Authors: Annie Klodd and Anne Sawyer, University of Minnesota Extension
- Minimize the amount of nitrogen applied in the fall; save it for the spring.
- Granular fertilizer is best applied as a broadcast directed to the vine rows
- If possible, avoid fertilizer application to the grassy aisles unless groundcover renovation is the intent
- Fertilizer application rates should be calculated based on soil and foliar tests. Use test reports from the current year or recent years.
During the growing season, grapevines allocate significant amounts of sugars and nutrients to the fruit, which is then removed from the vineyard when the fruit is harvested. If the soils are limited in nutrients like phosphorus (P), potassium (K), nitrogen (N) and magnesium (Mg), adding these nutrients back into the soil periodically is important to the continued productivity of the vineyard. Fertilizer amounts (rates) applied should be calculated based on soil and foliar test results, rather than applying an arbitrary rate.
Reasons to Consider Post-Harvest Fertilization
Other vineyard tasks are done for the year, and it is too early to begin dormant pruning. Therefore, growers likely have more spare time now than they do in the spring.
In terms of nutrients, grapes are in one of their most depleted states immediately following harvest, and in the early spring.
It is logistically easier to apply fertilizer in the fall when the ground is dry but not yet frozen, compared to the early spring when the vineyard may be impassable due to melting snow.
While applying fertilizer in the fall may not necessarily decrease the chance of vine winter injury, it will help the vines get off to a healthy start in the spring.
How to Apply Fall Fertilizers
As stated before, fertilizer rates should be determined based on soil tests, as well as foliar (petiole) tests if possible. Applying fertilizers without considering test data can cause growers to waste money and fuel on excess inputs, and can even cause soil nutrient toxicities, in which nutrient levels are so high that they negatively impact plant growth. Excess fertilizers are also tough on the environment.
The most effective and efficient method of vineyard fertilization is to apply it as a targeted broadcast directly underneath the vine rows. While many fertilizer spreaders broadcast the product over the whole area, it is generally unnecessary to apply fertilizer to the row middles (grassy strips) unless you specifically intend to improve the growth of the grass. Most grapevine roots grow within the vine rows (also called “herbicide strips”) rather than under the grass, so fertilizer applied to the grass has little impact on the grapevines.
Broadcast fertilizer spreaders vs. targeted broadcast spreaders
As mentioned above, most common fertilizer spreaders used in agriculture will broadcast the fertilizer in a certain radius behind the machine, which will of course apply the product to both the grass and vine rows. While this is not necessarily harmful, it is less efficient and more costly than using a targeted spreader (as explained above).
Specialized vineyard or orchard fertilizer spreaders can be purchased that are designed to direct the fertilizer under the vine rows.
General use broadcast spreaders can also be retrofitted to target vine rows, for those who do not wish to purchase a vineyard-specific spreader. This can be done by attaching a V-shaped bar on the back, or otherwise engineering a way to redirect the fertilizer at an angle so that it only hits the ground beneath the vines. One grower I spoke to recommended making and attaching a wooden V onto the back of a plastic spreader (such as those from Fleet Farm). He uses wood because he has found that frequent use can cause metal spreaders to rust. Of course, the methods of retrofitting a spreader will depend on the spreader you have and what tools are available to you.
Why not apply fall nitrogen?
For cold climate grapes, it is very important to eliminate or minimize nitrogen applications in the fall, especially while the leaves are still on the vines.
1) Nitrogen application in the fall can significantly increase the vines’ chances of severe winter injury.
After harvest, grapevines need to begin shutting down (senescing) in preparation for the winter. They stop growing, harden off green tissue, and move their energy and nutrients from the canopy down to the roots for winter storage. If nitrogen is applied in the fall, it encourages the vines to form new leaves, which is not a good thing. This interrupts the senescence process, which makes them unprepared for winter and therefore more vulnerable to winter injury.
2) Nitrogen applied in the fall may vanish before the spring
Nitrogen is highly mobile in the soil, meaning that it can be easily lost to the environment with water movement through the soil. Nitrogen can also be lost through volatilization (gaseous loss to the atmosphere). When water carries nitrogen down below the root zone (where the roots grow), the plant can no longer reach it and the N is lost to groundwater. This process is called “leaching.” If nitrogen is applied in the fall, it’s more likely to be lost to the environment than to be taken up by the plant. However, during the active growth season in the spring, the roots are actively growing and nutrients are in high demand by the plants. Therefore, fall-applied nitrogen is likely to be gone before the 2020 growing season starts.
What about other nutrients, like P and K?
Unlike N, P and K are immobile in the soil and are less likely to be lost to the environment with fall applications. Therefore, applying P and K in the fall will give vines a ready source of nutrients in the spring.
Some P and K fertilizers are available that do not contain N; read more at this link. However, many other fertilizers (like "N-P-K") and micronutrient sources contain some level of nitrogen. Therefore, it may be challenging to completely avoid fall nitrogen application if other nutrients are also being applied, particularly if using organic fertilizers. If this is the case, select a fertilizer product with very low N concentrations relative to the P and K concentrations, such as a 10-20-20 or 5-10-10 and wait until the leaves have fallen off the vines before applying it.
About the authors: Annie Klodd and Anne Sawyer are University of Minnesota Extension Educators for fruit and vegetable crops. Annie’s position focuses on assisting fruit farmers with topics involving fruit crop production, including grapes. Anne holds a PhD in soil science from UMN, and her position focuses on assisting farmers with soil health and farm food safety topics.