In nature, trees provide themselves with nutrients and thus maintain the nutrient cycle. In the garden, the prerequisites for this are usually not given, be it because the soil is too poor or there is competition from other trees. In the case of deciduous trees, a distinction is made between ornamental and fruit trees.
Fertilizing Deciduous Trees – The Basics
Deciduous trees can take on different functions in the garden, whether as shade providers, privacy screens, ornamental, or fruit trees. In addition to the main nutrients nitrogen, phosphorus, and potassium, all woody plants also need a sufficient amount of minerals and trace elements. Factors that influence the respective nutrient requirement are the tree species, location, nature of the soil and the size of the tree, light conditions, and weather. Trees with a greater depth of rooting can absorb significantly more nutrients. In nature, the soil is supplied with important nutrients through weathering processes. This creates a natural nutrient cycle. Here you can learn more about Tree Fertilization.
This could also work in the garden, but this process is often disturbed here. This is due, among other things, to the fact that in many cases falling leaves are regularly removed so that no rotting can take place on the spot and important nutrients are lost. These then have to be balanced out with an appropriate tree fertilizer.
Types of Tree Fertilizers
A basic distinction is made between two different fertilizers, organic and mineral fertilizers:
Organic fertilizer is created by the decomposition of naturally occurring organic substances. Plant nutrients, which are initially withdrawn from the soil for the plants to grow, are returned to it after they die, the natural cycle continues to close.
- Compost, manure, and plant manure are very good organic fertilizers
- Rock meal, horn shavings, and horn meal are also well suited
- Compost contains all the important nutrients and trace elements
- Rock flour consists of ground rock flour
- Horn shavings and horn meal are waste products from animal production
- Effect of organic fertilizers only sets in after the substances have decomposed
- It works more slowly, but over a longer period
- Over-fertilization is less common
Despite the positive properties of organic tree fertilizers, there are quite a few among amateur gardeners.
Followers of mineral fertilizers. The nutrients it contains are available to plants more quickly because they are more easily and quickly soluble. Results are visible after a short time. But the dosage is often not optimal. Over-fertilization occurs, which has a lasting impact on the environment because the soluble components are also flushed into the groundwater.
The exclusive use of mineral fertilizers, also known as artificial fertilizers, can weaken important soil organisms so that the soil is no longer adequately ventilated. The consequences are erosion damage and soil compaction. As tree fertilizer, especially in fruit trees, residues can accumulate in the ripe fruit and thus get into the food chain. In addition, an excess of nitrogen can reduce the yield.
Damage from over-fertilization can also be observed in other deciduous trees. The foliage can turn yellow, it burns. In addition, affected plants are generally much more susceptible to frost damage, pests, and diseases. Despite everything, mineral fertilizers can also provide deciduous trees with all the important nutrients, as long as the dosage is correct. Frequently used fertilizers are, for example, blue grain, lime, calcium ammonium nitrate, or so-called NPK fertilizers.
The Function of Individual Ingredients
Only if trees are adequately supplied with all the necessary nutrients can many new shoots, flowers, or fruits develop. The most important elements in both organic and mineral fertilizers are nitrogen, phosphorus, and potassium. In addition, minerals such as sulfur, calcium, and magnesium and trace elements such as iron, manganese, copper, and zinc are required. Each of these elements fulfills a specific function for the tree’s metabolism. They are in direct interaction with each other and should always be present in the soil in a balanced relationship.
- Phosphorus fertilizers preferably suitable for fruit trees that both bloom and bear fruit
- Phosphorus promotes the development of flowers, fruits, and seeds
- Supports the formation of healthy and strong roots
- Required by flowering and fruit-bearing deciduous trees
- Too high a nitrogen content in the soil can hinder the absorption of phosphorus
- Too much phosphorus leads to stunted growth
- It hinders the absorption of important trace elements
- Optimal phosphorus content in rich floral flora, fruit set, and fruit ripeness
Above all, nitrogen stimulates the growth of green parts of plants. In principle, nitrogen-based fertilizers, so-called ‘green manures’, are suitable for trees that are particularly concerned with leaves. The nitrogen used in mineral tree fertilizers is usually produced synthetically. A nitrogen deficiency slows the growth and causes leaves to turn pale green or yellow. An excess of nitrogen leads to soft, unstable tissue and puffy leaves. It usually also brings with it increased exposure to nitrates. Optimal nitrogen content in the soil manifests itself in normal growth and lush green foliage.
Potassium is a natural component of the soil. It ensures the formation of a stable plant structure, promotes the formation of roots, tubers, and fruits and their firmness. It is also required for the transport of water and nutrients and it makes the plants more resistant to frost and pests. An undersupply of potassium leads to various deficiency symptoms such as stunted growth, wilted, flaccid, and discolored leaves as well as increased susceptibility to diseases. Too much potassium can stunt growth and cause burns to the roots, leaf damage, and stunted growth. If the tree looks healthy, is growing stably and quickly, the potassium content is ideal.
