Fertilize correctly – but how?
It's well known that proper fertilization is important for optimal plant growth. But which fertilizer should be used for which plants, and, above all, how much, and should it be mineral or organic?
Fertilize correctly
In the garden, plants can largely satisfy their hunger from the soil's existing nutrient reserves. In organic farming, the soil is regularly supplied with organic matter such as compost, manure, and green manure to ensure a rich supply. Soil organisms break down everything, thus releasing the nutrients important to the crops. In pots, this process only works to a limited extent. Here, the soil volume, which also acts as a storage reservoir, is small. Therefore, additional nutrients must be added. This has the advantage of allowing the specific needs of the plant species to be met.
Who needs how much fertilizer?
There are three groups of plants: those with low, medium, and high nutrient requirements. The amount of nitrogen a plant consumes before harvest is used as a basis. This depends on the length of the growing season and the amount of mass produced by the plant.
The individual groups are referred to as:
Low-nutrient feeders: Most herbs as well as radishes, lamb's lettuce, beans and peas.
Medium feeders: Carrot, beetroot, most lettuce, onions, parsnip, spinach, chard, turnips and pak choi.
Heavy feeders: Potatoes, tomatoes, peppers, chilies, zucchini, cucumbers, pumpkins, and kohlrabi. All other types of cabbage, such as broccoli, red and white cabbage, cauliflower, etc., are also included, but they are not considered here because it's hardly worth growing them in pots.
The amount of fertilizer applied to each plant group:
Plant group nitrogen (n)/m2 nitrogen (n)/10 l soil
Low-nutrient feeders4-7g 1-3g
Medium feeders7-10g 3-6g
Heavy feeders10-14g 6-9g
The amount of fertilizer depends on the type of vegetable and its nutrient requirements, divided into three groups: weak, medium, and strong. Fertilizer recommendations from manufacturers often follow a similar breakdown. For Cuxin vegetable fertilizer, for example, the manufacturer recommends an application rate per square meter of 50 g for legumes (i.e., beans and peas), 100 g for root, tuber, and leafy vegetables, and 150 g for fruit vegetables (i.e., tomatoes, peppers, cucumbers, and zucchini). The recommended amounts always depend on the formulation of the individual fertilizer, so you should always read the instructions carefully. The fertilizer amount usually refers to grams per square meter of bed area. If you can't find any information on the packaging, you'll have to calculate the fertilizer dosage yourself. The values in the table are helpful for this: If a medium-feeder, such as lettuce, is to receive 8 grams of nitrogen using a commercially available organic fertilizer, first check the packaging to see what percentage of nitrogen the fertilizer contains. This information is hidden on the packaging in the number sequence: 7–14–12. It indicates the proportion of N (= nitrogen) – P (= phosphorus) – K (= potassium) in this order. For example, if the content is 7 percent nitrogen, the application rate of fertilizer is 114 grams per square meter (8 times 100 divided by 7). Based on a 10-liter pot, 57 grams of the same ready-to-use fertilizer would be required to provide the medium-feeding plant, in this case lettuce, with 4 grams of nitrogen (4 times 100 divided by 7). The amount of fertilizer would then be mixed with the entire potting soil.
Mineral or organic?
The nutrients in mineral nitrogen fertilizers dissolve quickly in the water and are then immediately available to the plants. In organic vegetable cultivation, however, mineral fertilizers are completely avoided. This is the only way to achieve optimal quality – and this is the motivation of many hobby gardeners. To produce healthy vegetables, nutrient supply and demand must be in balance. Over time, you get a feel for this. For example, if the crop looks puny, its leaves are light green and remain small, it is usually lacking in nitrogen. In this case, organic liquid fertilizers, which are also used in organic farming, such as BioTrissol and Provita Vinasse, provide quick relief. Both are products made from fermented sugar beet molasses that contain trace elements in addition to nitrogen, phosphorus, and potassium. They are mixed in measured doses into the irrigation water to provide targeted, short-term fertilization for crops. This benefits tomatoes, which need a lot of nutrients during fruit formation. It can also help lettuces that lack the strength to sprout again after multiple harvests. When a crop is finally harvested and the bed or pot is prepared for the next crop, it's important to ensure that there are still sufficient nutrients in the soil. Measuring these would be time-consuming. Therefore, the approximate nutrient removal is used as a basis, a method that is also widely used among professional vegetable gardeners. The table shows that approximately 1.8 grams of nitrogen were consumed or removed from the soil per kilogram of harvested lettuce.
Example: The basic supply for a medium-feeder, in our example lettuce, was 4 g of nitrogen per 10 l of soil. Based on a container with a capacity of 100 l, we initially applied 40 g of nitrogen. This corresponded to just under 600 grams of an organic fertilizer with 7% N (100 Å~ 40 divided by 7 = 571 g). According to the table, the lettuce harvest removed about 1.8 g of nitrogen per kilogram from the soil. If, for example, 5 kg of lettuce were harvested, the nutrient removal amounted to 9 g of nitrogen, which now needs to be reapplied. If the basic supply is to be increased back to the initial value, just under 130 grams of the organic fertilizer (100 Å~ 9 divided by 7 = 129 g) must be mixed into the soil.
Nutrient deprivation of individual species:
– Chives 5.0 g N/kg harvest
– Parsley 4.5 g N/kg harvest
– Lamb's lettuce 4.5 g N/kg harvest
– Rocket 4.0 g N/kg harvest quantity
– Spinach 3.6 g N/kg harvest quantity
– Chard 3.2 g N/kg harvest quantity
– Basil 3.0 g N/kg harvest quantity
– Peppers 3.0 g N/kg harvest quantity
– Dill 3.0 g N/kg harvest quantity
– Kohlrabi 2.8 g N/kg harvest quantity
– Beetroot 2.8 g N/kg harvest quantity
– Pak Choi 2.5 g N/kg harvest quantity
– Bush bean 2.5 g N/kg harvest quantity
– Radishes 2.0 g N/kg harvest quantity
– May turnips 2.0 g N/kg harvest quantity
– Lettuce 1.8 g N/kg harvest quantity
– Early carrots 1.7 g N/kg harvest
– Zucchini 1.6 g N/kg harvest
– Radish 1.5 g N/kg harvest quantity
– Tomato 1.5 g N/kg harvest quantity
Withdrawal through harvesting
The above figures only reflect the nutrient removal from the crop during harvest. Harvest residues are not taken into account. This explains why tomato fruits remove only a small amount of nitrogen from the soil. The high nutrient requirements of tomatoes, a heavy feeder, are primarily needed for plant growth. In our case, we are ignoring harvest residues to avoid the risk of over-fertilizing the vegetable. Some of the residues, such as the roots, remain in the soil anyway and gradually return to soil.
The content of this article is from the book:
Michael Breckwoldt
Self-catering balcony
Price: €(D) 15.00 / €(A) 15.50 / sFr 21.90
ISBN 978-3-8354-1628-4
BLV Publishing House
The proven month-by-month concept is now also available for balconies. "The Self-Sufficiency Balcony" (publisher: blv) takes care of the planning, with a highly practical and user-oriented approach. The author explains the basics to beginners, so they can immediately get started growing on their balcony. Each month, his book provides cultivation plans for a small raised bed, three vegetable bags, and five pots. The sowing calendar and seed shopping lists ensure a bountiful harvest in the smallest of spaces.
