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Stopping P fertilizer decreases soil inorganic P concentrations

 

If long-term phosphorus (P) fertilization increases soil P to concentrations exceeding plant requirements, water quality problems can result from P loss in runoff. Stopping P fertilization should decrease soil P; however, understanding changes in soil P forms and concentrations is necessary to manage P without reducing crop yields.

In the May-June issue of the Journal of Environmental Quality, researchers report on a study using grazed grassland plots from Northern Ireland. Plots received the same rate of inorganic P fertilizer from 1994-1999, different (zero to very high) rates from 2000-2005, and no P fertilizer from 2005-2010. Samples from 1994, 2000, 2005 and 2010 were analyzed in detail to characterize soil P forms and concentrations.

At the highest P fertilizer rates, soil P concentrations were increased in 2005, but decreased to 2000 levels by 2010 after fertilization stopped. Plots that received no P fertilizer from 2000 onward had soil P concentrations drop back to 1994 levels. Concentrations of soil organic P forms were essentially unchanged over the study period; only inorganic P changed.

This study shows that excess fertilizer P is stored in soil, and can be reduced when fertilization stops. However, it may take many years to see decreased soil P concentrations depending on the rate of P application.

Read the full paper in JEQ. Free preview June 16 - June 23

Grazed grassland plots in Northern Ireland were used to study soil P forms (with P-NMR spectroscopy) and pools (such as soil Olsen P) when P fertilizer was applied and then stopped.