By fertilizing, you can increase soil fertility and ensure healthy growth for your tree. And not only that: fertilizing can improve the soil and enhance soil life. And on a small scale, it also helps to store carbon in the soil.

Sounds like a really cool thing, doesn't it?

If you have your own garden and are thinking about fertilizing your trees, it is helpful to check in advance why you want to fertilize your trees. Depending on your motivation, different approaches and means are needed. Here are the most important reasons for fertilizing a tree:

• To provide the tree with the necessary nutrients
• To improve the soil and enrich soil life
• Storing carbon in the soil

For example: If you want to provide your tree with the nutrients it needs in the short term, you have a few simple options available. For example, you could consider using a purchased fertilizer. If you also want to improve the soil and enrich the soil life, you can mulch. If you mainly want to store carbon in the soil, you can buy biochar and bury it.

And then there are methods that cover all three goals simultaneously. Let's start with the basics: the ground.

Soil. Basis of life.

Healthy soil is an essential prerequisite for healthy trees. The federal government's definition of soil is:

"Soil is the product of the transformation of mineral and organic substances, permeated with water, air and living organisms, created under the influence of environmental factors on the earth's surface and evolving over time, with its own morphological organization, which is capable of serving as a habitat for higher plants. This enables soil to form a basis for life for animals and humans."

Here are the most important functions of the soil:

• Soil forms the basis for plants to grow and thrive. Plants extract nutrients and elements such as nitrogen, potassium and phosphorus from the soil and bind them. When these plants die, their parts are decomposed, converted and mineralized by soil organisms. These components are then used again by new plants.
• The soil is inhabited by numerous living creatures and forms an important habitat for microorganisms and small animals.
• The soil filters rainwater and thus creates clean drinking water.
• Soil is also the largest carbon store on Earth, storing more carbon than all the trees in the world combined.

Have a soil sample taken

Since most of us cannot read the soil directly, it is worth having a soil sample taken. Because if you fertilize the soil, you are interfering with a complex process and can also cause some damage. Many soils are already over-fertilized, especially with nitrogen and phosphate. If you add more of these through one-sided fertilization, the trees can suffer.

In addition to a soil analysis, you will also receive specific tips on how to fertilize. You will also find out the pH value of your soil.

Provide the tree with the necessary nutrients

Nutrients are an important basis for trees to grow well. If these are missing or only present in small quantities, this nutrient deficiency is usually evident in leaf discoloration, poor growth, low harvest and even disease. However, since trees are among the toughest and longest-living living things, they can also cope with nutrient deficiencies over a longer period of time without their deficiency becoming obvious.

The main nutrients for trees include nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulphur (S) and calcium (Ca). There are also a number of trace elements that are crucial for metabolism and growth, and of which a tree only needs small amounts. This group includes iron (Fe), copper (Cu), boron (B), zinc (Zn), molybdenum (Mo), silicon (Si) and manganese (Mn). It is important that these nutrients are in a healthy balance in the soil.

Important: A nutrient deficiency can not only be caused by a lack of fertilization, but also by an unbalanced supply of nutrients. We therefore recommend that you find out in advance which nutrients your soil needs. This is even more important if you use purchased fertilizer. If you only use organic fertilizer (such as your own compost, mulch or plant broth), an unbalanced supply of nutrients is much less likely.

Individual nutrients also have an effect on each other. For example, a plant needs potassium to be able to absorb nitrogen from the soil. If there is too much potassium in the soil, however, this can impair magnesium absorption, even if this element is available in sufficient quantities.

It is also important to note that each type of tree has different needs. We therefore recommend that you find out about the specific needs of your trees.

Which trees need additional nutrients?

In general, it can be said that most trees do not require any additional nutrients. However, there are exceptions:

• Trees on barren ground
• If you want to consistently reap a rich harvest over a long period of time
• In very tidy gardens, where, for example, autumn leaves are removed immediately
• For trees that were replanted several years ago without soil improvement
• If the tree shows signs of nutrient deficiency

How do I recognize a nutrient deficiency in my tree?

• A typical sign of a nutrient deficiency is leaf discoloration before the natural autumn coloring. Individual leaf sections or entire leaves begin to turn yellow. The yellowish discoloration can then turn brown.
• Pale foliage, sometimes with dark colored veins
• Early leaf shedding
• In addition to leaf discoloration, growth disorders can also occur. The tree then does not develop well.
• Another sign of nutrient deficiency is small or few fruits.
• Decline in flowering

These signs are also similar to the signs of drought stress. If your tree shows these signs, you can also give it water first. You can find more information about drought stress in this article.

