It shows that the yield improvement is mainly due to the soil nutrient amelioration (especially nitrogen nutrient) and the interaction of mutual supplement of N and P nutrient.
指出杨树刺槐混交林生产力提高主要是因为林地土壤养分（特别是N素养分）的极大改善及树种间N、P养分的互补。
Interaction effect of n, p, K is positive to soybean's growth. According to these characteristics, we can provide theoretical basis for reasonable technology of applying fertilizer.
P、K三要素及元素之间互作对大豆生长均为正效应，由此可为合理施肥提供理论依据。
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