Last updated on October 23rd, 2023 at 08:15 pm
We have already dealt with macronutrients, fertilizers and soil conditions. However, nutrients are a very complex subject; on the one hand, they must be in the right ratio to each other, and on the other hand, there are other decisive factors for their plant availability. If these factors are not in balance, it can lead to problems and disturbances in the development of your plants.
Plant availability: what it is all about.
This term describes whether and to what extent the nutrients dissolved in the soil can be absorbed by the plant via the roots. The actual quantity of the nutrient in question is of secondary importance, since it must be present in the required quantity, but this is of no use if the plant cannot absorb it.
When it comes to plant availability, both the necessary nutrients, the right amount of water, the soil condition and the ecosystem in the soil come together. Balanced and active soil life is essential for healthy roots and can be negatively affected by an imbalance of any of the other factors mentioned. Since soil texture affects soil moisture, which in turn determines nutrient uptake, you also need to keep a close eye on your watering habits. Here you will find a great guide to proper watering and in this article you can refresh your knowledge of soil texture again if needed.
The pH value holds the power
As far as plant availability is concerned, it is above all the pH value that holds the reins. This is because the nutrients in the soil are either coupled to the soil crumbs as salts or are already present in dissolved form due to the soil moisture. The substances bound to the soil must first be dissolved by the irrigation or rainwater so that they can find their way into the plant via the roots. The pH value influences how strongly the substances are bound to the substrate and whether (or how well) they can be dissolved at all. In addition, different plant species have different preferences in this regard, as their nutrient requirements vary among themselves. In this article you will also find instructions on how to determine the pH value of the garden soil.
Overview of pH values
- below 6.5: the soil is acidic
- 6.5 to 7.5: the soil is neutral
- above 7.5: The soil is alkaline
Balance in the soil – health for the plants
We already know that the right balance of nutrients in the soil is the key to healthy plants. Too many nutrients can downright poison our plants, but a deficiency is just as harmful and causes them to wither. Another important point is that the excess of one substance can block the absorption of another, causing a deficiency. Of course, it also works the other way around – certain substances can favor each other in plant availability. This phenomenon is also known as nutrient interaction.
Most plants like a neutral to slightly acidic soil. However, there are candidates, such as strawberries or blueberries, that find an even more acidic soil great. Most cabbage varieties, on the other hand, need just the opposite and feel comfortable in a more alkaline soil. Soil life also depends on pH and the nutrients present. Soil organisms, on the whole, tend to do poorly with acidic soils. This is one of the reasons why many macronutrients have poorer plant availability at a low pH. Fungi, however, are not so picky, which is why they make up the bulk of microorganisms present in such sites.
The pH value is quite variable and can change over time due to human intervention (monoculture, overfertilization, polluted wastewater), but also weather conditions (e.g. persistent heavy rain or long dry periods). For this reason alone, you should try to bring variety into your garden design and not always grow the same plants in one spot, but rotate them annually.
Macronutrients and pH – Now we’re getting down to the nitty gritty!
Now let’s take a look at the most important macronutrients and see exactly at which value which substance is most available and whether (and how) different substances might support each other or – on the contrary – compete with each other. In addition, you should end up with a better understanding of the processes in the soil and thus a deeper understanding of fertilizing in general. Since this is a very complex topic and we could certainly fill an entire book with it, let’s just focus on the most relevant points. Also, if you have purchased a new piece of gardening or your plants are giving you a lot of trouble, it makes sense to send a soil sample to a lab. Usually good nurseries, sometimes even hardware stores, offer such services.
Terms: Antagonism and synergism
Synergism: Positive interaction of the different nutrients. One substance helps the other to be better absorbed or metabolized by the plant.
Antagonism: Just the opposite – one nutrient can block the other, or reduce its availability to the plant.
Nitrogen (symbol: N for Latin nitrogenium) is very important to the structure of the plant itself and has the best plant availability in a neutral to alkaline soil (pH about 6.5 to 8.5). It can be in the form of ammonium or nitrate, the latter being more available to plants. Nitrate, in fact, is the variant dissolved in the soil and thus freely mobile, whereas ammonium is firmly bound to soil and clay crumbs and must first be converted into nitrate by microorganisms and dissolved in water. Important: In order to fulfill its function in the plant, nitrogen must be accompanied by sulfur and the latter must therefore be available in sufficient quantities. Here, then, is a synergism between nitrogen and sulfur. Good to know: The classification into weak, medium and strong growers is based on the nitrogen requirements of the plants. So, high growers, such as cabbage varieties or tomatoes, have a particularly high nitrogen requirement.
