Knowledge‐intensive approaches have been proposed to manage the variability in indigenous nutrient supplies (IS) in irrigated rice (Oryza sativa L.) systems. On‐farm experiments were conducted at 155 locations in seven domains of Asia to quantify the variability of soil properties, grain yield, and nutrient uptake in N, P, and K omission plots (0‐N, 0‐P, and 0‐K, respectively). Except for pH, coefficients of variation of soil properties within a domain ranged from 17 to 43%. Similar ranges were measured for grain yield and plant nutrient uptake in nutrient omission plots, which served as crop‐based estimates of indigenous N, P, and K supply. Soil properties showed little association with plant nutrient uptake or grain yield in nutrient omission plots. Mean grain yields in nutrient omission plots increased in the order 0‐N (3.9 Mg ha−1) < 0‐K (5.1 Mg ha−1) ≤ 0‐P (5.2 Mg ha−1). Soils, climate, and crop management caused large variability of IS among irrigated rice domains, years, growing seasons, and fields within a domain. Grain yield and nutrient uptake in omission plots were mostly higher in high‐yielding than in low‐yielding climatic seasons. No changes in indigenous N supply occurred for periods of 4 to 6 yr in the same seasons. Grain yields in nutrient omission plots were strongly correlated with each other and also with the yield in the fertilized farmers' fields. Fertilizer recommendations should be fine‐tuned to spatial domains with relatively uniform agroecological characteristics, cropping practices, and socioeconomic conditions. Within such domains, season‐specific management of the IS variability can include field‐specific approaches.
Nutrient supplies from indigenous sources (IS) can be estimated by measuring plant nutrient uptake in nutrient omission plots. On‐farm experiments were conducted in irrigated rice (Oryza sativa L.) domains of Asia to evaluate relationships of plant N, P, and K uptake with soil tests or grain yield measured in N, P, and K omission (0‐N, 0‐P, and 0‐K, respectively) plots and to develop guidelines for the use of omission plots in site‐specific management. Relationships between grain yield or nutrient accumulation and soil tests were scattered. Only 17% of the variation in plant N uptake in 0‐N plots was explained by total soil organic C. Extractable Olsen P explained 34% of plant P uptake in 0‐P plots, whereas 1 M ammonium acetate K showed no common relationship with plant K uptake in 0‐K plots. With good calibration, indigenous supply of N (INS), P (IPS), and K (IKS) can be estimated from grain yields in omission plots with a precision of about ±5 to 10 kg N ha−1, ±2 to 3 kg P ha−1, and ±10 to 20 kg K ha−1, respectively. Sampling requirements for estimating domain‐specific IS values depend on the homogeneity of the domain of interest. For irrigated rice domains of about 100 to 200 km2, grain yield in omission plots should be measured in at least one high‐yielding season in about 10 farms to estimate the domain mean INS, IPS, and IKS. Future research should focus on developing geospatial techniques for delineating fertilizer recommendation domains based on biophysical and socioeconomic characteristics that determine yield potential, IS, and response to fertilizer.
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