Potato Grower

26 POTATO GROWER | JUNE 2019 Managing fertilizer applications for improved quality The End Goal Diggin' In Diggin' In FERTILITY | By Mike Thornton & Nora Olsen Questions are often asked about how fertilizer applications affect overall tuber quality, especially as it relates to skin set, net development and bruise susceptibility. This is an area that has been the subject of a lot of research, starting almost a century ago. While there are numerous reports on the impact of both macro and micronutrients on tuber quality, most of the consistent results are associated with just four main nutrients—nitrogen, phosphorus, potassium and calcium. This article takes a more in-depth look at each of these nutrients and the role they play in quality. nItrogen Nitrogen fertilizer timing and amount can have big effects on bruise susceptibility due to the relatively large impact on maturity. Inadequate nitrogen can result in early crop senescence and an increase in susceptibility to blackspot bruise if tubers sit under dying or dead vines for a long period prior to harvest. In contrast, excessive nitrogen (especially late in the season) can delay crop maturity, resulting in increased susceptibility to skinning and shatter bruise. We believe that late nitrogen applications can also contribute to increased susceptibility to blackspot bruise in situations where vines remain lush and a lot of water is used toward the end of the season when the root system is starting to decline. This can increase the risk for dehydration of tubers prior to vine kill, resulting in greater blackspot bruise susceptibility. So how much is too much nitrogen, and how late in the season is too late to apply? It is hard to make sweeping generalizations due to all the differences in yield potential, crop rotations and varieties grown, but there are some studies that give a clear picture of the consequences of over-fertilizing with nitrogen. Jeff Stark of the University of Idaho led a study in the mid-1990s that looked at yield and quality on over 40 commercial Russet Burbank potato fields in southeastern Idaho, and related the results to fertilizer applications. They reported that both total yield and carton yield peaked at around 300 to 350 pounds per acre of total available nitrogen (pre-plant soil N + fertilizer N). Applying higher amounts of nitrogen did not increase yield, but reduced tuber size to the point that fewer of the tubers were big enough to be packed in cartons. They also found that applying nitrogen late in the season had negative impacts on net development (i.e., poor skin development). Fields that had petiole nitrate levels above the recommended level of 15,000 parts per million by the second week of August had much poorer netting than those with lower petiole levels. PhosPhorous Phosphorous tends to have the opposite effect on maturity and net development compared to nitrogen. Higher nitrogen rates and later applications of nitrogen require higher soil phosphorus concentrations to maximize net development. However, keep in mind that phosphorus is primarily taken up by the root tips during active growth. Once the root system stops growing and starts to decline, the uptake of phosphorus by the plant is greatly reduced. This means that application of phosphorus fertilizer after mid-tuber bulking is not very effective. PotassIum One of the earliest reports on blackspot bruise noted a direct relationship between the amount of potassium

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