fertilizer, particularly potash, if legumes like clover 

 and alfalfa are to be grown. If it is a fine-textured podzol, then we 

 also know that the subsoil will probably be rather tight and impervious. 

 This may require special treatment. 



On the other hand, if the soil is a chernozem, that will immediately 

 stamp it as being well supplied with nutrients and an excellent medium 

 for plant growth. It will need no lime and probably no fertilizer for 

 many years. The first fertilizer needed will probably be phosphate. It 

 should be an excellent wheat and alfalfa soil. 



When fertilizer treatment of soils becomes necessary because of 

 either a low level of available nutrients to start with or because of 

 exhaustive cropping, then recognition that plant nutrients in soils exist 

 in several categories of availability, and that transformation from one 

 category to another takes place at a rate dependent on a number of 

 specific conditions, will be basic to any satisfactory program of de- 

 termining and interpreting the nutrient status of the soils. Originally 

 the nutrient status was determined for the most part by total analysis. 

 Then when the results did not correlate at all with crop growth and 

 fertilizer response, attempts were made to determine the more readily 

 available portions of the nutrient elements. Because existing knowledge 

 of the actual forms in which the nutrients exist in soils and of the man- 

 ner in which they change from one form to another was meager indeed, 

 progress was slow, and faith in the eventual success of soil testing was 

 abandoned by many workers in this field over a long period of time. 



Advances in knowledge during the past 25 years with respect to 

 exchangeable bases, to types of minerals and compounds that exist in 

 soils, and to conditions (including in particular the pH of the soil) 

 which determine rate and type of transformation from one form to 

 another have all contributed greatly to the development of better and 



