Plants have needs. The soil in which they grow has needs. We as gardeners have needs…we want to harvest, and do a lot of it!. And it is up to us to consider and attend to the environmental factors that will determine whether our garden will be successful in the long term. We are going to get into some sciencey stuff here, such as the connections between the nitrogen requirements of your plants and the nitrogen concentration in your soil. We think that’s mega-cool. When it all comes down to it though, the most important thing to pay attention to when growing vegetables is the distinction between heavy-feeders, moderate-feeders and weak-feeders.
The term “heavy-feeder” sounds a bit like an affectionate euphemism for someone with an excessively large appetite – and with plants, that’s exactly what it is. In your garden, the heavy-feeders are the big boys. The chonks. Thicc with two Cs. And it is very important that you consider where you plant them. Weak-feeders on the other hand require much less to feel full. This is a popular framework and useful way to illustrate the nutrient requirements of different plant species in the vegetable patch.
But wait…feeders? …eaters? I thought plants are the ones that get eaten? Who is eating whom here?
We'll get into all that and much, much more in just a few moments.
Basically, plants get their food from the air and from the ground. They need light, water (H2O), carbon dioxide (CO2), oxygen (O2) and various other nutrients for healthy growth – and ultimately a rich harvest. In addition to phosphorus, calcium, potassium, magnesium and sulphur, they also need various trace elements. However, the distinction between heavy-, moderate- and weak-consumers is based on the amount of nitrogen that a specific plant draws from the soil in order to survive.
Your plants use nitrogen for leaf growth. If they lack nitrogen in the vegetative phase, the phase in which they form leaves, they remain small, grow weaker roots, develop yellow leaves and produce smaller fruits. Adequate nitrogen supply is also essential for the absorption of other nutrients. If you only grow heavy-feeders in your garden for years at a time, they withdraw a corresponding amount of nitrogen from the soil and leach it out. Then there is nothing left for subsequent cultures.
But be careful: you CAN have too much of a good thing. Too much nitrogen weakens plants, making them susceptible to diseases and pests.
You can only fertilise the soil with the right amount of nitrogen or select your plants according to the nitrogen content in the soil if you know the individual nitrogen requirements of your plants. In this way, you can plan the planting in your garden in such a way that you can get by with as little fertiliser as possible, or even do without additional fertiliser altogether.
In tables like the following, you can see which plants are heavy-eaters, that is, which plants draw a lot of nitrogen from the soil, and which, as weak-consumers, have little to do with taking or returning nitrogen to the soil.
If you don't remember all this, or if you simply want to bring some quick notes with you into the garden, you can use the following rough guidelines as a guide:
Heavy-consumers are often fast-growing plants with large fruits, for example many pumpkin and nightshade plants such as cucumbers, aubergines and tomatoes.
Many leaf and root vegetables such as lettuce and carrots are moderate-consumers.
Small, leafy vegetables such as rocket and lamb's lettuce, as well as plants with small fruits such as strawberries, radishes, plus flowers and herbs tend to be weak-consumers.
Legumes, i.e. peas and beans, are also weak-consumers and even enrich the soil with nitrogen.
All plants and animals need nitrogen to live. It's a good thing that nitrogen is the main component of the air we breathe.
The only problem: Neither plants nor animals can absorb the simple, gaseous nitrogen from the air (N2) directly. These organisms can only access Nitrogen through the preparatory work of special soil bacteria, which convert nitrogen first into ammonium, then into nitrite and nitrate. These are water-soluble nitrogen compounds, which plants can use for the production of proteins. These proteins in turn are ingested by animals and humans through food.
The binding of nitrogen from the air into water-soluble compounds is called nitrogen fixation. Nitrogen-fixing bacteria can be found in animal excrement, for example, but cyanobacteria, or “blue-green algae”, also have the ability to bind nitrogen from the air.
In addition, nitrogen from the air can be fixed by chemical processes (ammonium and nitrate as fertilisers), lightning (nitrate) or combustion. When fossil fuels are burned (e.g. in engines), nitrogen is converted into nitrogen oxides and released.
Legumes such as beans, peas, lentils, lupins, clover, etc., live in symbiosis with certain nitrogen-fixing bacteria called nodule bacteria. These attach to the the plant’s roots. When we grow plants like this, we also encourage the build-up of nitrogen compounds in the soil – and we have a protein-rich meal on the table for our plant-kiddos.
Some researchers even see the cultivation of legumes as a solution to the global food crisis. Legumes also thrive on extremely poor soil and are not dependent on animal fertilisers or nitrate from the soil, since they can absorb nitrogen directly from the air via symbiosis with the root bacteria.
Strictly speaking, legumes are not only weak consumers, but even soil-improvers, if we take into account the work of their nodule bacteria as well.
After legumes and other plants have died, the nitrogen compounds return to the soil to feed subsequent plants. The use of these deceased plants is called green manure, and it can be very useful, provided we or our livestock do not eat it all.
Other benefits of green manure:
Roots loosen up the soil and save laborious digging
Protection against erosion through strengthening roots
Plant residues provide food for earthworms and other soil organisms and help them to produce humus
Dead plants form a protective layer of mulch.
Example: Clover. As a legume, clover already feeds nitrogen into the soil during its growing season. If left standing, the nitrogen compounds left by the plant as proteins are carried back into the soil with the help of microbes, bacteria and fungi, where bacteria in turn convert them into ammonium, nitrite and nitrate.
Pro-tip: Green manure in autumn protects your bed over the winter, loosens it up and supplies it with nitrogen for the spring, so the heavy-consumers have a place to get their food!
