254 PLANT BIOCHEMISTRY 



cal condition of the soil by improving texture, moisture-holding capac- 

 ity, and aeration. Organic matter increases the buffering capacity of 

 soil and combines with inorganic soil constituents to prevent their 

 loss by leaching but releases these substances for the use of plants. 

 Finally, organic matter is the sole storehouse of nitrogen. 



The importance of humus for the maintenance of soil productivity 

 has been recognized for many years. Repeated observations have 

 shown that virgin soils containing large amounts of organic matter in 

 the form of humus may yield large crops for the first few years of cul- 

 tivation. These crop yields gradually decrease with the number of 

 years of cultivation and crop removal. One of the chief reasons for 

 this loss of fertility is the loss of humus from the soil under con- 

 tinuous cropping. 



However, the reader should remember that neither humus nor 

 organic matter of any kind is essential to plant growth. Plants can 

 be grown in sand cultures with properly balanced nutrient solutions 

 containing no organic compounds, and plants can be grown in water 

 solutions containing only inorganic compounds. The principal impor- 

 tance of soil organic matter, or humus, is its effect on the soil as a 

 medium for plant growth. 



Nitrogen Fixation 



Nitrogen is one of the elements essential to the maintenance of 

 soil fertility. The lack of sufficient quantities of available nitrogen 

 in soils has long been recognized as a limiting factor in crop produc- 

 tion. An ample supply of readily available nitrogen stimulates vege- 

 tative growth of plants. 



Most of the nitrogen found in soils occurs in combination with the 

 soil organic matter. While in this form it is comparatively insoluble 

 and not available for plant growth. However, the nitrogen of organic 

 matter may eventually be liberated for utilization by plants through 

 the action of microorganisms. All soils contain a small proportion of 

 soil nitrogen in the form of relatively simple compounds such as 

 amino acids, ammonium salts, and nitrates. It is from these com- 

 pounds, particularly from ammonium salts and nitrates, that higher 

 plants obtain their nitrogen. The total quantity of nitrogen found 

 in mineral soils is usually quite low, varying from 0.1 to 0.5 per cent. 



Atmospheric nitrogen is the ultimate source of soil nitrogen. The 

 various processes by which this inert gas is changed into organic and 

 inorganic compounds are biochemical. Some of these processes will 

 now be considered. 



