Managing Potatoes for Minimal Nitrogen Loss

Problem:

Potatoes require high amounts of nitrogen during a short period of time. Many varieties, in addition, have limited root systems. High amounts of nitrogen need to be in the root zone and, in most cases, some of this nitrogen will be unused. Several areas of the San Luis Valley have a high water table and, where this is the case, there is the possibility that some of this nitrogen could get into the unconfined water aquifer. This would be a problem because it is difficult to remove from these areas. The EPA standard for safe drinking water is 10 ppm NO3-N. This is exceeded in some areas of the Valley, and cases of methemogloinemia (blue babies) have been the result. As residents of the Valley, we should be concerned with this situation. We should also try to avoid over-regulation by government agencies.

Proactive management to reduce any possible nitrogen contamination of the groundwater can be a solution to these problems. Many of the methods used to minimize nitrogen loss are the same methods that result in high yields and high quality crops. Over application of nitrogen can result in immature tubers, poor tuber storability, an increase in tuber bruising, low yields, poor tuber size, certain disease problems and poor color in red potatoes.

Management Guidelines

Crop Rotation

This can be one of the best ways to reduce excess nitrogen in the soil. After a potato crop, there is an average of 40 to 100 units/acre of nitrate-nitrogen in the first 2 feet of soil, depending on the previous potato variety grown and condition of this crop. A small grain crop, carrots, canola, and other deep rooted crops can use this excess nitrogen. There is an average of 2-10 units/acre of nitrate-nitrogen in the soil following a grain crop. Heavy winter moisture and over watering the grain crop early, before its root system develops, can push this nitrogen too low into the soil to be used.

Variety Selection

Potato varieties vary a lot in their nitrogen needs, the size of their root systems, and the ability of their root system to uptake nitrogen from the soil.

Differences in Potential for Nitrogen Loss per Variety

Fertilizer Recommendations

The use of soil testing, realistic yield goals, a proper crop nitrogen multiplier to obtain total nitrogen needs of the crop, and subtracting available nitrogen from this total need can result in very good nitrogen management of many crops. Potatoes are a high value crop, and are not very efficient at extracting soil nitrogen. As a result, all the nitrogen needed for high production by a potato crop must be applied during that season. In some varieties even more must be applied. Experience has shown that a potato crop does not respond to carryover nitrogen in the soil. This can occur even if the soil test result indicate very high soil nitrogen levels. One cannot depend on the potato crop to use this carryover nitrogen. An example of this is in potato on potato rotations where it is necessary to apply almost the same amount of nitrogen the succeeding potato years. An exception is large rooted varieties with lower relative nitrogen needs such as Russet Nuggets do effectively use this soil carryover nitrogen.

Fertilizer Application

Applying nitrogen close to the period of greatest demand can reduce the potential of losing nitrogen from the root zone. This nitrogen is not being stored in the soil where water can leach it. Banding fertilizer reduces the potential of losing nitrogen from the crop root zone. If nitrogen is banded in an area where excess water doesn't occur like in the hill. Broadcasting nitrogen before hilling concentrates the nitrogen in the hill instead of leaving it in the furrow where early season leaching occurs before the potato root system develops into the furrow, which may never effectively occur in some varieties. All these above practices in addition to being potentially good for the groundwater are also good practices for top potato production. Injecting nitrogen though the irrigation system can also apply the nitrogen according to the crop's needs which can reduce groundwater leaching as well. Although, early nitrogen fertigations can result in some nitrogen lost in the crop root zone as nitrogen applied with the irrigation water is applied to the furrows where with little early roots leaching can occur. In most cases most of the nitrogen a potato crop needs is applied early in the season and waiting too long to apply nitrogen can hurt potato yields so the potential to lose some nitrogen out of the root zone always exists. In season monitoring using tissue analysis should be used to adjust injected nitrogen.

Irrigation Management

Proper irrigation management can reduce the amount of excess water applied to a field that deep percolates below the crop root zone and can leach nitrogen. A small grain crop following potatoes can pick up some of this nitrogen below the potato root zone, but within the small grain root zone. During periods of potato foliage growth and early tuber bulking when the vines are still very healthy and green the potential to over water is low. Early season and as the potato crop matures it become easy to over water. Monitoring soil moisture in the root zone, adjusting irrigations for changing crop needs and weather, and adjusting for rainfall are important methods for good irrigation water management. Over watering a potato crop not only can lead to possible leaching nitrogen below the root zone, but can harm a potato crop in other ways like disease problems.

A Healthy Crop is Important to use all the Nitrogen Applied

The amount of nitrogen left over after a potato crop can vary by large amounts depending on how healthy the crop was and its ability to use applied nitrogen. There are many management practices to produce a high yielding potato crop such as: Good potato seed and planting, proper fertilization and crop uptake, good irrigation water management, good soil preparation, and good disease, weed, and pest management. People over apply nitrogen to try to correct other agronomic problems such as hail, frost, vertilcilium and poor disease control and in many cases this does not work. This excess nitrogen can be lost to the root zone.

Note: This information should only be used as a guide. Adjustments for local conditions must always be made.