Fertilization Guide for Walnut Orchards

Effective fertilization plays a key role in the health, growth, and productivity of walnut orchards. This guide focuses on the three most important nutrients for walnut trees: Nitrogen (N), Phosphorus (P), and Potassium (K). We explore the specific nutritional needs of walnut trees, covering fertilization strategies for both young and mature trees, optimal application methods, and techniques for monitoring nutrient levels. By understanding and implementing these practices, growers can enhance nut yields, maintain tree health, and support the long-term success of their walnut orchards. Whether you're an experienced orchardist or new to walnut cultivation, this comprehensive guide offers valuable insights to improve your fertilization practices.

The Role of Nitrogen in Plant Growth and Development

Nitrogen is the most important nutrient in walnut trees. Trees need nitrogen to grow strong and produce walnuts. Let's explore why nitrogen is so important and how growers can use it wisely.

Imagine nitrogen as a versatile building block that plants use for almost everything. About 80% of the nitrogen a plant takes in goes into making proteins. These proteins help build plant structures, create energy, and even defend against pests and diseases.

Nitrogen is also crucial for other important jobs in plants. It helps create DNA, which is like a plant's instruction manual. It's part of chlorophyll, the green pigment that allows plants to turn sunlight into walnuts. Nitrogen even helps plants make hormones, which are like chemical messengers that tell different parts of the plant what to do.

When Plants Don't Get Enough (or Get Too Much) Nitrogen

The process of nitrogen uptake requires energy from the tree, and trees primarily take up nitrogen when they need it for growth and nut production. Nitrogen can come from various sources, including synthetic fertilizers, organic materials such as cover crops, compost, and manure, as well as nitrate present in groundwater.

Plants need just the right amount of nitrogen. Too little, and they can't grow properly. Their leaves might turn yellow, and they won't produce as much walnuts. But too much nitrogen can also cause problems.

When plants get too much nitrogen, they can grow too fast. This might sound good, but trees with too much nitrogen might grow lots of leaves but not enough fruit or nuts. They might also have trouble preparing for winter. High nitrogen levels can delay leaf drop in autumn, interfering with the plant's natural dormancy cycle. This can increase the plant's vulnerability to cold weather and more susceptible to frost damage, potentially leading to significant crop losses.

The Four R's of Nitrogen Management

To optimize nitrogen use efficiency, you should adhere to the "Four R's" of nitrogen management:

1. Right Rate: Apply nitrogen at a rate that matches the tree's demand.
2. Right Time: Time nitrogen applications to coincide with the tree's nitrogen demand.
3. Right Place: Apply nitrogen where the majority of the tree's roots are located.
4. Right Source: Select the appropriate form of nitrogen based on your specific orchard conditions and management goals.

Nitrogen Requirements in Young Walnut Trees

In the early years of the orchard, young trees need sufficient nutrients to grow their trunk, roots, branches, leaves. However, using too much Nitrogen can also cause problems.

In extreme cases, applying far too much can damage or kill the tree's delicate new growth. Excess fertilizer leads to overly rapid growth, resulting in trees that are too tall and thin. Such growth patterns can reduce the amount of walnuts it can support as the tree matures. So you need to make sure that you are applying the right amount of fertilizers.

In most cases, the young trees would still grow fine even if you don't give them any extra nitrogen in the first couple of years. Because nitrogen is already available in irrigation water, if you are using groundwater. When you irrigate the trees you will be giving them some amount of nitrogen.

Based on the latest research, we recommend the following application rates:

1. First year: 100 grams of nitrogen per tree over the course of the season
2. Second and third year: 200 grams of nitrogen per tree over the course of the season

Other than Nitrogen, usually young trees don't need any other fertilization or nutrients such as Phosphorus or Potassium.

Young orchards require more frequent applications of nitrogen to support rapid growth without risking nutrient leaching. For best results, follow a "spoon-feeding" approach. Apply nitrogen in smaller, more frequent doses throughout the growing season. This way, you can ensure that the trees are taking and using nitrogen in the most effective way, and you can reduce the risk of nutrient loss through leaching.

Nitrogen Requirements in Mature Walnut Trees

As walnut trees mature, their nitrogen uptake patterns change. Older trees tend to be less aggressive in their nitrogen absorption and utilization. This shift reflects the changing priorities of the tree from rapid growth to maintenance and reproduction. Understanding these age-related differences in nitrogen uptake has important implications for fertilization strategies:

1. Mature orchards might benefit from more targeted, moderate nitrogen applications timed to support key physiological processes like bud break (when tree wakes up in the spring), leaves growing to their full size, and nut fill (when the kernel of the walnut starts to grow and fill up the space inside the walnut shell)
2. Fertilization plans should be adaptive, changing as the orchard ages to reflect the evolving nutritional needs of the trees.

Starting from the fourth year, apply 400 grams of nitrogen per tree, plus 13 kilograms per ton of harvested walnuts. This is the total amount you should apply throughout the season. You can refer to the table below.

Remember, for every ton of nuts harvested with shells, about 13 kg of nitrogen leaves the orchard. At the very least, growers need to put this amount back into the soil to keep growing healthy crops.

