ISOTOPE STUDIES ON THE COMPARATIVE EFFICIENCY OF NITROGENOUS SOURCES

In a growth chamber experiment with 15N-labeled KNO3, (NH4)2SO4 and urea at 75 and 150 kg N/ha and NH4NO3 at 150 kg N/ha, N application produced significant responses of dry matter yield and total N uptake by shoot and root of barley in chernozemic dark brown Elstow silt loam and deep black Hoey clay soil. Total N removal per pot and isotope-derived criteria, i.e., percentage N derived from fertilizer, ''A'' value and percentage fertilizer N utilization, indicated that KNO3 was the most efficient and urea the least. At 75 kg N/ha, the recovery of fertilizer N in shoot and root was 47, 42 and 34% in Elstow silt loam and 65, 54 and 50% in Holey clay with KNO3, (NH4)2SO4 and urea, respectively. At 150 ppm N, these values were 62, 46 and 45% in Elstow silt loam and 75, 51 and 53% in Hoey clay and that of ammonium nitrogen from NH4NO3, 17 and 43% in the 2 soils, respectively. The percentage excess of 15N in soil total N after the harvest of barley showed that urea was tied up more in the soil, causing lower efficiency for crop recovery. The contribution of root portion for different parameters in such evaluation studies needed equal consideration. The availability of fertilizer N for crop utilization was more in Hoey clay than in Elstow silt loam. Immobilization and losses of fertilizer N were greater in Elstow silt loam.