By: Annie Klodd, University of Minnesota Extension
Throughout the growing season, we see our grapevines grow and change immensely. Berries turn from green to red, and shoots grow from inches to feet in a matter of weeks.
What we do not see is all the behind-the-scenes work – the vines are transporting numerous nutrients from the soil to the canopy, and then moving them from the leaves to the fruit during ripening. Then we harvest that fruit, removing a portion of the vines’ nutrients. Grapevines are in their most depleted state in the fall and early spring.
To keep the vines productive over their lifespan, we do tests to see if the soil needs more nutrients to replace those lost. Based on those tests, we may add critical amendments to the soil. On the flipside, if the soil is already rich in key nutrients, soil tests save money and the environment by telling us when fertilizer is not needed.
Some growers wish to use the fall as a time for applying nutrients. Harvest is over, but it is too early to prune. It seems like a good opportunity to check something off the to-do list. Before you place your fertilizer order, you show know: What nutrients your vineyard needs, how much is needed, and whether those nutrients are best applied in the fall or the spring.
Reasons to Consider Fertilizing After Harvest
First, convenience. Other vineyard tasks are done for the year. It is too early to begin dormant pruning. Growers usually have more spare time now than they do in the spring. That is, if they are not tired of being in the vineyard.
Secondly, grapevine biology. From the grapevine’s point of view, it is in one of its most nutrient-depleted states immediately following harvest, and in the early spring. This is because much of the nutrients it has accumulated have been used up to produce fruit, and that fruit has just been removed from the system.
Third, logistics. In temperate climates like my area in Minnesota, the soil is wet and spongy in the spring and dry and firm in the fall. 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 melting snow may make the vineyard impassable.
Applying certain fertilizers in the fall can give the vines a healthy start in the spring. However, one nutrient in particular is best applied in the spring – nitrogen, due to its tendency to leach out of the system. Read on for suggestions on when to apply nitrogen.
How to Apply Fall Fertilizers
First, do a fall soil test, especially if it has been over 5 years since your last one. Calculate your fertilizer rates and the type of fertilizer based on soil test and foliar test reports. Foliar tests need to be taken at bloom or veraison, but soil tests can be taken in the fall.
I cannot understate the importance of soil and foliar nutrient testing. These tests are the best way to understand what the soil is lacking, what it has plenty of, and how well the vine is taking up each nutrient. If nutrient testing seems intimidating, just contact you state university soil testing lab or a private lab – they will tell you how to proceed. It’s easy!
Test your soil during or shortly after the harvest season. Give yourself 2-3 weeks between when the sample is submitted and the likely first hard frost, in order to receive the results and make an appropriate fertilizer application before the ground is covered in snow.
After receiving your test report, enlist the help of an Extension Educator or trusted consultant to decide what nutrients are needed and at what rates. Key nutrients include phosphorus, potassium, calcium, magnesium, boron, and iron, amoung others. Nitrogen is also very important, but is usually not included in soil tests. Your test report will also include soil pH, organic matter percentage, and possibly your soil’s cation exchange capacity.
Once you know what to apply and how much, you are ready to go. The most common method of granular fertilizer application is broadcast application using a targeted vineyard spreader. This type of spreader applies it under the rows, avoiding the grass aisles, maximizing efficiency and minimizing cost. There is generally no need to fertilize the grass, as most vine roots are located in the rows.
Some growers prefer to incorporate their fertilizer through cultivation or banding it about three inches deep using tillage equipment. The advantage is better incorporating nutrients that are not very mobile in the soil, like potassium and phosphorus. The challenge is that it can cut some of the grapevine roots and requires more niche equipment.
Acquiring a Vineyard Fertilizer Spreader
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. Applying fertilizer only to the rows will dramatically decrease the amount of product needed.
Specialized vineyard or orchard fertilizer spreaders are available commercially but may be cost-prohibited for smaller vineyards. One work-around for this would be to hire a custom fertilizer applicator.
A second solution would be to make your own. A general-use broadcast fertilizer spreader can also be retrofitted to target vine rows. This can be done by attaching a V-shaped bar on the back of the spreader where the fertilizer is ejected, or otherwise engineering a way to redirect the fertilizer at an angle so that it only hits the ground beneath the vines. In Minnesota, some growers build and attach a wooden “V” onto the back of a plastic spreader. Wood and metal can both be used for this purpose. 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?
When it comes to nitrogen applications, it is best to wait until spring. For cold climate grapes, which are my specialty, it is very important to eliminate or minimize nitrogen applications in the fall. Avoiding late-summer or fall nitrogen application is especially critical while the vines are still actively growing.
The first reason is that nitrogen application in the fall can significantly increase the vine’s chances of severe winter injury.
After harvest, grapevines need to begin 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 shoot growth late in the season, which is not a good thing. This interrupts the senescence process and makes the vines less prepared for winter and therefore more vulnerable to winter injury.
Secondly, 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 growing zone, the plant can no longer reach it and the nitrogen is lost to groundwater. This process is called “leaching.”
If nitrogen is applied in the fall, it is more likely to be lost to the environment than to be taken up by the plant. This is because the roots are not actively absorbing nutrients. However, during the active growth season in the spring, the roots are actively growing and nutrients are in high demand by the plants. Fall-applied nitrogen is likely to be gone before the next growing season starts.
What about other nutrients, like phosphorus and potassium?
Phosphorus and potassium, two key nutrients for grapevines, are less mobile in the soil and are less likely to be lost by the spring if applied in the fall. Applying these key nutrients in the fall will give vines a ready source of nutrients in the spring.
Many common “all-purpose” 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 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. Some P and K fertilizers are available that do not contain nitrogen. Consult with your fertilizer supplier about specific product options based on your soil test results.
Most of the vineyards I work with have high levels of phosphorus and potassium and do not need to add more. A recent review of Minnesota soil test reports from University of Minnesota Extension showed that many of our cultivated soils have excessive levels of potassium. Excess potassium threatens local waterways, as it can run off from agricultural fields and residential properties. Always consult your soil test results before adding nutrients that your soil may not need.
Here are some key tips for fertilizing in the fall:
1. Minimize the amount of nitrogen applied in the fall; save it for the spring.
2. Granular fertilizer is best applied as a broadcast directed to the vine rows.
3. If possible, avoid fertilizer application to the grassy aisles.
4. Calculate fertilizer needs based on soil and foliar tests. Only apply nutrients if needed.
Nitrogen Fertilization in the Vineyard. Dr. Joe Fiola. University of Maryland Extension, 2021.
Nutrient Management for Fruit and Vegetable Crop Production. Dr. Carl Rosen. University of Minnesota Extension, 2005.