The use of SPAD 502 chlorophyll meter for dynamically optimising the nitrogen supply in potato crop: first results

An experiment was carried out in 2000 on potato cv. Primura to test the usefulness of SPAD threshold values in decision making for supplemental nitrogen supply. A non-fertilized control was compared with nine fertilized treatments receiving 30 kg ha-1 of N (urea 46% N) at crop emergence plus supplementary nitro- gen which was applied as follows: i) five treatments provided increasing rates of N (30, 90, 150, 210 or 270 kg N ha-1, urea 46% N) distributed 18 days after emergence, (conventional fertilization, CF); ii) four treatments provided supplemental nitrogen application (urea, 46% N) whenever the SPAD values fell below the critical value of 39 SPAD (dynamic fertilization, DF). This critical level corresponds to 10% deviation from threshold value of 44-45 SPAD calculated in a previous work. Each time, increasing rates of N were distributed to the soil (30, 60 or 90 kg N ha-1 - 30 DF, 60 DF and 90 DF), or 9.2 kg N ha-1 was applied via foliar spray (foliar DF). The number of N applications was 4, 3, 3 and 5, and the total amount of N applied to the crop was 120, 150, 210 and 76 kg N ha-1, for the 30 DF, 60 DF, 90 DF and foliar DF treatments, respectively. Yield response to CF treatments showed best result with 120 and 180 kg N ha-1 (120 CF = 30+90 kg N ha-1; 180 CF = 30+150 kg N ha-1). Yield of DF treatments did not differ significantly from 120 CF and 180 CF. This yield was achieved with a noticeable reduction in total N application for foliar DF treatment (reduction of 37% and 58% as compared to 120 CF and 180 CF, respectively).