Supply of phosphorus and nitrogen affects both growth and development rates in onion

The hypothesis that soil anion storage capacity (ASC) directly influences the response of onion yield to soil Olsen P status was tested. Onions were grown to maturity in three different soils in the same tunnel house. Each soil received three rates of phosphorus (P) fertiliser 1 month before planting, and at planting soil ASC and Olsen P status was measured. Although bulb yield increased with Olsen P (over the range 15–95 μg g−1), ASC (which varied from 10 to 70%) had no influence on this response. This supports recent findings with young onions grown in small pots, but contradicts previous advice to vegetable growers. The results show that low to moderate soil Olsen P values and low initial availability of N, slow early plant growth and development. Delays in maturity due low P input may allow growth to catch up, at least partially, later in the season. This has implications for the design and execution of field experiments to test P fertilisers for onions and suggests that visible responses to P fertiliser in young onion crops may be misleading.