FERTILIZER NEEDS BEST DETERMINED BY TESTS 67 



must get its necessary amount of plant-food elements from 

 less soil with less ease and in less time than oats. A richer 

 soil, therefore, is necessary for barley to enable it to secure 

 its requirements. 



Fertilizer Needs Best Determined by Tests. — Since the table 

 gives the amounts of the important plant-food elements actually 

 contained in crops harvested, it is useful in showing the draft that 

 harvested crops make on soils. The figures are not to be taken to 

 indicate exactly how much and in what proportion the fertilizing 

 elements, in the form of fertilizers, should be supphed to crops 

 growing on any particular soil. The only sure way to determine 

 the fertilizer needs of any crop on a particular soil is by fertilizer 

 tests — ^for the following reasons: 



1. The condition of the soil with respect to the ''availability" 

 of the elements is difficult to determine by chemical analyses. 



2. The feeding power of certain plants may not be well 

 imderstood. 



3. Some plants vary in their chemical composition and hence 

 in their requirements of some of the elements at different stages 

 in their growth. Corn and potatoes, for example, contain the most 

 potassium when well matured, w^hile wheat contains the most 

 when just heading out.^ Thus the potassium content of a corn 

 crop when cut is a very good index of the potassium needs of corn, 

 whereas the potassium content of a harvested wheat crop does not 

 indicate the true potassium requirements of that crop. 



It has been found that a turnip crop requires much more phos- 

 phorus during, its growth than is indicated by its chemical compo- 

 sition at harvest time. 



4. Some harvested crops consist of the entire growth, while 

 others consist only of the edible or usable portion. This fact must 

 be remembered when the table is studied. For example, in com- 

 paring a twelve-ton corn crop and a fifteen-ton sugar beet crop, 

 it would appear that the corn crop is the greater feeder on all the 

 elements. The com crop consists of the entire growth; the sugar 

 beet^crop only the marketable roots. In case of the fifteen-ton 

 beet crop at least 6.2 tons of green leaves and tops are left on the 

 field, containing about forty pounds of nitrogen, 6.5 pounds of 

 phosphorus, 12.5 pounds of potassium and 30.5 pounds of calcium. 

 Thus to grow a fifteen-ton sugar beet crop the soil on each acre 



^ The amount not needed in later growth or development goes back into 

 the son, either washed from the leaves or migrates through the roots. 



