FERTILIZERS 15 



steamed bone and tankage. The rock phosphates are the 

 principal source of phosphoric acid for use in gardening. 



18. Complete Fertilizers. Fertilizers containing all three 

 of the principal elements of plant food, nitrogen, phosphoric 

 acid and potash, are called coni'plete fertilizers. The per- 

 centages of nitrogen, phosphoric acid, and potash, vary 

 with the crops for which they are intended, nitrogen being 

 higher, for example, in a fertilizer for asparagus or onions, 

 while a fertilizer for the root crops would be lower in nitrogen 

 and higher in potash. 



A fertilizer containing 4% nitrogen, 8% phosphoric 

 acid, and 10% potash, is considered the basic fertiUzer, 

 but as has been indicated, the percentages vary with 

 the need of the crops to be fertilized. 



The rate of application of complete fertilizers is governed 

 by crop requirements and soil conditions. The amount 

 varies from a few hundred pounds to a ton or more per acre 

 for a single crop. The average application per season in 

 intensive gardening is 1500 to 2000 pounds per acre, although 

 the amount applied during the season may be as high as 

 4000 pounds or more. 



19. Effects of Chemical Fertilizers on the Soil. Exces- 

 sive applications of chemical fertilizers tend to cause soil 

 acidity. In instances where the supply of humus is not 

 kept up, the soil will become hard and unproductive, as well 

 as acid. 



To remedy these conditions, the supply of humus must 

 be maintained and liberal applications of lime made as 

 required. The amount of lime to apply varies with soil 

 conditions; a ton to the acre is usually sufficient for most 

 soils, although as much as three tons per acre are sometimes 

 required to neutrafize an acid soil. 



20. Effects of Lime. Soil acidity is neutralized by 



