276 SOIL AND FERTILIZER ANALYSIS 



good for certain special soils, but there are no methods 

 of general applicability. Pure water, or water charged with 

 carbon dioxide, might seem an excellent solvent, but against 

 this solvent as against others for that matter, though in 

 lesser measure, the power of chemical and physical absorp- 

 tion acts to prevent the extraction of plant food that may 

 be readily available to plants in the soil. Moreover, in all 

 these methods it is not the amount of plant food available 

 at any one time which nourishes the crop, it is the plant 

 food available from day to day throughout the growing 

 season. It is the rate at which plant food becomes avail- 

 able that determines crop growth. 



But soil analysis is of very great value to the soil chemist 

 in determining changes which take place in the soil under 

 certain conditions, and in comparing one soil with another, 

 all of which work is valuable in studying the effect of 

 fertilizers, the effect of changing physical conditions, and 

 the effect of cropping. The knowledge so obtained can later 

 be practically applied to help the farmer obtain better and 

 larger crops. 



211. Lime Requirement. — Since acidity is a very prevalent 

 condition of many soils, and since it needs correction in 

 most instances, numerous efforts have been made to deter- 

 mine the amount of acid in a soil; or, which is more direct, 

 to determine the amount of lime necessarv to neutralize 

 acidity to a given depth. The method which has given the 

 best results is one devised by F. P. Veitch. Equal weights 

 of soil are treated with different amounts of lime water 

 until one amount is found which leaves the soil slightly 

 alkaline. Knowing the weight of soil in the sample, the 

 weight of lime applied, and the weight of an acre of soil to 

 the given depth, the amount of lime in pounds per acre 

 needed to correct the acidity can be calculated. The results 

 are only approximate at best, but the method serves to 

 compare the relative amounts of lime needed on different 

 soils, or on a soil under different treatments. It is not 

 sufficiently accurate to tell a farmer just how much lime to 

 put on a given field, although it may be a guide to the expert 

 in determining the amount. 



