Changes in Soil Test P and K with Site and Soil Differences.

A long-term grid sampling study that included records on P and K fertilizer rates and yield was conducted in one Illinois field from 1961-1999, and in another Illinois field from 1982-2001. Soil test P and K was increased due to P and K applications until 1988. From 1988 until the end of the study no additional P or K was applied. This enabled the determination of P and K soil test buildup and draw-down rates at each site. Due to the presence of grid sample P and K values, an estimate of differences in P and K buildup and draw-down could also be conducted. At one site, P buildup rate was 1 kg ha-1 for each 5.4 kg ha-1 P2O5 applied. P draw-down rate was 1 kg ha-1 for each 7.9 kg ha-1 crop removal. At this site, K buildup was 1 kg ha-1 for each 6.0 kg ha-1 K2O applied. K draw-down rate was 1 kg ha-1 for each 1.7 kg ha-1 crop removal. At the other site, P buildup rate was 1 kg ha-1 for each 5.8 kg ha-1 P2O5 applied. P draw-down rate was 1 kg ha-1 for each 15.0 kg ha-1 crop removal. At this site, K buildup was 1 kg ha-1 for each 7.1 kg ha-1 K2O applied. K draw-down rate was 1 kg ha-1 for each 3.0 kg ha-1 crop removal. Differences in buildup and draw-down rates were also noted within fields between soil types.