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Issue No. 9, Article 6/May 23, 2008

Nitrogen and Phosphorus for Wheat

This spring continues to be unusually wet and cool. These conditions have caused concern, and rightfully so, for wheat producers. In the last week there have been many reports of stunted growth and entire wheat plants showing pale or yellow leaves. What is causing these symptoms? In hope of finding an answer, many farmers are sending samples to be tested for disease. When the results are negative, the next obvious step is to investigate tissue nutrient content, with nitrogen (N) and phosphorus (P) being the nutrients of interest, for two main reasons: wheat is very sensitive to N and P deficiency, and stunted growth is a typical symptom of P deficiency and yellow chlorotic leaves are typical of N deficiency.

Although plants are stunted, the suspicion of P deficiency can be dispelled because P deficiencies do not fit the other symptoms being reported. Deficient wheat will tend to show very dark green leaves, and purple tissue will start to develop in the tips of lower leaves, progressing along the leaf margins until the entire leaf blade shows purpling. Nitrogen deficiency, on the other hand, is more difficult to diagnose by visual examination under the weather conditions that have prevailed for most of the spring. However, some N deficiency symptoms should be possible to detect visually. Typically, unless wheat is under severe N deficiency, newer leaves remain green while older leaves tend to be lighter in color and may die. This is because under deficient supply, N is remobilized within the plant to newer tissues. Still, if it is not clear by visual examination whether the symptoms indicate N deficiency, there are two important points to consider before deciding to apply expensive nitrogen.

First, how was the crop fertilized? Typically, a small portion of the total N application (depending on the N-supplying power of the soil between 15 and 30 lb N/acre) is done prior to seeding in the fall along with a full application of P. In the spring, or late winter, the remaining N is top-dressed. Top-dressing is generally most effective when done as close as possible to the time that regrowth begins. Some people prefer to split N applications in the spring. In most years splitting the application does not result in increased seed yield. In wet springs, such as this year, there is greater chance for the split method to be more effective than a single application. However, if most of the N was applied this year at appropriate rates, there is little reason to suspect that the severe yellow symptoms observed in the field are related to N deficiency, the reason being that poor weather conditions are creating a very stressful environment for wheat production this year.

This is the second point I would like to discuss. This spring has been unusually wet and cool. Some of the most important factors influencing nutrient availability and plant growth (including root growth that is important for nutrient uptake) are temperature, water availability, and soil oxygen levels. With the exception of water availability, all other factors have been marginal for wheat to grow and take up nutrients present in the soil. Low light intensity caused by substantial cloud cover has exacerbated the problem by limiting photosynthetic activity necessary to generate the required energy for growth and nutrient uptake. I suspect that most of the yellowing we see now in wheat fields will disappear if the soils dry a little and more sunlight starts to shine. If weather conditions improve quickly and some N remains to be applied, it can be supplied up to heading time. However, it is critical to realize that the response to N declines rapidly after the start of the flag leaf stage, and some yield potential has likely been lost at this point. Rates thus should be adjusted accordingly. If N was already applied at rates close to the recommended maximum, it is very unlikely that a yield response to additional N would occur.--Fabián G. Fernández

Fabián Fernández

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