The rationale behind fertilisation in the vegetable garden

The benefits of covering soil with organic fertiliser

Market gardens are known for their particularly intensive farming systems. Certain vegetable crops such as radishes, lettuces, turnips, beetroots and others are grown in succession on the same plot, which leads to significant losses of mineral salts that must be identified and corrected if deficiencies are to be avoided. All major elements are affected, as well as trace elements, whose reserves can be quickly depleted.

Industrial complete fertilisers are mainly intended to supplement nitrogen, phosphorus and potassium requirements. Some industrial fertilisers also contain boron, magnesium and even a few other elements. However, many vegetable plants are sensitive to one or more deficiencies in manganese, molybdenum, iron, copper and others. It is recognised that a good organic fertiliser can provide all these elements. But not just any organic fertiliser. The materials used for composting must contain all the elements that plants need. The more a growing soil is amended with properly prepared compost, the more trace element deficiencies are avoided.

Organic fertiliser application and assessment of the nutritional status of vegetable crops

The overall requirements of each vegetable crop have been defined in a number of studies by organisations such as CTIFL for essential elements, particularly nitrogen, phosphorus and potassium. These studies show significant disparities between each vegetable crop. Certain plants such as aubergines, courgettes, spinach, leeks, potatoes and tomatoes are heavy consumers of nutrients. For a given type of vegetable, there are also significant disparities between certain varieties, which are influenced to a greater or lesser extent by the growing season, soil type, etc

Fortunately, we now know that the richer the soil is in organic matter, the lower the risk of deficiencies, especially for trace elements. For amateur gardeners, who generally have limited space, these deficiencies can be avoided by adding at least 100 kg/are of compost, which is still affordable and/or feasible for gardeners to acquire and/or produce. However, the assimilable form of nitrogen disappears quickly, absorbed by plants or through leaching, which requires frequent checks and, if necessary, corrections to avoid a deficiency in this element (see chapter the issue of nitrogen assimilation in organic farming).

Some consequences of the ban on mineral fertilisers in organic farming.

In organic farming, synthetic fertilisers are prohibited, so farmers only use organic fertilisers in the hope of meeting all the plants’ mineral salt requirements, which is an impossible task. Nitrogen management is particularly problematic.

Nitrates are gradually released, regardless of the plants’ needs, even after harvesting. When nitrogen demand is high, organic fertiliser is unable to meet it, and when this demand collapses, the nitrates from organic fertilisers are lost.

As nitrates are highly soluble in water, nitrate losses are inevitable. Even the slightest rainfall will cause these nitrates to leach into the groundwater.

With organic base fertiliser used as the sole source of nitrogen, depending on the volume applied, either there is an excess production of nitrates with a risk of toxicity when vegetable plants are not in the phase of maximum nitrogen assimilation, or a nitrogen deficiency occurs when plants need it most.

he role of organic and mineral fertilisers in integrated agriculture

Microorganisms play an essential role in the fertility of cultivated soil. There are several billion bacteria and several million fungi in one gram of soil. This population of microorganisms contains between 10,000 and 100,000 (2) different species, many of which still have poorly understood characteristics.

Bacteria and fungi are the only organisms capable of transforming organic matter and mineral matter into a form that can be assimilated by plants. Bacteria are also responsible for fixing atmospheric nitrogen. Without microorganisms, life on earth would be impossible.

The diversity and structure of bacterial communities and the networks of biotic interactions between bacterial taxa are influenced by natural factors (especially soil type) and entropic factors. Generally speaking, there are fewer bacteria in cultivated soils when organic inputs are insufficient.

Rebuilding and maintaining microbial flora is therefore essential. To this end, organic fertilisers are indispensable, as are certain agricultural practices such as plant cover, reducing synthetic or organic pesticides when they have a negative impact on soil biodiversity, and adding amendments to improve soil structure, texture and pH…

The composition, quantity and method of application of organic fertiliser must meet three objectives :

• Maintaining soil biodiversity and benefiting from the ecosystem services it provides.
• Feeding plants.
• Form C.A.H.s that will fix nutrients not absorbed by plants.

In integrated agriculture, apart from potassium, phosphorus and other elements, organic fertiliser generally aims to meet the minimum nitrogen requirements of vegetable crops, with the rest being added as nitrogen requirements increase during the crop cycle. The aim is not to build up a nitrogen reserve using organic fertilisers alone, as these have a major drawback: they are made up of various nitrogen compounds that take varying amounts of time to mineralise and are very difficult to predict.

The addition of organic fertiliser is often supplemented with a complete NPK fertiliser to optimise soil fertility, particularly in terms of potassium and phosphorus (unless laboratory analysis shows that the soil already contains sufficient reserves of these two elements). The more potassium and phosphorus-rich compost is applied, the less synthetic NPK fertiliser is needed. Thus, when the soil is well supplied with compost, with some exceptions, corrections during the crop cycle mainly concern nitrogen.

A regulated supply of organic nitrogen throughout the growing cycle is possible using farm manure, and especially products rich in ammonia from the methanisation of organic waste. If these products are still difficult to find for amateur gardeners, they can fall back on preparations of nettles or macerated grass, which are rich in nitrates.

Commercially available organic fertilisers rich in nitrogen contain dried blood, sea guano, feather meal and bone meal (e.g. N.P.K. 11-4-3) and are approved for use in organic farming. They can be extremely useful, especially at the start of planting. However, as they also contain phosphorus and potassium, their use to reduce a specific nitrogen deficiency is not always appropriate. This type of fertiliser should not be used if the soil is too rich in phosphorus and/or potassium, as demonstrated by laboratory analysis.

