Effects of N rates and harvest dates on the efficiency of 15N-labelled fertilizer on early harvested potatoes (Solanum tuberosum L.)

The efficiency of 15N-labelled fertilizer on early harvested potaotes (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 15NH415NO3 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 f...