When to Fertilize
Deciduous trees use their falling leaves to provide themselves with all the essential nutrients. You can help them by using a rake to distribute the surrounding leaves on the tree grate and a little beyond it. The rest is done by microorganisms in the soil. When fertilizing deciduous trees, less is more.
- Deciduous trees don’t necessarily fertilize every year
- Every two years is sufficient
- Only fertilize during the growing season
- At the beginning of shoots in March / April and at the end of shoots around June 24th
- The effect of organic fertilizers is delayed
- We recommend a lead time of 3 – 4 weeks
- Depending on the moisture content and temperature of the soil
- Mineral fertilizers are water-soluble and immediately available to plants
In autumn and winter, during the rest period, fertilizers are completely avoided, because then plants do not absorb any nutrients. If you fertilize at other times, you run the risk that the plants will not mature. The soft shoots are sensitive to frost and can be damaged. The frequency and timing of fertilization also depend on the age of the tree.
Young trees that were supplied with compost when they were planted do not need any additional fertilizer this year. The compost provides him with all the important nutrients in the first year. A thick layer of mulch on the root area prevents competition from other plants that could deprive the tree of nutrients.
Instructions for Fertilizing
Before starting to fertilize, it is advisable to determine the actual nutrient requirement using a soil analysis in the area of the tree grate. If the result is certain, one should first consider that the root system of a deciduous tree is usually always slightly wider than the crown, so that there are also fine so-called suction roots outside the crown eaves.
If the tree slice is not overgrown or open, you can spread a thin layer of fertilizer over the entire area and again a little further. Then you work it lightly into the ground with a rake. A layer of mulch is then spread over it, which should be renewed every year.
To fertilize deciduous trees that grow on the lawn or a meadow, it usually makes no sense to simply sprinkle the fertilizer on. It should be introduced below the sward by making small holes at regular intervals on the tree grate with a lawn aerator, adding the fertilizer, and, if necessary, slurrying it with water.
If possible, fruit trees should not stand on the lawn or in the meadow; without an open tree disc, the food competition from the grasses is very strong, especially for young trees. Furthermore should never be used on dry soil, it would burn the roots. It is better to apply it after a rain shower or with irrigation water, depending on the type of fertilizer.
Deciduous trees that do not bear fruit can largely supply themselves with nutrients through the fall of leaves in autumn. Fruit trees have a slightly higher nutritional requirement. Leaving fallen fruit lying around fruit trees is also a natural fertilizer. For many hobby gardeners, compost is the best organic fertilizer.
- Spread fresh or ripe compost on tree grate every 3 – 5 years
- Work lightly into the ground
- If necessary, add some primary rock flour
- Put a layer of mulch on top of the compost
- Lawn cuttings, bark mulch, or wood chips are suitable
- Apply 100-140 g of nitrogen fertilizer to stone fruit trees
- For pome fruit, 70 – 100 g per tree is sufficient
- Reduce the amount of fertilizer for young plants by around 75%
- A little wood ash can increase the potassium levels in the soil
- Add important trace elements with algae lime or rock flour
Compost is less suitable as a fertilizer for deciduous trees that are sensitive to lime, such as rhododendrons, dogwoods, or magnolia. Another good organic fertilizer is manure, in the form of horse, sheep, cattle, rabbit, or chicken manure. It should be noted that manure should never be applied fresh, but only when it has rotted well. It should be spread over the area in question every three years in autumn and buried shallow. Incidentally, cattle manure is also available in the form of pellets.
With mineral fertilizers, stronger deficits in the soil can be compensated relatively quickly. Although they do not ensure a build-up of humus, they still supply the trees with all the nutrients they need. The main problem here is the correct dosage, so that over-supply or over-fertilization occurs relatively quickly, which is more harmful than good for the tree.
The most common mineral fertilizers for deciduous trees, especially for fruit trees, are calcium ammonium nitrate and blue grain. These fertilizers are best applied to the soil in two separate doses. As a rule, 15-20 g of the fertilizer is sufficient. The need for older trees is slightly higher. Quantities of 50 – 60 g are recommended here.
Recognize Nutritional Deficiencies
Signs of a nutritional deficiency can be, for example, declining growth. If leaves noticeably lose color and become more and more bright, this can indicate chlorosis. The reason for this is usually a lack of mineral nutrients such as magnesium and iron. But there are also plants, so-called pointer plants, that indicate a deficiency. These include the nettle, the occurrence of which is particularly high on very nitrogen-rich soils. On the other hand, hollow teeth and chamomile can be signs of nitrogen deficiency.
Sorrel, horsetail, and bracken feel particularly good on soils that are poor in lime. Heather, sorrel, and daisies love nutrient-poor soils. To be sure whether there is a deficiency or an excess, there is no avoiding a corresponding soil analysis, which is ideally repeated every few years.