On-farm assessment of organic amendments effects on nutrient status and nutrient use efficiency of organic rice fields in Northeastern Japan

Animal compost, organic fertilizer and straw management are among the factors affecting soil fertility in organic cropping systems. However, few studies have compared the effects of organic amendments on soil fertility of organic paddy fields. We conducted a survey of organic farmers' paddy fields to compare soil nutrients (N, P, K, Ca and Mg), nutrient budgets and partial factor productivity of N (PFPN), and to relate them to organic amendments. The study fields were located in the Higashi Okitama area (38°N, 140°E) of Yamagata prefecture, Japan, and organically managed by a group of farmers using their management standard. Soil samples from 40 rice fields were collected in October 2001. The fields were grouped into three types in terms of organic amendments: application of cattle/chicken compost and straw incorporation (CA), application of cattle composts and straw removal (CR), and straw incorporation only (FA). Purchased organic fertilizers were applied to all the fields. Most of the fields had received the same managements for 5 years or more prior to the survey. Organic amendments had significant effects on pools (the amount of nutrients to 0.4 m depth) of Bray-2 P and NH4OAC extractable K, input, output and balances of the nutrients. Annual surpluses of N, P, K, Ca and Mg (+100, +102, +130, +133 and +33 kg ha−1, respectively) were found in the CA field due to the compost application, but K balance of the CR field was −27 kg ha−1. Nutrient budgets were well balanced in the FA field, but NH4OAC extractable K in the FA soil was lower than the medium range of the conventional soil diagnosis by 37 mg kg−1, suggesting K depletion. The pools of Bray-2 P and NH4OAC extractable K were correlated with balances of P and K (r = 0.597 and 0.540, P < 0.001), but those of NNM, total N and NH4OAC extractable Ca and Mg were not related with nutrient balances (P > 0.05). PFPN of the FA fields (89 kg kg−1) was twice as high as that of the CR and CA fields (45 and 37 kg kg−1). The former value was comparable to PFPN (78 kg kg−1) of conventional rice fields with mineral fertilizer use of the surrounding area. Two alternative scenarios for fine-tuning nutrient inputs in organic farming practices are better compost quality and reallocation of chicken/cattle compost among organic rice fields.