70 



CROPS AS FEEDERS 



Fruit cannot be produced without the formation of leaves and 

 new wood. This requires additional amounts of the plant-food 

 elements between twenty and fifty pounds of nitrogen, two to 

 four pounds of phosphorus, ten to twenty-five pounds of potassium 

 and twenty-five to eighty pounds of calcium per acre. 



THE SUPPLY OF NITROGEN, PHOSPHORUS AND POTASSIUM IN SOILS 



Having studied the draft that crops make upon the important 

 elements of the soil, let us now consider the soil reserve or supply 

 of these elements. Since calcium and lime are fully discussed in 

 relation to acid soils in Chapter XIII, our attention here is directed 

 to nitrogen, phosphorus and potassium. 



Supply Varies in Different Soils. It is to be expected that 

 chemical analyses should show great variations in the amounts 

 of nitrogen, phosphorus and potassium contained in different classes 

 and types of soils mentioned in Chapter II (Fig. 28). The amounts 

 of these elements in any particular soil expressed in terms of per 

 cent mean nothing or very little to the average person when he 

 has no standard for making comparisons. 



Simple Standards for Comparing Soils. The following table, 

 based on the averages of many analyses, will be of much help to 

 the beginner in judging soils of similar classes and types when 

 only chemical analyses are given. 



The Supply of Nitrogen, Phosphorus, and Potassium in Soils 



Some soils may be rich in one or two of these elements and poor 

 in others. 



It is not to be inferred that a silt loam is not a productive soil 

 unless it contains at least one-quarter of one per cent nitrogen, 

 one-tenth of one per cent phosphorus and two per cent potassium. 



