No. 10 — May 27, 2019

Light Green Grapevine Leaves and Saturated Vineyard Soils

Saturated soil conditions from persistent rainfall events has some grape growers wondering if all the applied nitrogen has been washed away or leached away. There is the visual observation that light green leaves are the result of low amounts of available nitrogen. Some soil applied nitrogen was likely lost over the past few weeks of rainy weather conditions. Another factor that is likely limiting nutrient uptake are saturated soils. Saturated soils do not have air spaces that allows roots to respire. There is a lack of oxygen in the soil that results in changes of nutrient availability, microbial activity, reduced plant respiration and the buildup of carbon dioxide and ethylene in the root and soil interface. Low oxygen reduces the amount energy available from the breakdown of sugar molecules. This in turn results in a reduction of plant growth. Saturated soils also result in denitrification or the loss nitrate (NO3-) through a process that results in the formation of molecular nitrogen (N2). The denitrification process is carried out by anaerobic bacteria in the soil. These bacteria can perform the denitrification process when oxygen concentrations are very low.

Grapevines nitrogen needs should be based on the vigor of the vines and recalling the nitrogen needs of the grapevines. During the period of bud break to bloom the grapevines are using stored nitrogen in the permanent wood (trunks, cordons and spurs) and roots. From bloom to veraison the grapevines depend on nitrogen uptake from the roots. During the period of bloom to veraison the demand for nitrogen is the greatest. If you applied all your nitrogen already this season and your grapevines have experienced substantial rainfall then I strongly suggest that you take petiole samples at bloom. This would allow you to use the results to determine if nitrogen is needed. Although some soil applied nitrogen may have been lost through runoff and leaching there will be nitrogen available through mineralization.

Mineralization is the release of organic carbon from organic matter into a nitrogen form available to plants. The more organic matter the soil contains the more nitrogen will be available to the grapevines. Soil organic matter of 1% will contribute approximately 20 to 30 lbs of nitrogen/acre/year to the grapevines. Soils with 2% organic matter will contribute 40 to 60 lbs of nitrogen/acre/year to the grapevines. The rate of mineralization is dependent on soil moisture and temperature. With increasing soil temperatures and moist soil conditions the mineralization process will be taking place. However, the release of nitrogen from organic matter is slower in wet saturated soils compared to well aerated soils.

Foliar nitrogen applications can be applied to correct a deficiency. However, there are a couple of precautions. Do not apply foliar nitrogen at bloom as this will likely reduce fruit set. Do not mix foliar nitrogen with some pesticides and do not apply when air temperatures are above 90°F with high humidity as this can result in phytotoxicity. I strongly encourage foliar nitrogen treatments to be applied alone and not tank mixed with pesticides.

The best time to apply nitrogen to the soil is at bloom and six weeks after bloom. This timing corresponds to when the grapevines demand for nitrogen is the greatest. Early season nitrogen applications prior to bud break are more prone to leaching. Also grapevine root uptake of nitrogen in the early season is minimal since grapevines are using stored nitrogen in the roots, trunk, cordon, and spurs. In regards to nitrogen and leaching it would be good to remember that the month of May in Missouri typically receives the greatest rainfall. On average, 5 to 6 inches of rainfall occurs in May. Therefore, you may want to consider split nitrogen applications. Apply half the needed nitrogen around bloom and half the needed nitrogen 4 to 6 weeks after bloom, especially if long range weather forecasts predict sustained rainfall events.

There are many forms of nitrogen fertilizers available for soil application. Some of these nitrogen fertilizers will increase soil pH whereas others will decrease soil pH (Table 1). In addition, the type of nitrogen fertilizer determines how readily available the nitrogen is to the grapevines. Urea fertilizer is not readily available for plant uptake. Soil microbes convert urea into nitrogen forms for root absorption. In comparison calcium nitrate fertilizer applied to moist soil disassociates quickly providing nitrate which can be absorbed by the roots.

Urea (46-0-0) is the most cost effective soil applied nitrogen fertilizer. If you are considering applying urea fertilizer to your grapevines, time the application just prior to a rainfall event. Rainfall will dissolve the fertilizer and distribute the fertilizer into the soil and reduce the amount of nitrogen loss due to volatilization.

Be prepared for extensive weed growth if soil applied nitrogen fertilizer is applied. Weeds and inter-row grass will respond with rapid growth to applied nitrogen fertilizers. Be ready to control weed growth after a nitrogen fertilizer application, especially with moist soil conditions persisting.

Table 1. Three commonly available nitrogen fertilizers and their affect on soil pH and nitrogen availability to grapevines.

Fertilizer Nitrogen content Effect on soil pH Availability to grapevines
Ammonium sulfate 21-0-0 Acidifying/decrease Readily available
Calcium nitrate 15.5-0-0 Alkaline/increase Readily available
Urea 46-0-0 Acidifying/decrease Microbial breakdown needed before nitrogen available

In summary, if grapevine leaves are light green in color use the following tools to determine nitrogen needs.

  • Take petiole samples at bloom to determine nitrogen needs
  • Use you past experience to gauge grapevine vigor prior to nitrogen application.
  • Nitrogen fertilizer applied prior to bud-break has likely leached from the root zone and or washed away through surface water runoff.
    • Apply soil applied nitrogen at bloom when the grapevine roots are actively absorbing nitrogen.
  • If nitrogen is needed, select a nitrogen fertilizer that fulfills the needs of the grapevine and the needs of the vineyard soil. Ammonium sulfate will add sulfur to the soil and calcium nitrate will add calcium to the soil.
  • If rain events persist, be ready to control weeds after a nitrogen fertilizer application.

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