Determination of critical nitrogen concentrations of potato (Solanum tuberosum L. cv. Sebago) grown in sand culture

Abstract

A gamma x cubic response surface model was used to predict the dry matter yield of potato cv. Sebago over the 12-week growth period in sand culture with nitrogen (N) levels of 2, 7, 14, 29 and 43 mmol N/L. At each 2-week sampling period after emergence, dry matter yield relative to maximum was plotted against tissue N concentration to derive diagnostic petiole, petiole sap, leaf nitrate-N and leaf total N in youngest fully opened leaf (YFOL), youngest fully expanded leaf (YFEL) and oldest green leaf (OL) and for total N in bulked leaves. Critical concentrations corresponding to 90% maximum yield are presented. Tissue nitrate was much more responsive than leaf total N to applied N over the 2-14 mmol/L range where positive growth responses to N were recorded. Plants grown with 2 mmol N/L were severely N deficient and growth was depressed. Tissue nitrate concentrations in these plants from 4 weeks after emergence onwards were negligible, while leaf total N concentrations exceeded 2.36%. Salt toxicity occurred at 29 and 43 mmol NIL, and it sometimes reduced tissue N concentrations so that adequacy and toxicity concentrations overlapped. Critical tissue N concentrations declined over the growth period, the largest decline occurring for nitrate. Critical tissue N concentrations for YFEL, from 2 weeks after emergence to final harvest were: petiole sap nitrate-N, 1.2-0.2 g/L; petiole nitrate-N, 2.1-0.1%; leaf nitrate-N, 0.44-0.08%. Critical tissue nitrate concentrations clearly differentiated between inadequate and adequate N application levels. Critical leaf total N concentrations only differentiated between inadequate and marginal N application rates, except for OL when inadequate and marginally adequate (80-90% maximum yield) concentrations were not different (P>0.05). Nitrogen application level affected (P<0.05) leaf potassium, phosphorus, calcium (Ca), magnesium (Mg) and sulfur concentrations. The largest effects were recorded for Ca and Mg where increasing N application level reduced leaf nutrient concentration. Petiole sap nitrate concentrations can be used as a rapid field test for distinguishing between a deficient and an adequate N supply. Where concentrations exceed critical values, they can be interpreted as such because N fertiliser toxicity rarely occurs under field conditions.