CHAPTER VII 



AVAILABLE PLANT FOOD 



"AVAILABLE plant food" is an expression much used in connec- 

 tion with commercial fertilizers, and the argument is commonly 

 made that because the soil does net contain available plant food, 

 we should therefore apply available plant food in commercial fer- 

 tilizers. Instead of following this advice, however, the farmer 

 should, as a very general rule, adopt a system of farming that will 

 make available the plant food in the soil so far as practicable, and, 

 if any element is actually deficient in the soil, apply that element 

 in cheap form and in positively larger quantities than will be re- 

 moved in large crops; and then make it, too, available by his 

 method of farming. 



There are three methods of determining with some degree of 

 satisfaction which elements, if any, are deficient in the soil: 



First, we may compute from the composition of the soil and the 

 requirements of crops the probable durability of a soil with reference 

 to any element of plant food. Thus, we may determine that the 

 unglaciated yellow silt loam surface soil of Illinois, Kentucky, 

 Tennessee, and other adjoining states, contains sufficient nitrogen 

 for less than 20 large corn crops if only the grain were removed; 

 while the potassium in the late Wisconsin brown silt loam is suffi- 

 cient for more than 2300 such crops. 



Second, we can assume for a rough estimation that the equiva- 

 lent of 2 per cent of the nitrogen, i per cent of the phosphorus, and 

 | of i per cent of the total potassium contained in the surface soil 

 can be made available during one season by practical methods of 

 farming. Of course, the percentage that can be made available will 

 vary very much with different seasons, with different soils, and for 

 different crops; and yet with normal soils and seasons and for ordi- 

 nary crops the above percentages represent roughly about the 



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