ORGANIC CONSTITUENTS OF SOILS 151 



7. Furnishes Nitrogen to Crops. The only source of nitrogen 

 for our non-leguminous. crops is organic matter. Nitrogen starva- 

 tion goes' hand in hand with low organic content in soils. This is 

 evidenced by the yellowish-green color of corn, oats, or wheat on 

 eroded land deficient in organic matter in contrast to the dark green 

 color where this constituent is abundant. It supplies nitrogen, the 

 most expensive food element used by plants, one that we cannot 

 afford to buy for ordinary farm crops. A 100-bushel crop of corn 

 per acre requires 150 pounds of nitrogen, the commercial value of 

 which at 15 cents per pound is about $22.50. Other crops require 

 somewhat similar amounts. Legumes are independent of organic 

 matter, as they obtain their nitrogen from the air. 



8. Binds Soil Particles Together. On sandy soils well-decom- 

 posed organic matter binds the sand grains together and reduces 

 movement by wind. It also increases the water-holding capacity, as 

 seen before. 



Losses of Organic Matter. The amount of organic matter 

 in the surface stratum of the ordinary upland soils varies from 

 15 to 60 tons per acre. This has required thousands of years 

 for its accumulation, but through the systems of cropping generally 

 practiced it is being removed from the soils much more rapidly 

 than it ever accumulated. 



(a) By Cropping. The amount of organic matter removed 

 annually from a soil well supplied with it in reasonably active form, 

 such as brown silt loam, is not far from three-fourths to one ton per 

 acre. A large portion of this is used indirectly by the crop, while the 

 remainder is lost by the natural processes described below. In com- 

 paring a virgin prairie soil with the same soil after cropping for 

 sixty years, it was found that the organic-matter content of the soil 

 has been reduced approximately fifty tons per acre. Of course, in 

 soils with a smaller amount of organic matter the total removed is 

 necessarily less. The amount removed depends to some extent upon 

 the crop grown. The inter-tilled crops use more nitrogen, and more 

 organic matter would be decomposed to produce it than non-tilled 

 crops. It must be remembered that loss of nitrates either by crop- 

 ping or leaching means loss of organic matter from the soil. 



(b) By Erosion. Organic matter may be removed from the 

 soil by erosion. Very few regions are so flat or have the soil so well 

 protected that there is not more or less erosion taking place, and in 

 the more rolling areas this becomes a very active agent in the 

 removal of the organic matter along with the soil. In this way in 