How do you properly fertilize trees?

Here are the most important guidelines for fertilizing trees:

• Do not use fertilizer carelessly and ideally have a soil sample taken beforehand. This is especially important when you buy fertilizer.
• Fertilizer should ideally be applied in spring at the beginning of the growing season when your tree begins to form new shoots. You should stop fertilizing in autumn.
• If you fertilize in one year, you should wait at least a year before fertilizing again.
• If possible, use only organic fertilizers.
• We only recommend mineral fertilization in cases of severe deficiency symptoms, such as iron or potassium deficiency. Otherwise, garden trees can quickly become over-supplied with special tree fertilizers.

Organic fertilizer vs. mineral fertilizer

Basically, there are two types of fertilizer: organic and mineral fertilizer. Organic fertilizer is created by the slow decomposition of organic substances, for example through composting or mulching.

Organic fertilization increases the amount of humus, improves the soil and improves soil life. Organic fertilizers not only contain individual elements such as nitrogen, but usually a whole range of important nutrients and trace elements that a tree needs.

Mineral fertilizers, on the other hand, are usually designed for specific substances. For example, there is a fertilizer containing nitrogen, phosphorus and potassium, also known as NPK fertilizer.

Important: Mineral fertilizers should be used with more caution than organic fertilizers. With organic fertilizers, it is much more difficult to achieve one-sided over-fertilization than with mineral fertilizers.

mulching

Mulching is a word that is often only known to gardeners. Unjustly, because it is something very simple and reflects a principle from nature. In forests, for example, the soil is covered with a layer of leaves.

Mulching is the covering of soil with plant material. It is one way to help improve soil quality. A layer of mulch also improves soil life and protects the soil from drying out.

A layer of mulch brings you these advantages:

• Slowly and steadily delivers nutrients to the soil and thus builds it up
• Provides food for soil organisms
• Promotes the favourable crumb structure of the soil. This also allows the soil to absorb water better
• Protects the soil from drying out
• Balances soil moisture
• Suppresses other plants (also called weeds)
• Prevents surface soil loss (run-off of nutrients)

What materials can you use for mulching?

• Laub
• grass clippings
• shredded bark material 
• semi-mature compost covered with a thin layer of organic material
• chopped straw
• Nettles or other medicinal wild herbs

How to use mulch:

• Green material is only applied thinly, otherwise it will easily rot. It can be renewed more often
• Dry material can be applied 2 to 10 cm thick.
• In heavy soils, mulch carefully to prevent waterlogging and rot.

You can find more information about mulching trees in this article. 

Make your own compost

You create a compost for the entire garden ecosystem. In doing so, you promote local biodiversity, improve the soil and store carbon in the earth. At the same time, you minimize waste from the kitchen by returning it to the earth. 

If you have space in your garden, we definitely recommend that you create your own compost. You can then use the mature compost to fertilize your trees. You can find the most important information about this in this Articles.

Storing biochar

Due to its porous structure, biochar has a large surface area. This means it can store water and nutrients particularly well. If you store Terra Preta in the soil, you can also store it in a small amount.ßstably help to store carbon in the soil. You can find more information here. 

Pay attention to the needs of your tree

Keep in mind that different tree species have different nutrient needs. Fertilization should always be adjusted based on the specific type of tree and its environment.

Fertilizing city trees

In some cities, tree sponsors can fertilize their sponsored trees. If you have taken on a tree sponsorship and would like to fertilize a city tree, first check with the responsible authority whether this is allowed in your city. However, there are also sponsorship programs where the tree sponsors only water their treesßIn any case, you should not fertilize trees in public green spaces without a tree sponsorship. 

You can find more information about tree sponsorship in this article.

Insert tree watering bags and fertilize trees

Tree watering bags are a useful tool for watering trees efficiently. The water is released into the soil drop by drop, allowing it to seep into the deeper layers of the soil where the tree can absorb it. 

You can consider the following points when fertilizing trees and using tree watering bags:

• If you use liquid fertilizer, make sure that it does not attack the material of the bag. Most tree watering bags are made of PVC or PE.
• You should not put fertilizer with solid components in your bag because this will clog the small holes. 
• You should also not put fertilizer teas such as nettle manure in your bag. They cause the holes to become clogged and the bags to stink. This will become a problem when you store the bags for the winter. Use manure and fertilizer teas when you remove the bag from the tree for a few days.
• For mulch and compost, a tree watering bag is a good addition as it keeps the area around the tree ring moist. Remove the tree watering bag every now and then for a few days to prevent mold from forming.