Relative Phosphorus Fertilizer Efficiency of Rapeseed and Soybean Cakes Across Different Soils

Cake fertilizers are phosphorus-rich organic fertilizers that are commonly used in horticulture. Soil plays a crucial role in determining the effectiveness of phosphorus fertilizer. Comparative data on the relative phosphorus efficiency (rPE) of cake fertilizers across contrasting soils are scarce in the international literature. Information on the mechanisms that control phosphorus supply is also limited. This study examined the rPE of rapeseed and soybean cakes in three soils using ryegrass growth experiments and investigated the main factors affecting their phosphorus efficiency. The results showed that the rPE of rapeseed cake did not differ significantly among the three soils, with an average value of 71%. In contrast, the rPE of soybean cake showed a clear soil-dependent pattern, with the highest rPE in red soil (67%), followed by fluvo-aquic soil (47%), and the lowest in yellow-brown soil (32%). In red soil, there was no significant difference in rPE between the two cakes. Water-soluble phosphorus content of cake fertilizers and soil phosphatase activity are key factors affecting rPE. Owing to its low water-soluble phosphorus content, the phosphorus supplied by soybean cake is predominantly mobilized through soil phosphatase–mediated mineralization of organic phosphorus. In phosphorus fertilization practices, both cake water-soluble phosphorus content and soil phosphatase activity should be considered. In soils with low phosphatase activity, cake fertilizers with a higher water-soluble phosphorus content should be prioritized.