102 THE BOOK OF CORN 



clay, lime and vegetable matter, and which contain in 

 themselves naturally such an abundance of plant food 

 as to supply the needs of plants for very long- periods. 

 As a broad general rule, therefore, sandy soils are 

 deficient in practically all of the constituent elements, 

 while peaty soils are deficient in the mineral elements. 

 The various clay soils are usually rich in potash, shale 

 and limestone soils are rich in lime and phosphoric acid, 

 and alluvial soils rich in all the plant food elements. 



Guides in Use of Plant Food — The previous crop- 

 ping also is a reasonable guide as to the probable 

 necessities of the plant. If, for example, wheat or corn 

 or hay, or any one crop has been grown continuously 

 for a long time, and no manure has been returned, then 

 the chances are that in order to continue their growth 

 on soils not naturally very rich, all of the constituents 

 will be required, as the crop will remove the same con- 

 stituents in the same proportion year after year, thus 

 reducing very rapidly the store of active fertility. If, 

 on the other hand, crops have been grown in rotation 

 which take from the soil the constituents in different 

 amounts and proportions, and also introduce by their 

 roots and stubble considerable vegetable matter, the 

 apparent exhaustion will be extended, and the applica- 

 tion of one or two of the constituents will meet 

 the needs. 



In the next place, the character of the crop, and 

 the period in which it makes its most rapid growth, 

 will guide as to the application required. If the period 

 of growth is short, a greater abundance of available 

 food will be required than if the growth extends 

 through a long period. The food requirements for 

 particular seasons are also important if the maximum 

 growth is in the very early season, as April and May, 

 as is the case with wheat, and the grasses — a larger 

 amount of nitrogen will be required than for a crop 



