. Efficient use of nitrogen on crop land in the Northeast . Jmji h tt-A 7»U- ck-rmnmittrmk g â >- « i -W^- Â¥ cwl ' f U ^>%Lridt-«- !-â¢. -I 6 8 0 12 2 6 8 10 12 2 MONTHS Figure 14. Distribution of rainfall with time and distribution of nitrate-N concentration with soil depth and time beneath shade tobacco in 1975. (Data of Starr and DeRoo, Connecticut) quently, the rate of fertilizer N uptake dropped to nearly zero, resulting in a low overall total fertilizer efficiency of about 35% from the first application and 20% from the second. Owing to the low leaching loss observed
. Efficient use of nitrogen on crop land in the Northeast . Jmji h tt-A 7»U- ck-rmnmittrmk g â >- « i -W^- Â¥ cwl ' f U ^>%Lridt-«- !-â¢. -I 6 8 0 12 2 6 8 10 12 2 MONTHS Figure 14. Distribution of rainfall with time and distribution of nitrate-N concentration with soil depth and time beneath shade tobacco in 1975. (Data of Starr and DeRoo, Connecticut) quently, the rate of fertilizer N uptake dropped to nearly zero, resulting in a low overall total fertilizer efficiency of about 35% from the first application and 20% from the second. Owing to the low leaching loss observed in these experiments, the low efficiency must be due to a large degree of immobilization and/ or denitrification. Studies in Maine The impact of rates of N fertilizer applied to po- tatoes on the soil solution levels and on the leaching losses of NO3-N to groundwater was investigated at Presque Isle, Maine, from 1973 to 1978. The soil at the site was classified as a Caribou loam (coarse- loamy, mixed, frigid Typic Haplorthod). Samples of soil solution were removed from the B and C horizons at respective depths of about 45 cm and 120 cm. Ka- tahdin potatoes were grown in an alternate year ro- tation with buckwheat. Rates of N applied to potatoes were 0, 168 and 224 kg/ha. No N was applied to the buckwheat. Samples of soil solution were removed at bi-weekly intervals beginning at spring thaw and con- tinuing until freeze-up. Potato yields were measured annually. Potato tubers were the only material re- moved from the plots during the rotation; all other organic residues were incorporated into the soil. Tuber yields obtained over a 6-year period are shown in Table 8. The addition of 168 kg N/ha re- sulted in tuber yields that were not significantly dif- ferent from those at 224 kg/ha. The lack of response by Katahdin potatoes to the 224 kg/ha rates of N as compared to 168 kg/ha indicated that the additional
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