Project Details:

Phosphorus is an important element which must be maintained at a sufficient level to sustain the maximum crop productivity. Even the soils in the coastal Pacific Northwest, which are among the highest in total P concentration in the nation, are no exception to this. The amount of phosphorus that should be applied to achieve maximum crop productivity is generally high in acidic soils and even higher in regions where the soil temperature is cold in the spring and the growing season is relatively short.

Under most crop production systems, phosphorus is typically banded to reduce the P sorption by soil particles and to lower the P fertilizer rate needed for maximum crop yields. Phosphorus in the band maintains a higher availability even with a modest rate of P (50 to 100 lb P₂O₅ per acre) applied at the beginning of the growing season. This results from the small volume of the soil it contacts with, which invariably markedly increases the solution P concentration and the P saturation on the soil particle within an area adjacent to the band. The amount of the applied P removed by the crop in the year of application at the modest rates is usually less than 30% so that a substantial proportion of the P applied remains in the band following harvest.

Conventional tillage involving primary and secondary tillage in the spring for seed bed preparation, however, will mix the soil in the plow layer and destroy the band of residual P. Although a new band of P will be applied again at the time of seeding, a continuation of such a practice will gradually enrich the surface soil with P over time. In soils prone to surface erosion, the P-enriched soil particles may be transported to the surface waters and sediments, making them the largest depository of P. The resulting effect on eutrophication of aquatic systems and water quality is well recognized.

Maintaining a small P-enriched zone through repeated band applications of P fertilizer each year below the depth of tillage would minimize P enrichment of whole surface soil and the amount of P that may be transported to surface waters by erosion. Planting into the P-enriched zone each year should sustain strong seedling growth that otherwise would reduce yields or delay plant maturity, which is difficult to be accommodated particularly in the region with a relatively short growing season. A ridge tillage system would be conducive to maintaining a P-enriched zone. Also, the shallow tillage involved in the ridge building results in less disturbance of surface soil than in conventional tillage, thereby reducing the rate of soil organic matter oxidization and depletion. With these considerations, we propose to examine and validate the effectiveness of combining ridge tillage with pin-pointed band applications of P fertilizer, to sustain the crop productivity without causing P enrichment in the whole surface soil. A wide range of P rates will be used so that the P rate and P concentration in the P-enriched zone at which the maximum crop yield is attained can be determined. The crop rotation system to be used in this study will be corn-corn-bean. Bean is included in the study because it has a high P requirement, and it is planted in the third year after a moderate amount of P has been applied.