However, soils cultivated using organic farming methods are often imbalanced in terms of potassium and phosphorus. Many amateur gardeners are surprised when they discover through laboratory analysis that their soil is too rich in certain major elements such as phosphorus (often fixed in a form that cannot be absorbed by plants), or another element whose excess causes an induced deficiency.

Synthetic fertilisers: an unjustified aversion

Synthetic fertilisers are often viewed with aversion on the grounds that they harm the environment (pollution of soil, groundwater and rivers, volatilisation of ammonia, etc.). These negative effects are in fact the result of misuse of these products, rather than their intrinsic properties (in particular the lack of a strategy specifying inputs in relation to changing needs)..

These fertilisers are also criticised for being too fast-acting. For medium-release fertilisers, the process of nitrification by bacteria from ammonium actually takes between one and several weeks, depending on soil temperature and moisture content. However, this process from ammonium also occurs with organic fertilisers.

As with organic fertilisers, synthetic fertilisers must be used in accordance with a “code of good agricultural practice”, a classic expression often found in texts dealing with agriculture. Since synthetic fertilisers are more rapid-acting than organic fertilisers, it is sufficient to divide the application according to the needs of the plants throughout their growing cycle to avoid nitrate losses through leaching. Ultimately, this is not complicated for the amateur gardener.

When soil has a good supply of CAH, if plants do not absorb all of the non-nitrogen mineral salts provided by fertilisers, a large part of the surplus is fixed by these C.A.H.s. Plant roots draw on these reserves as and when needed. Losses are therefore very low, except for nitrate, which is not fixed by CAH. This is also why nitrogen fertiliser should not be used as a base fertiliser.

Contrary to the widespread belief in certain non-scientific agroecology circles that soils are depleted by the high yields produced by synthetic fertilisers, the opposite is true. Increased yields improve soil fertility potential, simply because they produce more surface organic residues and underground litter that will be recycled. Soil depletion (particularly in carbon) occurs when organic residues are no longer recycled. This is particularly the case when farmers sell their crop residues (especially straw) to other farmers or livestock breeders for financial reasons.

Why it is necessary to provide nutrients during critical periods of the crop cycle !

Vegetable plants have intense and immediate nutrient requirements during certain periods of their growing cycle. For example, tomatoes need more potassium during flowering and fruit set. Other crops are sensitive to phosphorus inputs, such as seed vegetables, while others require more nitrogen, such as leafy vegetables. It is well known that if you want to promote rooting in young plants, the soil must be rich in available phosphorus.

The aim of fertilisation for each vegetable crop is to avoid overall and sudden deficiencies that could occur during their nutrition. Chemical fertilisers, particularly foliar fertilisers, have the advantage of offering very precise mineral salt compositions to meet the varied needs of plants and correct deficiencies that could occur during their growth period. It is more difficult to meet these needs precisely with organic fertilisers, except for nitrogen, if solutions such as dried blood powders (NPK = 14.0.0) are used.

Guano, used in organic farming as a boost, also provides nitrogen, but it is very rich in potash and phosphorus and its use can lead to induced deficiencies.

In addition, certain vegetables such as cucumbers cannot tolerate fertilisers that are too rich in ammoniacal nitrogen. The form of nitrogen supplied during the growth of certain plants is therefore also very important and is taken into account in integrated farming.

Management of nitrogen, potassium, and phosphorus during the crop cycle

Nitrogen nutrition in plants is one of the most important factors in production. In the event of nitrogen deficiency, losses can reach up to 90% of expected production. In principle, a crop’s nitrogen requirement is defined as the amount of nitrogen to be supplied in relation to the slightly lower amount that will actually be absorbed by that crop, in order to achieve optimal production without limiting factors. For an amateur gardener, it is not easy to estimate these two quantities of nitrogen, but we can still get a good idea of the amount of nitrogen to be applied by measuring the nitrate content of the soil using laboratory test strips

As with other nutrients, nitrogen requirements vary throughout the crop cycle. Determining these requirements makes it possible to identify the stages of high demand. For example, in spring, lettuce absorbs large amounts of nitrogen for about 40 days after planting, then absorption decreases until harvest. This nitrogen demand is lower in autumn (1). From sowing to peak demand, lettuce’s nitrogen requirement increases from 0 to more than 4 kg/ha in spring and from 0 to less than 3 kg/ha in autumn.

For carrots, nitrogen demand peaks in summer crops 70 days after sowing (which usually corresponds to August for outdoor crops) and then decreases very slightly. Over this period, the nitrogen demand of carrots increases from 0.5 kg/ha to over 3 kg/ha.

As soon as the temperature drops below 10 to 12°C, soil microflora activity decreases and mineralisation slows down significantly. In general, fertilisers with a fairly low nitrogen content should be used. Otherwise, there is a risk of toxicity, as plants require less nitrogen. When the temperature rises again, plants require more nitrogen and a synthetic fertiliser with a higher nitrogen content can then be applied to support growth.

As for potassium, for crops that are sensitive to a deficiency in this element and when the soil has sufficient nitrogen and phosphorus, potassium is applied before planting, especially in autumn-winter for perennial crops. Potassium is spread alone or together with phosphorus in the form of a binary fertiliser if phosphorus is also deficient

After planting crops, potassium can be applied in the form of potassium sulphate between rows using a hoe (also known as a cultivator) or a rake. Potassium sulphate can be found in some garden centres or online.

It is recommended to split the potassium fertiliser, for example for legumes that follow another crop such as winter garlic, at a rate of 2/3 in the autumn and 1/3 after the garlic harvest.

Examples of fertilisation are given in the following article: Examples of sustainable fertilisation for some vegetable plants.