EFFECTS OF N RATES AND HARVEST DATES ON THE EFFICIENCY OF N-15-LABELED FERTILIZER ON EARLY HARVESTED POTATOES (SOLANUM-TUBEROSUM L)

The efficiency of N-15-labelled fertilizer on early harvested potatoes (Solanum tuberosum L., 'Norland') was studied in field experiments on two soil series (Saint-Damase loamy sand and Soulanges sandy loam) during a 2-yr period. The (NH4)-N-15 (NO3)-N-15 fertilizer was band applied at 0 and 140 kg N ha-1 in 1985 and 0, 70, 140 kg ha-1 in 1986 and four harvests were made during the growing season. The foliage was in full canopy development at 65 or 75 d and its dry matter yield increased significantly with the application of fertilizer-N. At this time about 70 % of the total N uptake was in the foliage. From this peak, foliage N decreased gradually with time to about 28 % at 95 or 100 d as N was transferred to the tubers. The N concentration in tubers was nearly constant during the growing season. As tuber dry matter increased at each successive harvest, N uptake increased proportionally. Between the first two harvest dates, from 65 to 75 d, the average rates of N accumulation in tubers were 2.1 and 4.5 kg N ha-1 d-1 for the control and N fertilized plot, respectively. The root dry matter and N concentration increased with fertilizer-N and were generally lowest on the final harvest day. Marketable tuber yield responded to N fertilization on the severely N-deficient fields in 1985. But in 1986, the 140 kg N ha-1 treatment delayed the growth of marketable tubers. The percentage of N derived from fertilizer (Ndff) was also very high in 1985 fields and was at a maximum at 65 d (64-69 %) with the 140 kg N ha-1 treatment. These values were lower for the same treatments in 1986 fields (39 %) because of the larger amounts of available N in soils. The maximum coefficient of utilization (ICU) of labelled fertilizer N was 72-76 % in 1985 and 63-68 % in 1986. At the final harvest, about 36-50 % of the applied fertilizer-N was found in the tubers.