Yield gaps and nutrient balances in intensive, rice-based cropping systems on degraded soils in the Red River Delta of Vietnam

Alerted by reports about stagnating or even declining yields in intensive, irrigated rice-based systems, we analyzed productivity trends and P and K balances in a continuous cropping experiment on a degraded Acrisol of the Red River Delta in Vietnam. The trial, established in early 1998 and continued to 2003, included three cropping systems (rice–soybean–rice, soybean–rice–maize, and rice–rice–maize) and seven combinations of N, P and K and FYM. Potassium was the most yield-limiting macronutrient, and regular K applications were required to make investments in the application of other mineral nutrients profitable. In the absence of K application, the average yield gap for rice ranged from 1.2 to 2.2 Mg ha−1depending on season. For soybean and maize the yield gap resulting from K omission averaged 0.9 and 3.4 Mg ha−1, respectively. This yield gap was less when N or P was omitted. With NPK application, positive apparent K balances of the cropping systems of +14 to +37 kg K ha−1 yr−1 were achieved. Yield trends for 5 years were not significant and ranged from −0.04 to +0.13 Mg ha−1 yr−1 in rice, from −0.02 to 0.0 Mg ha−1 yr−1 in soybean, and from −0.04 to 0.0 Mg ha−1 yr−1 in maize. The application of FYM improved grain yield and K balances more in upland crops than in rice. The break-even costs for the application of FYM ranged from 24 to 31 US$ Mg−1 FYM depending on the cropping system. While present NPK fertilizer recommendations for degraded soils appear to maintain current average yield levels of 3.8 Mg ha−1 for rice and maize, and 1.2 Mg ha−1 for soybean, modifications of the nutrient management including the application of nutrients other than NPK are likely to be required to sustain higher yield levels in the future.