Effects of Nitrogen, Phosphorus and Potassium Rates on Yield and Quality on Durum Wheat in a Two-year Rotation

Within the environmental impact reduction perspective, this work was aimed at assessing for durum wheat, grown within two crop rotations, the minimum allowable value of fertilising formulae to obtain adequate crop yields and quality for the operators of the whole production chain. Six fertilising formulae N1 P1 K1, N1 P2 K1, N2 P1 K1, N2 P2 K1, N3 P1 K1, N3 P2 K1, obtained from the factorial combination of 3 nitrogen levels (N = 50-100-150 kg ha-1) and 2 phosphorus rates (P2O5 = 50-100 kg ha-1) were compared against an unfertilised control N0P0K0. The potassium rate was the same for all fertilising formulae (K2O = 100 kg ha-1). The treatments were studied within a comparative trial between 2 two-year rotations: potato - wheat + bean (A1) and potato + purslane - wheat + bean (A2) aimed at testing the purslane potential and its ability to use the available residual nitrogen after potato cropping to reduce deep percolation losses. The best yield results were observed for wheat in the rotation that did not include purslane after potato (A1). In agreement with the literature, the nitrogen fertilisation had a positive effect on wheat grain yield also in the same cropping season as a consequence of a diversified action on the major yield components. The highest grain yields were observed in both rotations at the rate of 150 kg ha-1 of N, without any change with the rates of P. The protein content, which is higher in rotation A1, shows in both rotations the lowest value in the unfertilised control and improves with the increase in fertilisation, reaching the highest value (about 6% more) with the rate N3. No significant difference was observed between the rates N1 and N2; this would suggest a lower efficiency of the latter, at least for protein accumulation. A similar trend was observed in the gluten content that does not show any significant difference between N0 and N1, whereas it shows increases significantly different with the two higher rates. The inclusion of purslane that could use the residual nitrogen after potato crop so as to reduce deep percolation losses seems to induce a significant reduction of all yield parameters of wheat crop. As to the effect of nitrogen fertilisation, an application of 100 kg ha-1 is shown to produce a satisfactory yield response, but higher inputs of N (150 kg ha-1) are necessary to achieve good quality standards, at least in terms of protein and gluten content. As regards phosphorus fertilisation, the application of 50 kg ha-1 of P2O5 is shown to produce a satisfactory response.