Fig. 4: Average growing season concentration of phosphate (PO₄^3--P) collected in lysimeters versus the plot soil P level in that same growing season.

P tests for each study (each panel) are different and represent the suggested test for that region. P-AL and Bray-P are used for acidic soils, and Olsen-P for calcareous/alkaline soils⁶⁴. Similarly, the vertical grey line in each panel represents regional recommendations. Each dot represents one plot in a given study year. Colors in (a) match Figs. 2 and 5 –input treatments. In panels b and c, shapes represent farms and gardens, and the colors match Fig. 3. Given shared site history, one would expect soil P levels to be more similar within one farm (b) or garden area (c) which is why these were used as symbols. The black line in panel b is a linear regression line (p < 0.01).