BIOLOGY AND AGRICULTURE 487 



and carbon dioxide to act chemically on the soil; it loosens the 

 soil grains to permit easy root growth and exposes new stores of 

 plant food for them to absorb. Loosening the top layers by 

 frequent tillage also forms a protective layer which retains water. 



Soil Composition. Plants can obtain oxygen, hydrogen, and 

 carbon from air and water, but must depend' on the soil for all 

 compounds of nitrogen, phosphorus, and potassium which are 

 just as essential in the making of protoplasm. 



To be fertile, a soil must contain compounds of these elements 

 in soluble form, available for plant use. The average soil contains 

 a supply of potassium compounds sufficient for 2000 years, phos- 

 phorous compounds to last for 130 years, but nitrogen compounds 

 only sufficient for 70 years' use. Yet nitrogen compounds are 

 more essential and used in greater quantity than either of the 

 others. 



Evidently the supply of nitrogen is the limiting factor in de- 

 termining how long a soil will remain productive; hence its return 

 to the soil is one of the greatest problems in agriculture. 



Maintaining the Soil. Every crop removes these essential 

 elements from the soil and erosion may rob it of as much more, 

 so man has learned to replace the removed substances by, 1. fer- 

 tilizers, 2. nitrifying bacteria, 3. crop rotation. 



1. Fertilizers obtain potash as potassium chloride and sul- 

 phate, largely from German deposits. Phosphorous compounds 

 are obtained from bone ash and the phosphate rock found in 

 California and Florida. Nitrogen is supplied to the soil by 



(a) Natural manures. 



(b) Nitrate of soda from Chile. 



(c) Slaughter house wastes. 



(d) Ammonia compounds from coal distillation. 



(e) Action of nitrifying bacteria. 



A complete fertilizer should supply all three elements, but as 

 the soil often has enough of one or two, this is sometimes un- 

 necessary and analysis of the soil is the only sure way of deter- 

 mining its needs. 



2. Bacteria, found in nodules on the roots of clover, peas, al- 



