334 SOILS: FBOPEBTIES AND MANAGEMENT 



236. Deficiency of ingredients and manurial needs. — 



Many standards have been set for the minimum quantity 

 of each of tlie important soil constituents that must be 

 present in order to insure a productive soil. Experi- 

 ence has shown, however, that no definite standards hold 

 for all soils. By comparing analyses of soils of known 

 productivity with that of a soil under investigation it is 

 an easy matter to ascertain whether the soil contains a 

 large quantity of each agriculturally important ingredi- 

 ent; but when the quantity of any constituent is low, 

 it becomes a difficult matter to tell how this will affect 

 the agricultural value of the soil. Some soils will be 

 productive with 0.05 per cent of phosphoric anhydride, 

 while others are unproductive when all the plant nutrients 

 are present in ample quantity. 



The fact that the degree of productiveness of a soil 

 cannot always be gauged by its analysis gives rise to a 

 similar uncertainty with regard to its manurial needs. 

 A soil may contain potassium in very large quantities, 

 sufficient to produce crops for hundreds of years, as indi- 

 cated by a strong hydrochloric acid analysis, and yet a 

 potassium salt may be used with profit. On the other 

 hand, it is evident that as the content of any constituent 

 becomes less, the probable need for its application be- 

 comes greater, and a knowledge of the composition of 

 the soil thus suggests a practice without assuring its 

 success. An analysis of the hydrochloric acid extract, 

 therefore, cannot be taken as an infallible guide to the 

 fertilizer needs of a soil, and of itself should not be relied 

 upon ; but in connection with other knowledge, particu- 

 larly that derived from fertilizer tests, it may be useful. 



237. Partial solution with weak acids. — The difiiculty 

 in judging of the properties of a soil from the results of 