Phosphorus (P) has a fairly broad tolerance spectrum in the soil to pH, which should ideally be between 6 and 7 and is available to plants in dissolved form as phosphate. However, too much phosphorus in the soil can hinder the uptake of the important micronutrients iron and zinc. Here we have a nice example of antagonism between phosphorus and the aforementioned micronutrients. Too much calcium, in turn, can bind phosphorus tightly in the soil, making it unavailable to plants for extended periods of time. Since overfertilization with phosphate is quite common, and there is usually enough of it for pretty much all plant species in our gardens anyway, calcium (or even better, liming in the form of compost) can be helpful here. Raspberries and other berry fruits especially love phosphorus.
With potassium (K), the lower the pH, the poorer its plant availability. From a value of 6, this is optimal. Nitrogen helps with potassium uptake; however, it competes with magnesium because potassium hinders magnesium uptake. Too much calcium, in turn, inhibits potassium uptake. Potassium is generally the same as phosphorus: we often have too much of it in the garden soil rather than too little. Tomatoes love potassium, as do other high-yielding plants that produce fruit. It also has a positive effect on storability, for example in root vegetables.
Calcium (Ca) is best absorbed in the alkaline range and raises the pH value. It also has a stabilizing effect on the soil structure, which can be particularly beneficial in sandy soils, which usually have a fairly low pH. Microorganisms also think calcium is great, which is why it promotes soil life. Nevertheless, you should be very careful with calcium and only fertilize when there is a deficiency, because on the one hand it competes with many other nutrients and can either block their uptake or bind them so tightly in the soil that they are no longer available to plants. In addition, it has the property of releasing nutrients in the short term when administered in larger quantities, which is why, however, they are also washed out very quickly – and this results in a depleted soil. Generally, fruit trees, tomatoes, onions, cabbage varieties and some herbs, such as sage and oregano, like calcareous soils.
Magnesium (Mg) also has the best plant availability when the soil pH is in the alkaline range. However, too much calcium directly at the root blocks magnesium uptake, and the same is true with potassium. Interestingly, the reverse is not true: too much magnesium does not block potassium uptake. So when fertilizing with potassium, it definitely makes sense to also apply magnesium to prevent an imbalance from developing. Vegetable and fruit plants generally need a sufficient magnesium supply, especially on sandy soils you should keep an eye on the content. Cabbage – being a real “glutton” – also loves a lot of magnesium in the soil.
Sulfur (S) is present in the soil in organically bound form, for example as humus, plant residues or in microorganisms. However, it can only be absorbed in inorganic form (as sulfate) and for this purpose must first be converted into sulfate by an industrious troop of microorganisms by means of mineralization. As already described above, sulfur is the most important companion of nitrogen, because it significantly favors its plant availability. Root vegetables and legumes, for example all types of cabbage (how could it be otherwise), radishes, peas, but also onions, need sufficient sulfur for best harvest results.
Easily correct and prevent nutrient deficiencies
Once you have diagnosed a deficiency or an imbalance, the question naturally arises as to how you can remedy it. This is usually not a big deal, because most of the ingredients for homemade and organic fertilizer can be found in your kitchen or in your garden. So, fortunately, we can leave mineral fertilizers to the left.
A real magic ingredient in almost all cases is good old compost. It not only raises the pH value, but also provides pretty much all nutrients, especially a large portion of magnesium and sulfur. Wood ash from untreated (!) wood, straw or printed paper also provides a lot of magnesium and raises the pH. If you regularly indulge in coffee, we finally have some good news: Coffee grounds are rich in potassium, phosphorus, sulfur and nitrogen. Eggshells are – and this is really no surprise – great sources of lime. Horn shavings are rich in nitrogen, as is the well-known nettle liquid manure. Animal manure (especially poultry manure) also contains a lot of nitrogen, but almost too much, which is why these products should only be used with caution. In addition, biological algae fertilizers, which provide potassium, phosphorus and nitrogen, are available in stores. You can also prevent deficiencies by using slow-release fertilizers, which provide nutrients in a balanced ratio over a long period of time.