The results of the eighth year of the Dutch IMPHOS field trial IB 0013 on calcareous silt loam soil with potato are presented and discussed. At the first stages of crop growth, phosphorus fertilization had a positive effect on fresh and dry matter yields of biomass. At the final harvest the potato yield was significantly increased by phosphate fertilization. Without fertilization the yield was 87.7% of the fertilized treatments. A Pw value of 6-9 mg of P2O5 per litre of soil for potato is not sufficient. At this low phosphorus status of the soil potato requires fertilization. Phosphorus fertilization increased the phosphorus content of the potato tops and tubers. Besides, the phosphorus uptake by potato increased after fertilization. The general yield level of the crop was low. This is contributed to the delayed planting date, a sub-optimal supply of nitrogen and possibly the presence of a latent form of fungi disease, Phytophtora infectans. Based on the average total phosphorus uptake by sugar beet on the maintenance treatment (M), the 1999 maintenance phosphorus fertilization for spring barley is set at 8 kg of P per ha. This leads to an M treatment of 18.2 kg of P2O5 per ha, and 2M and 3M treatments of 36.5 and 54.7 kg of P2O5 per ha, respectively. The phosphorus status, determined as Pw value, of the plough layer (0-25 cm) of 2M and 3M treatments decreased compared to 1997. The phosphorus status of the M treatments and the zero treatments remained at the same level.
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