

Authors:  G. Gianquinto, P. Sambo, F. Pimpini 
Keywords:  Foliar fertilization, N fertilization, Solanum tuberosum L., SPAD Threshold Value, supplementary fertilization, tuber production 
Abstract:
An experiment was conducted on potato cv. ‘Primura’ to test the usefulness of SPAD threshold values in decision making for supplemental nitrogen supply in 2000. A nonfertilized control was compared with nine fertilized treatments receiving 30 kg ha^{1} of N (urea 46% N) at crop emergence plus supplementary nitrogen which was applied as follows: i) five treatments provided increasing rates of N (30, 90, 150, 210 o 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) when 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 4545 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). In these treatments, the number of N applications and the total amount of N distributed to the crop were 4, 3, 3 and 5, and 120, 150, 210 and 76 kg N ha^{1}, respectively.
Yield response to CF treatments were optimum with 180 kg N ha^{1} (180 CF = 30+150 kg N ha^{1}). Yield of DF treatments did not differ from 180 CF. These results, particularly 30 DF and foliar DF treatments had amount of N distributed reduced by 30% and 60% as compared to 180 CF.

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