High yield and high-quality potato production in any cropping system depend on adequate soil levels of nutrients being available. Nitrogen and potassium are applied at higher levels than phosphorus (P), but no nutrient is more valuable for growth and development of a potato crop than P, especially if it is limited by either rate or its unavailability because of various soil conditions.
Maintaining an adequate P supply is critical for potato plant development, tuber growth and enhancing tuber maturity. Phosphate deficiencies can significantly reduce tuber yield and size. Therefore, fertilization practices must be customized for a grower’s individual field characteristics and local conditions to maintain adequate P availability throughout the growing season.
Concentrations of soluble P in soils of the potato-producing regions in the Pacific Northwest are usually very low and must be constantly replenished from soil P sources during the growing season. Daily potato P uptake requirements typically range from 0.6 to 1.6 pounds of P2O5 per acre per day during the tuber-bulking phase. If these plant requirements are not met, total yield and U.S. No. 1 yield could suffer and negatively impact production economics.
In Western alkaline soils, the primary factors used in determining P fertilizer recommendations are soil test P concentration, amount of excess lime (CaCO3), and a yield goal. Excess soil lime increases P retention (tie up) on CaCO3 surfaces and increases P precipitation as Ca-P minerals, which reduces overall P availability to the plant. Regional potato P fertilizer recommendations recognize this limitation and adjust for excess lime content.
To manage for these P inefficiencies, growers have incorporated the following management strategies: increased rates, fall application of P into prepared beds, planter bands of concentrated amounts of P fertilizer, and recently by adding the Avail polymer to their P fertilizers. This is a unique polymer combination of two long-chain organic acids with a negative charge of -1800, compared to typical Western soils that have a negative charge ranging from about -5 to -25.
Avail’s mode of action is to a) surround the P fertilizer in the soil solution, b) complex, within its structure, antagonist cations (Ca, Mg, Al and Fe) that would normally tie up applied P and make it unavailable for plant uptake, c) thus increasing P availability to the plant within the growing season.
There are many reports across varied crops, including potatoes, that indicate positive responses to the addition of Avail to P fertility programs. These responses have also been reported across a wide range of soils and growing conditions; however there are also references in which the benefits of Avail have not been observed. A part of this article’s objective is to help explain why these differences might occur within a potato cropping system and how to best manage for positive responses.
A total of nine irrigated field trials were conducted in southeastern Idaho between 2004 and 2008. All of the trials were conducted with the Russet Burbank potato cultivar and all were conducted on calcareous soils with pH values ranging from 7.8 to 8.3; soil test P ranged from 17 ppm to 35 ppm and excess lime contents ranging from 1.0 to 9.7 percent. (For additional details contact the author).
Five summary trials (2004 and 2005) were conducted in growers’ fields near the University of Idaho’s Research and Extension Center outside of Aberdeen, Idaho. Each of the trials included a check with no P and then various P fertilizer rates, forms and timings, as well as in combinations with or without the Avail polymer. The long-term nature of the study was used to better understand year-to-year variability as well as the most valuable treatment combinations. It is also important to point out that all other necessary nutrients were applied as called for to avoid any nutrient deficiencies that would impact yield or quality.
An objective of these studies was to determine if there were differences observed in P uptake with treatments. Although total P was not altered in many of the experiments there was a noticeable change in petiole P concentrations between P sources treated with AVAIL and where none was applied (Figure 1).
Higher potato petiole P concentrations begin to explain the value of using an enhanced efficiency P material that allows plants to more easily access P from fertilizer materials being applied. The thought is that if a grower can access more P into the plant, it would be utilized for potential yield and quality changes.
Both yield and quality improvements were observed with most of the field trials being evaluated where the Avail polymer was incorporated either with dry MAP or liquid P sources (11-37-0). Total yield and No. 1 yield was improved in seven of the nine site years where these comparisons were made. A portion of these field trials are represented in this article. The non-responsive sites were directly related to soil P concentrations that would be classed as high and where P fertilizer levels would be or could be reduced. However, where levels of soil test P are at low to medium levels, Avail is a better alternative than is applying P that is not treated with the Avail polymer.
In summary, Avail increased total and No. 1 yields for selected P rate/source/timing combinations for most of the field trials conducted from 2004 to 2005. These trials indicate the highest benefits for both total yield and No. 1 yield was associated with the use of Avail when soil test P was not excessive and when moderate rates of P are being applied. Trial 1 had the highest soil test P and was the least responsive. It is clear from the range of responses across soils, years and P rates that potatoes will and do respond positively to Avail. For high soil test P, a reduced level (15 to 20 percent) of P fertilizer could be considered when Avail is being used as part of the P fertility program.