112 A YEAR IN AGRICULTURE 



Twelve to 15 per cent of phosphorus in rock phosphate is 

 equivalent to the 28 and 34 per cent of the so-called phos- 

 phoric acid. It must also be said, in advising the use of 

 the natural rock, that it must be used liberally and in con- 

 nection with plenty of decaying organic matter. The phos- 

 phorus in the raw rock must be made available, and this is 

 done through the decomposition of organic matter. About 

 one ton of rock phosphate once every four or five years incor- 

 porated in the soil with a green manure crop or barnyard 

 manure is the economic way of using this mineral plant-food. 

 As was said before, in the decomposition of these organic 

 materials, carbonic acid and possibly other acids are formed, 

 and these furnish a solvent for the phosphorus compound in 

 the natural rock. 



Other forms of phosphorus, such as raw bone, steamed 

 bone, and acid phosphate, are more quickly available than 

 the rock phosphate and give quicker results when used, but 

 they are more expensive when purchased in sufficient quan- 

 tity to supply the need. These soluble forms of phosphorus 

 may be practical and economical in truck farming, garden- 

 ing, and in starting such crops as alfalfa and other legumes. 



Potassium. The tables given in a preceding paragraph 

 show that potassium is an abundant element in normal soils. 

 It is found in greater abundance in clay soils than in sandy 

 or peat soils. As it exists in clay soils it is often unavailable 

 and must be made soluble for the plants' use by proper 

 tillage and by the use of organic matter. By the use of 

 active organic matter, as explained above, large quantities 

 of potassium otherwise not available are made available to 



