Show NET SOIL NITROGEN LOSS 23 TONS A YEAR f science shows that plant growth is absolutely dependent upon nitrogen as well wall as upon other elements without available nitrogen in the soil plant and animal life would cease to exist in the 1938 yearbook of agriculture soils and men alen it is estimated that farm soils in the united states each year lose nearly 23 million tons of nitrogen through harvested crops grazing erosion and leaching only a little tle m more 0 re than 16 million tons are given back to the soil in the form of fertilizers manures rainfall irrigation waters and legume crops soils of this country have from 2000 to pounds of nitrogen per acre down to a depth of 40 inches binche jn in areas studied by department part ment scientists it is estimated that the air above an acre of soil however contains from to tons of free nitrogen although it must follow what is known as the nitrogen cycle before becoming available to plants and animals there are two ways by which free nitrogen of the air may be fixed in the soil the yearbook authors point out one is natural and the other artificial lightning discharges units nitrogen and ox oxygen y to form oxides of nitrogen these unite with moisture in the air to form nitrous and nitric acid which enter the soil with rain by other natural processes bacteria free in the soil arid and in the roots of legumes also transform air nitro gen into fixed nitrogen in artificial fixation man does what lightning and soil bacteria do he brings free nitrogen into combination with other elements mans success is 13 shown by the fact that in 1900 two thirds of the worlds worlds nitrogen supply was obtained from nitrate deposits in chile thirty four years later 74 percent of the supply was obtained from the air and fixed in the form of chemical fertilizers but because of the great net lo 10 loss s of soil nitrogen the yearbook advocates a wider and more effective use of legume crops |