Observation of An Apparent Increase in P Availability for Potatoes Following a Green Manure.

Large fluctuations in fertilizer prices over the past several years have given cause for producers and researchers to look for ways to more efficiently use fertilizer materials. As part of this process, data from recent P response studies with potatoes in Maine were reviewed to make a preliminary evaluation of how well the current recommendation system predicted experimental results. Data from 14 trials (11 on-farm studies and one research station trial with ‘Atlantic’ potato, and two on-farm studies with ‘Russet Burbank’ potato) were evaluated. A quadratic regression was fit to each data set, and an optimum rate estimated as the point where the slope of the estimated return matched the incremental cost of P fertilizer. Where potatoes followed a small grain crop, the current recommendation system approximated the estimated optima from experimental data fairly well. Where potatoes followed a green manure or hay crop, the recommendation system tended to over-predict P requirements relative to what experimental data showed as economic optimums. In fact, there was no case in the data sets reviewed where potatoes following a green manure or clover plow-down showed an economic response to P. It appears that soil P availability following a green manure is greater than the soil P measurement would indicate – this may be because of a better environment for root growth following a green manure, improved mycorrhizae development, or it may be because P was not removed in the harvest of grain from the previous crop. Further work should be done with controlled studies directly comparing P response of potatoes following a green manure, versus a small grain, crop. As a first approximation to improve P recommendations for potatoes in Maine, one option would be to give a credit of 25 lb P per acre for potatoes following a green manure.