Effect of organic, inorganic, and combined organic and inorganic P fertilization on plant P uptake and soil P pools

Phosphorus (P) is a limited resource, and its efficient use is a main task in sustainable agriculture. In a 6-year field experiment on a loamy-sand soil poor in P, the effects of organic, inorganic, and combined organic-inorganic fertilization on crop yield, P uptake into grain, and soil properties (organic matter [OM] content, pH, water-extractable P [Pw], double lactate-extractable P [Pdl], oxalate-extractable P [Pox], P-sorption capacity [PSC], and degree of P saturation [DPS]) were investigated for the maritime climate in northeast Germany. Nine treatments were compared: a control treatment without fertilizer application, two organic fertilizers (cattle manure [CM] and biowaste compost [BC]; applied at a rate of 30 t ha⁻¹ in autumn 1998 and 2001), application of triple-superphosphate (TSP; applied once a year either in autumn or in spring to evaluate the effects of application date), and combinations of organic and inorganic fertilizations. Several winter and spring crops (oilseed rape, barley, wheat) were cultivated according to good agricultural practice. The 6 year-average yield and P uptake were significantly higher for fertilized plots than for nonfertilized plots. Although the combination of organic x inorganic fertilizers resulted in higher soil P contents, significant yield increases were only found when organic fertilization was combined with TSP in spring. Small effects of P supply on yield in some years indicate that plant-available soil P (despite of low Pdl values) was sufficient for crop growth. Phosphorus supply affected soil Pdl and Pw more than the parameters measured in the oxalate extract (Pox, PSC, DPS). In general, periodically applied cattle manure and biowaste compost had the same effect on yield, P uptake, and soil P status as annually applied soluble mineral P.