Example Calculation of Nitrogen Needs

To give an example, let's calculate how much Nitrogen you need to apply to compensate for the loss of nitrogen due to harvest:

1. Estimate your expected yield (e.g., 3 tons per hectare)
2. Calculate total nitrogen removal: 3 tons × 13 kg N/ ton = 39 kg N/hectare
3. Account for nitrogen use efficiency (e.g., 70%): 39 kg ÷ 0.70 = 55 kg N/hectare
4. Divide the total by four to determine monthly applications: 55 kg ÷ 4 = 13.75 kg N/hectare/month

The Best Time to Apply Nitrogen Fertilizers

Walnut trees need the most nitrogen between middle of May through August. This is the time from when new shoots start growing until the nuts are almost fully developed. An even distribution of nitrogen applications across these four months helps maintain a steady supply of nitrogen, matching the tree's needs.

You should avoid early spring Nitrogen applications, particularly in April. While roots may be growing during this month, the nitrogen used by trees during this time comes entirely from storage.

To avoid damage from freezing, we also recommend not to apply fertilizer after August. When the tree continues growing late in the season those parts could be vulnerable to cold weather. Additionally, any nitrogen not absorbed by the leaves before they fall could be washed away during the winter.

When more nitrogen is present in the soil than plants can absorb, and too much water from irrigation or rain carries it deeper into the soil. This causes the nitrogen to go beyond the root zone of plants, and the plant roots cannot reach the nutrients anymore. This is called nitrogen leaching.

Many areas in Europe experience significant rainfall in April, increasing the risk of nitrogen leaching. Similarly, it's important to avoid late-season applications, particularly in September. By this time, the risk of nitrogen leaching increases, and there's less new root growth, reducing the tree's capacity to take up nitrogen efficiently.

You can apply nitrogen fertilizer either as a dry product or mixed with irrigation water, but be sure not to apply it close to the tree trunk.

Leaf Sampling to Evaluate Nitrogen Levels

If you would like to follow a more scientific approach to understand if your trees are getting the right amount of Nitrogen, you can do leaf sampling every year. You take samples from your tree leaves in July and send it to a laboratory for analysis. Based on the results you can take action to get the nitrogen content of the leaves to the optimal range, which is between 2.3% to 2.7%.

When leaf nitrogen levels fall below 2.3%, trees may be entering a deficient state. This deficiency can lead to reduced yields and overall tree vigor. Conversely, levels above 2.7% may indicate excessive nitrogen application, which can lead to unnecessary costs and potential environmental issues due to nitrogen leaching or runoff. It's important to note that visible symptoms of nitrogen deficiency, such as yellowing leaves, only appear after the deficiency has already begun to impact yields. Therefore, regular leaf sampling allows growers to detect and correct nutritional imbalances before they manifest as visible symptoms or yield reductions.

Phosphorus Requirements (P) in Walnut Orchards

Phosphorus is another nutrient that walnut trees use. However, it is not as important as Nitrogen. Usually soils have sufficient Phosphorus in them to support walnut trees. However, trees lose some Phosphorus with every harvest and it is beneficial to add it back to the orchard.

Once you start harvesting considerable amounts of walnuts from your trees (usually after the 4th year) you can start applying Phosphorus. As a general rule of thumb, orchard loses 4 kg of Phosphorus for every 1 ton of harvest. You can add it back to the orchard by applying this amount of Phosphorus. If trees show symptoms of P deficiency, you can consider applying a higher amount.

The ideal time to add phosphorus fertilizer to soil is during the plant's inactive season, typically fall or winter. You can start phosphorus applications in late October to early November.

Potassium Requirements (K) in Walnut Orchards

Potassium (K) is essential for walnut tree health, fruit development, and disease resistance. Effective management of this nutrient is important for maintaining high yields and quality in walnut orchards.

Walnut harvesting removes significant amounts of potassium from the orchard system. For every ton of in-shell walnuts harvested, about 5.5 kg of Potassium is removed from the orchard. However, the hulls contain even more potassium. If these hulls are not returned to the field, an additional 15.9-18.1 kg of K per tonne of nuts is lost.

The way hulls are managed greatly impacts the orchard's potassium balance. Returning hulls to the field recycles potassium back into the soil, reducing the need for additional fertilization. Conversely, removing hulls results in greater potassium loss and necessitates higher fertilizer applications.

To sustain optimal potassium levels, you should replace at least the amount removed during harvest. Also, you should conduct regular soil tests and annual leaf analyses to monitor potassium levels and adjust your fertilization strategies.

Application Rates

Application rates vary based on soil type and the trees' potassium status. K₂O is a form of potassium fertilizer. For potassium-deficient trees on sandy soils, applying 250 kg K₂O/hectare annually is recommended. On heavier clay or silt loam soils, up to 1000 kg K₂O/hectare may be required to correct severe deficiencies. Once deficiency is corrected, annual applications of 250 kg K₂O/hectare typically maintain adequate levels.

Response Time

Trees usually don't show visible responses to potassium fertilization until the second season after application. This delayed response emphasizes the importance of consistent, proactive potassium management.