The only question that remains is: What happens to all the nitrogen that these beans and their friends diligently store in the soil? Does it stay there forever?
Well, no. The cycle is completed by bacteria, which redissolve the nitrogen compounds and release nitrogen (N2) into the air.
Graphic: The nitrogen cycle with legumes greatly simplified: an illustrated bean plant, the soil life is visible. Nitrogen molecules (N2) are converted into ammonium (NH4+) by nodule bacteria on the roots of legumes. Other bacteria convert this first into nitrite (NO2-) and then into nitrate (NO3-), which is absorbed by the pea plant. If the plant dies, decomposers (fungi, bacteria) decompose the nitrogen compounds back into ammonium, which in turn is converted into nitrite and nitrate. Finally, other bacteria convert the nitrate back into molecular nitrogen and the cycle can begin again.
So, it’s back to the classroom with us. Let’s talk about the four-field economy.
This approach to gardening had already been practised by our ancestors and is more important today than ever. It works very simply: An area is divided into four “fields” on which, after a year of green manure, heavy-eaters are planted alternately in the first year, moderate-eaters follow in the second year and in the third year weak-eaters have to be content with the remains of the nitrogen fixed in the soil from the first three rotations. The cycle starts again with green manure. This is left on the field and incorporated into the soil.
Modern agriculture looks different due to our high demand for cheap energy and food: For decades, intensive forms of farming have been leaching the soil. Too often, fast-growing crops are grown one after the other, which overtax the soil because they require large amounts of nutrients and nitrogen.
To compensate for the constant lack of nutrients, nitrogen compounds such as nitrate are applied to the fields in the form of artificial fertilisers. The soil cannot absorb such enormous amounts of nitrate. Too much of it gets into bodies of water and groundwater through rain, which in turn poses a threat to people, animals and plants.
Intensive animal husbandry also contributes to fields being over-fertilised with nitrogen. Animal manure also contains nitrate, which is harmless in small amounts and enriches the soil with valuable nitrogen compounds. However, a large herd of cattle produces a lot of manure, which in turn is spread on the fields. However, the area of these fields is usually not in proportion to the amount of excrement spread, then over-fertilisation is inevitable.
A return to the balanced cycle of the four-field economy would regenerate our soils and protect our groundwater from contamination.
Graphic: Crop rotation and four-field management: Field 1 Green manure made from lamb's lettuce and clover, Field 2 weak consumers (rucola, peas, strawberries, beans), Field 3 moderate-consumers (beetroot, lettuce, carrots, kohlrabi), Field 4 heavy consumers (tomatoes, cauliflower, zucchini, Paprika). The planting rotates one field further each year (shown with arrows).
The four-field economy took the leap into the vegetable patch via the cottage gardens, which had also been used very intensively in the past. But because there is hardly a garden that big that one would call it a "field", one usually speaks more generally of "crop rotation" and "crop sequencing". Here, too, the garden is divided into four fields or segments; Green manure, heavy-feeders, moderate-feeders and weak-feeders are alternated annually or within a season.
Every plant has its own nitrogen requirements. An oversupply of nitrogen leads to poorer growth and lower yields, as does an undersupply. This means that it is quite possible that there are too many nutrients in the soil for moderate-feeders in the first year after fertilisation, and that they grow less well as a result.
However, some of the moderate eaters on the list, such as kohlrabi or lettuce, still require a relatively large number of nutrients. In addition, green manure does not introduce such large amounts of nitrogen into the soil that would pose an over-fertilisation problem to moderate-consumers. The situation is different with artificial fertiliser or manure. Here you should be a little more economical with fertilisation and use it in a targeted manner. Try to reduce it to the needs of moderate-consumers. In the next year, weak consumers can then be cultivated again.
Of course, you don't need to create four fields on your balcony to provide your plants with the right amount of nutrients. But here, too, the classification into heavy-consumers, moderate-consumers and weak-consumers can help you to estimate the nitrogen requirements of your plants.
So you should enrich your soil with compost, manure or organic fertiliser once or twice per season if you want to grow heavy feeders in a pot. Nettle manure is also suitable for fertilising heavy feeders and also keeps pests away. Weak feeders do not require additional fertilisation. Moderate-feeders should only be fertilised once at the beginning of the season, because potted plants in particular are quickly over-fertilised if you give them too many nutrients. This damages the plants and ultimately your health if excess nitrate accumulates in lettuce leaves, for example.
This brings us to one very important point: Water your plants well, so that they can absorb the water-soluble nitrogen compounds. This will help ensure that they grow healthy and strong.
The small space in raised beds makes it not so easy to alternate between heavy-, moderate- and weak-feeders. It's the heavy- and moderate-consumers that we want to harvest year after year: tomatoes, cucumbers, peppers, maybe a few carrots and lettuce.
So that the soil in your raised bed is not leached out, you should also plant a few legumes (lepidoptera) between rows and years and fertilise your heavy-feeders with an organic fertiliser, compost or manure at the beginning of the season.
If you have several raised beds, you can alternate between heavy-, moderate- and weak-feeders in a rotating movement every year.
Raised bed from above with lettuce, celery and cabbage, next to a light blue wooden bench
You should not uproot perennial plants after a year to keep up with the crop rotation, even if they are heavy-consumers.
Most perennial plants consume less nutrients than annuals anyway, with notable exceptions being rhubarb and asparagus. If your perennial plants show signs of nitrogen deficiency such as stunted growth, lower yields, yellow leaves or pest infestation, then fertilisation makes sense. After about seven years, however, the rhubarb and asparagus should move to a new location.
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