Managing nitrogen in irrigated potato production requires a lot of information. Petiole samples tested each week help determine the current nitrogen status of the crop. Early in the season the petiole nitrate levels are expected to be high (20,000-30,000 ppm) and the soil nitrogen level (0-18″) is usually higher than 80 lb/a. As the potato crop goes through rapid vegetative growth and tuber bulking begins, petiole levels are expected to drop, but sometimes they can drop very quickly. This is especially true when temperatures warm up quickly after a period of cool weather. This causes explosive growth of the vines resulting in a dilution of the nitrate in the potato petiole. This explosive growth can cause the petiole test level to drop to very low levels (5000 ppm or lower). Even with these low levels, the canopy can look very lush and green. Growers get concerned when this happens and wonder if they should apply more nitrogen to the potato crop based on the low petiole test level. At this point it is important to have a soil nitrogen test as backup information (0-18″ sample from the hill). Soil samples should be collected from the same areas the petiole samples were collected and tested for nitrate and ammonium nitrogen. If the total soil “N” level (ammonium plus nitrate) is higher than 60 lb/a mid-season and 40 lb/a late-season, there is no need to apply additional nitrogen. If the grower ignores the fact that the soil nitrogen level is adequate and applies additional nitrogen, the result is usually delayed maturity and increased problems in storage. If the soil total N level is low, an application of N may be required depending on how late in the season this happens.
This past season AGVISE staff sampled an irrigated potato field each week to follow the petiole nitrate and soil nitrogen level from start to finish. Nitrogen was applied through sprinkler irrigation several times. The figures above show the petiole N levels and soil N levels through the season. There were two times during the season when the soil nitrogen (nitrate plus ammonium) fell below the adequate range (see figure). Nitrogen applied through the irrigation system increased the soil N level first and later you can see how the petiole nitrate level increased. Even with irrigation, it takes a few days for the fertilizer “N” to be converted to nitrate and taken up by the potato plants.
It is important to remember that large unexpected rainfall events can cause N to be lost from the soil during the season from these coarse textured soils. Petiole nitrate testing is a good tool to tell you the recent “N” status of the plant, but a soil test may also be necessary as a backup to make sure that there is enough “N” in the soil to take care of the plants needs going forward. Irrigated potato production is a very expensive proposition and we need to use all of the tools at our disposal to stay on top of nitrogen management.