PLANT FOOD IN THE SOIL 67 



Magnesium, a silver white solid. 



Iron, a silver gray solid. 



Of these elements the nitrogen, sulphur, phos- 

 phorus, potassium, calcium, magnesium, and iron 

 must not only exist in the soil but must also be 

 there in such form that the plant can use them. 

 The plant does not use them in their simple ele- 

 mentary form but in various compounds. These 

 compounds must be soluble in water or in weak 

 acids. 



Of these seven elements of plant food the nitro- 

 gen, phosphorus, and potassium and calcium are of 

 particular importance to the farmer, because they 

 do not always exist in the soil in sufficient available 

 quantities to produce profitable crops. Professor 

 Roberts, of Cornell University, tells us that an aver- 

 age acre of soil eight inches deep contains three 

 thousand pounds of nitrogen. The nitrogen exists 

 largely in the humus of the soil and it is only as 

 the humus decays that the nitrogen is made avail- 

 able. Here is another reason for keeping the soil 

 well supplied with organic matter. The decay of 

 this organic matter is hastened by working the soil ; 

 therefore good tillage helps to supply the plant with 

 nitrogen. 



If the nitrogen becomes available when there is 

 no crop on the soil it will be washed out by rains 

 and so lost. Therefore the soil, especially if it is 

 sandy, should be covered with a crop the year 

 through. Many lands lose large amounts of plant 

 food by being left bare through the fall and winter, 



