Phosphorus (P) is an essential plant nutrient and P fertilizers are added to supplement the soil's available P. There are economic and environmental benefits to making informed decisions about P fertilizer use. The under-application of P fertilizer can result in reduced yields, while over-application adds to input costs, with economic losses resulting from both scenarios. From an environmental perspective, excessive P going into streams and lakes can result in toxic algal blooms.
A few frequent questions about P fertilizer use are: Does P fertilizer always result in a positive yield response? How much yield increase is expected with applied P? What is the likelihood of yield penalty if P fertilizer is not applied?
How much yield increase is expected with applied P?
It depends on the Mehlich-3 soil test P level. The Mehlich-3 soil P measures the readily available soil P for crop uptake. Culman et al. (2023) classified Mehlich-3 soil P levels into five categories: <10, 10–20, 20–30, 30–40, and >40 ppm to evaluate the yield increase for each soil P category.
The crop yields were presented as Relative Yield, which refers to the yield with no P application divided by the maximum yield obtained across all P treatments. In other words, 100% relative yield means no yield increment with added P. The lower the relative yield, the higher the yield increment.
Generally, as the soil test P levels decreased, the yield increment increased with P input (Table 1). When the soil test P was less than 10 ppm, the median relative yield was 87%. As the soil test P level increased above the critical level of 20 ppm, the median relative yield ranged from 97% to 99%, signifying minimal yield increment with added P.
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