ORGANIC MATTER 55 



nitrogenous. As the growth of these organisms is very- 

 necessary in making the various sorts of plant nutrients 

 available, the supply of organic matter for this purpose is 

 of great importance. 



58. Organic matter forms acids. — Finally, organic matter 

 in its very last stages of decomposition continues to serve 

 the plant by producing carbonic acid gas, which, dissolved 

 in soil water, is an excellent solvent for many mineral sub- 

 stances needed by plants. It is estimated that in an acre 

 of soil sixteen inches deep, sixty-eight pounds of carbon 

 dioxide are produced annually from the decomposition of 

 organic matter when present in ordinary quantity. There 

 are also other organic acids formed by the rotting of or- 

 ganic matter that serve to dissolve the inorganic matter of 

 soils. The combinations of these organic acids with min- 

 eral substances form readily available plant-food materials. 

 Another final product of nitrogenous organic matter is 

 nitrate, which is the most 'available form of nitrogen for 

 many plants. 



59. Injurious effect of organic matter. — The injury that 

 organic matter may cause is probably not of very frequent- 

 occurrence and is unimportant as compared with its benefi- 

 cial action. Two effects have been noted : 



(1) Undecomposed organic matter may cause a soil to 

 dry out quickly by preventing it from settling sufficiently 

 to establish water connection with the subsoil and by leav- 

 ing large air spaces that allow a rapid movement of air 

 through them which dries out the soil. 



(2) Partially decomposed organic matter may form prod- 

 ucts that are poisonous to some agricultural plants or that 

 interfere with the operations of those microorganisms that 

 are beneficial to plant growth. 



60. Management of soil with respect to organic matter. — 

 The first step in the control of organic matter in soil is to 



