Features Fertilizer Seed & Chemical
Improving nitrogen management

For petiole analysis: first choose last fully expanded leaf.

Good nitrogen management has never been more important. Rising energy costs are driving up fertilizer costs, and environmental issues are receiving more publicity than ever. This makes fine-tuning fertilizer nitrogen applications an important business decision for potato growers.

The challenge growers face is that the right amount of fertilizer nitrogen to apply can be quite different in different fields or in different years. That is because not all nitrogen used by the crop comes from fertilizer. The soil also provides a lot of nitrogen to the crop. However, the amount of nitrogen supplied by the soil varies depending on the crop rotation, manure management practices, soil properties and even weather conditions.

To help growers meet this challenge, government and university researchers, provincial crop specialists and industry agronomists from New Brunswick, Prince Edward Island, Nova Scotia and Maine reviewed the latest research findings. The result was new fertilizer nitrogen recommendations for potato production in Atlantic Canada. Also new are guidelines for petiole nitrate testing.

Remove leaflets from petiole.

The new recommendations reduce the guess work when choosing fertilizer N rates by using a nitrogen credit system. Credits are used to account for nitrogen supplied to the potato crop by a previous legume crop, by manure application, and from soil organic matter. The recommended nitrogen rate for a specific potato variety is adjusted using average nitrogen credit values to determine the required fertilizer application rate for a particular field.

For a legume crop, the nitrogen credit depends on the legume crop grown and whether it was a pure or mixed legume stand.

Credits are given for manure applied in the spring before planting and in the previous fall. The manure credit depends on the time of manure application (fall versus spring), the type of manure (solid versus liquid), the time between manure application and incorporation, and the soil conditions at the time of manure application (bare soil, crop residues, standing crop). A manure analysis is required to calculate the manure nitrogen credit.

Fertilizer nitrogen rates are adjusted downward on soils with a high soil organic matter because these soils provide more nitrogen to the crop.

These new recommendations will help growers apply the right amount of fertilizer to meet the nitrogen requirement of their potato crop. This will help growers meet their tuber yield and size targets while avoiding issues related to excess nitrogen application, such as low tuber specific gravity or difficulty in top killing vines. Good nitrogen management is also an effective way for growers to reduce the risk of nitrate leaching and greenhouse gas emissions from their farms.

Strip leaflets off petiole.

Some growers may be worried about cutting back on their fertilizer rates. One way to be sure the crop is getting enough nitrogen is to use petiole nitrate testing. This is where the new petiole nitrate testing guidelines come in.

By testing your petiole nitrate level every seven to 10 days, you can track the performance of your crop against standard curves that have been developed. If your petiole nitrate test value gets too low, additional nitrogen can be added as urea mixed in to fungicide spray applications.

Some care is required in interpreting petiole nitrate results. Test results will drop if the crop is short of water. Test results drop under these conditions because the nitrate cannot travel to the roots when the soil is dry. Application of foliar nitrogen to a potato crop under drought conditions will not help the crop survive the drought, and may result in an excessive nitrogen supply later in the year when more rainfall occurs.

Figure 1. Examples of petiole nitrate concentration results for a Russet Burbank crop in Atlantic Canada with excessive, optimal and deficient nitrogen fertility.

There is another way to use petiole nitrate testing. It can be used as a report card on nitrogen management in each field. A high petiole nitrate test late in the growing season just before top-kill means that the supply of nitrogen from the soil and from fertilizer was more than what the crop needed. If the petiole nitrate test is consistently high, growers can consider reducing fertilizer nitrogen inputs in future years.

While these new recommendations are an improvement over previous recommendations, they are not the final answer. Research continues on ways to better predict the supply of nitrogen from the soil. A new test for soil nitrogen supply is currently being examined to see if it can do a better job of predicting fertilizer nitrogen requirement than the nitrogen credit system. This would be the first test of its kind in Canada.

Fertility is an important factor in crop development and yield results. The new fertility recommendations and the petiole test help growers fine-tune their fertility programs to meet the needs of the crop and, hopefully, improve the bottom line. -end-

*Bernie Zebarth is a research scientist with Agriculture and Agri-Food Canada in Fredericton, New Brunswick.