PLANT FOOD 69 



(1) Nitrogen. Nitrogen is made available by the decay of 

 organic matter in the soil. The ammoniacal form is changed by micro- 

 scopic organisms present in the soil, into nitrous acid; other organ- 

 isms in turn change this to nitric acid, which when in union with the 

 mineral bases forms the nitrates which are the directly available 

 forms of nitrogen. As the nitrogenous organic compounds, such as 

 dung, urine, and green manure, as well as ammonium salts, are 

 finally changed to nitrates, it is evident that the corn plant growing 

 on a field which has been treated with manures of this character 

 draws its nitrogen supply from the nitrates of calcium, magnesium, 

 potassium and sodium, formed by the union of their decomposition 

 products with the bases in the soil. Nitrogen gathering bacteria living 

 in symbiotic relation with certain plants, namely, the legumes, draw 

 upon the abundant supply of nitrogen in the air, transforming it into 

 nitric acid, thus making it available for the plant. The element nitro- 

 gen enters largely into the formation of the grain. Sixteen per cent 

 of the elementary composition of protein is nitrogen. Experiments 

 have shown that corn grown on soils rich in nitrogen are higher in 

 protein content. 



(2) Phosphorus. Phosphorus constitutes a large proportion of 

 the ash of seeds. The amount of phosphorus (calculated as phos- 

 phoric acid) in the ash of the wheat kernel is 45 to 50 per cent, while 

 in the straw it is only 5 per cent.* 



Phosphorus is absorbed in the form of phosphates of calcium and 

 potassium. It enters into the formation of the proteins and is also 

 present in the inorganic compounds. 



In live stock farming phosphorus is more largely sold from the 

 farm than any other of the principal soil constituents necessary for 

 plant growth. Being used in the formation of bone and muscle the 

 per cent of phosphorus in a feed is of significance in feeding young 

 animals. In many sections of the corn belt it has already been found 

 profitable to add phosphorus to the soil in the form of some commer- 

 cial fertilizer. 



(3) Potassium. Potassium, usually spoken of as potash, K20, the 

 oxidized form, requires less serious consideration from the standpoint 

 of its ultimate depletion in the soil than either nitrogen or phos- 

 phorus. In the first place, there is already in all soils, except some 

 peaty-swamp soils, a large supply. Furthermore, the fact that it is 

 present in the straw rather than the grain of plants, guarantees, under 

 more modern methods of farming, its return to the soil each year. 



•"Agricultural Botany," Percivr.l. 



