A Study of the Validity of Laboratory Techniques in Appraising the Available Nitrogen Producing Capacity of Soils1
- C. B. Kresge and
- F. G. Merkle2
There is urgent need for a laboratory procedure capable of determining the status of the soil nitrogen, one which will predict the ability of a given soil to quickly replenish nitrates removed by cropping and leaching. Such a method would be useful in evaluating soil management systems and in determining the need for nitrogenous fertilizers. A laboratory method is described involving incubation under controlled conditions for 12 weeks with bi-weekly removal of nitrates produced. This method was found to be successful but time-consuming. Distillation of ammonia from soils in the presence of alkaline permanganate gave recoveries which were well correlated with incubation results. Both procedures showed that from 200 to 400 pounds out of the 2,000 to 4,000 pounds of total nitrogen in soils are biologically active. With these methods it was possible to show that long continued use of manure leads to a backlog of biologically active nitrogen. Acidity likewise favors accumulation while excessive liming without proper provisions for maintenance depletes the active organic nitrogen supply. Prolonged acidity causes a decline in microbiological activity, permitting potentially active forms of organic nitrogen to accumulate. When this acidity is corrected, as in the laboratory procedure described or as in tier II of the Jordan plots, the yields are higher than on the corresponding less acid plots.
It is suggested that either method is quite useful in evaluating soils and soil management systems, but of questionable value in determining the nitrogen fertilizer requirements for the current year. Nitrate delivery to a given crop depends, not only upon the presence of easily ammonifiable organic nitrogen in the soil, but also upon the soil conditions prevailing at the moment. The available nitrogen requirement of a given crop depends upon the adequacy of other growth factors. A very poor correlation was obtained in the field between response to nitrogen and the quantity of easily nitrifiable organic nitrogen in those soils.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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