FOURTEENTH ANNUAL YEAR BOOK— PART X. 715 



crops. The amounts in available form of the various elements which are 

 necessary for the growth of plants cannot be ascertained by chemical 

 analyses but must be determined by field experiments such as are de- 

 scribed farther on in this circular. 



As has been pointed out, chemical analyses merely give indications 

 of plant food deficiencies showing as they do the total plant food present. 



Of the ten chemical elements which have been found to be essential 

 for the growth of plants, five are supplied in sufficient amounts either 

 from the air or soil, and need not be applied to the soil. These five ele- 

 ments are carbon, hydrogen, oxygen, sulphur and iron. 



Nitrogen, phosphorus, potassium, calcium and magnesium may be 

 lacking in soils, however, and if lacking, crop yields will be deficient. 

 The two latter elements may be supplied by application* of lime as 

 calcic or dolomitic limestone, as has already been shown. If a soil is 

 acid these elements are probably lacking. 



Nitrogen. — A deficiency in nitrogen in a soil may be safely assumed if 

 there is a lack of humus or organic matter present. Conversely, if a 

 soil is dark in color it is well supplied with humus and will contain 

 a supply of nitrogen, although the absence of satisfactory physical condi- 

 tions may prevent the proper production of soluble nitrates. In such 

 a case the adjustment of the physical conditions would prove more profit- 

 able than an application of sodium nitrate or any other nitrogenous fer- 

 tilizer. 



When soils are deficient in humus and nitrogen, applications of barn- 

 yard manure should be made and leguminous crops should be grown and 

 used as green manure. Thus the organic matter and nitrogen content 

 are both increased and the use of any other nitrogenous material is un- 

 necessary for the production of common farm crops such as corn, oats 

 and hay. 



In some particular instances it might be worth while to make a small 

 application of sodium nitrate to act as a stimulent to start and encourage 

 the growth of a crop, but in general farming the use of commercial nitrog- 

 enous fertilizers on Iowa soils is unprofitable and unnecessary. 



In market gardening, however, the application of nitrogenous or other 

 artificial fertilizers is often of considerable value as here abundance of 

 available plant food is necessary for forcing the crops. 



Potassium. — The potassium content of most soils is considerably 

 greater than the nitrogen or phosphorus content and there is much less 

 danger of a deficiency of that element. That is the total amount of potas- 

 sium in soils is large and if conditions are satisfactory for the trans- 

 formation of the inactive into the active forms, applications of potassium 

 fertilizers are hardly necessary. 



Thus, if the humus content of the soil is high, its reaction is not 

 acid, and if the proper methods of tillage are practiced, the bacterial 

 activities which accomplish the solution of unavailable plant food con- 

 stituents are facilitated and abundant potassium in proper form is offered 

 to the crop. On soils, however, which show a deficiency in total potas- 

 sium as is true of some poor, light soils, and many peat soils, especially 

 those which are deep and are underlain with rock or sand, then an ap- 



