The reasoning behind fertilisation in the vegetable garden

The value of covering the soil with organic fertiliser

Vegetable farms are known for their particularly intensive farming systems. Some vegetable crops such as radishes, lettuce, turnips, beetroot and others are grown in succession on the same plot of land, which results in significant mineral exports that must be determined and corrected if deficiencies are to be avoided. All the major elements are concerned, including trace elements whose reserves can be rapidly depleted.

Industrial complete fertilisers are mainly intended to supplement the need for nitrogen, phosphorus and potassium. Some industrial fertilisers also contain boron, magnesium and even a few other elements. However, many vegetable plants are sensitive to one or more deficiencies of manganese, molybdenum, iron, copper, and others. It is well known that a good cover of organic fertilizer is able to provide all these elements. But not just any organic fertilizer. The more a cultivation soil is amended with properly prepared compost, the more micronutrient deficiency is avoided.

Organic bottom dressing and assessment of the nutrition status of vegetable plants.

The global needs of each vegetable crop have been defined in some studies by organisations such as CTIFL for the most important elements, notably nitrogen, phosphorus and potassium. These studies show strong disparities between each vegetable crop. Some plants such as aubergine, courgette, spinach, leek, potato and tomato are high consumers of nutrients. For a given type of vegetable, there are also major disparities between certain varieties, more or less modulated by the growing season, the nature of the soil, 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 a hobby gardener, who generally has a small area, these deficiencies can be avoided by providing at least 100 kg/are of compost, which the gardener can still acquire and/or produce. However, the assimilable form of nitrogen disappears rapidly when absorbed by the plants or by leaching, which requires frequent monitoring and, if necessary, corrections to avoid a deficiency in this element (see the chapter: the problem of nitrogen assimilation in organic farming).

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

In organic farming, synthetic fertilisers are banned, so the farmer only applies organic fertilisers in the hope of satisfying all the plants' mineral needs, which is an impossible task. Nitrogen management is the main problem. Nitrates are progressively released, whatever the needs of the plants, even after the harvest. When nitrogen demand is high, organic fertiliser is not able to meet it, and when nitrogen demand collapses, nitrates from organic fertiliser are lost. As nitrates are very soluble in water, nitrate losses are inevitable. Any rainfall will wash these nitrates into the water table. With an organic bottom dressing used as the only source of nitrogen, depending on the volume applied, either there is an excess of nitrate production with a risk of toxicity when the vegetable plants are not in the maximum nitrogen assimilation phase, or a nitrogen deficiency occurs when the plants need it most.

The role of organic and mineral fertilizers in integrated agriculture.

Micro-organisms play an essential role in the fertility of a crop soil. There are several billion bacteria and several million fungi in one gram of soil. This population of micro-organisms contains between 10,000 and 100,000 (2) different species, many of which have characteristics that are still poorly understood. Bacteria and fungi are the only organisms capable of transforming organic matter into mineral matter that can be assimilated by plants. Bacteria are also responsible for fixing atmospheric nitrogen. Without micro-organisms, 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 (especially soil type) and entropic factors. In general, there are fewer bacteria in cultivated soils when organic inputs are not sufficient.

Reconstructing and maintaining the microbial flora is therefore fundamental. To this end, organic fertilizers are essential, but also the use of certain agricultural practices such as plant cover, the reduction of synthetic or organic pesticides when they have a negative impact on soil biodiversity, the use of amendments to improve the structure and texture of the soil as well as the pH level...

The composition, quantity and manner in which organic fertiliser is applied should meet three objectives:

En integrated agriculture, apart from potassium, phosphorus and other elements, an organic fertilizer aims to meet the minimum nitrogen requirement of vegetable crops, with the remainder being added as nitrogen requirements increase during the crop cycle. We do not try to make a nitrogen reserve only with organic fertilizers, because the latter present a major defect; they are made of various nitrogenous compounds whose mineralization is more or less long and very difficult to predict.

Organic fertilisers are often supplemented by a complete N.P.K. fertiliser to optimise the fertility of the soil, particularly in terms of potassium and phosphorus. The more potassium and phosphorus rich compost is used, the less synthetic N.P.K. fertiliser is needed (unless the reserve of these two elements in the soil is sufficient as demonstrated by a laboratory analysis). Thus, when the soil is well supplied with compost, except in exceptional cases, corrections during the crop cycle mainly concern nitrogen.

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

There are commercially available nitrogen-rich organic fertilizers containing dried blood, marine guano, feather meal and bone meal (e.g. N.P.K. 11-4-3) approved for organic farming. They can be very useful, especially at the beginning of the plantation. However, as they also contain phosphorus and potassium, their use to reduce a specific nitrogen deficiency is not always adequate. The use of this type of fertilizer should be avoided if the soil is too rich in phosphorus and/or potassium as shown by a laboratory analysis. Soils grown organically are often unbalanced in potassium and phosphorus. Many amateur gardeners are surprised when they discover by a laboratory analysis that their soil is too rich in certain major elements such as phosphorus (often fixed in a form that cannot be assimilated by plants), or another element whose excess causes an induced deficiency.

Synthetic fertilisers; an unjustified aversion.

Synthetic fertilisers are often disliked on the grounds that they are harmful to the environment (soil, groundwater and river pollution, ammonia volatilisation, etc.). These negative effects are in fact the consequence of bad uses of these products, rather than their intrinsic properties (notably the absence of a strategy specifying the inputs in relation to the evolution of needs).

These fertilisers are also criticised for being too fast. In the case of medium-term fertilisers, the nitrification process by bacteria from ammonium does indeed take between one and several weeks, depending on the soil temperature and humidity. However, this process from ammonium also exists for organic fertilisers. As with organic fertilisers, synthetic fertilisers must be used according to a 'code of good agricultural practice', a classic expression often found in texts dealing with agriculture. Synthetic fertilisers are indeed faster than organic fertilisers, so it is sufficient to split the applications according to the needs of the plants throughout their crop cycle to avoid nitrate losses through infiltration. Nothing complicated in the end for the amateur gardener.

When a soil is well supplied with CAC, if the plants do not absorb all the non-nitrogenous mineral salts supplied by fertilisers, a large part of the surplus is fixed by these CAC. Plant roots draw on these reserves as and when they are needed. Losses are therefore very small, except for nitrate, which is not fixed by CACs. This is also the reason why nitrogen fertilizer should not be applied as a basal fertilizer.

Contrary to the widespread belief in some spheres of non-scientific agro-ecology that soils are depleted by the high yields provided by synthetic fertilisers, the opposite is true. Increased yields in turn improve the fertility potential of soils, simply because they produce more surface organic residues and subterranean litter to be recycled. Soil depletion (especially of carbon) occurs when organic residues are no longer recycled. This is particularly the case when the farmer sells his crop residues (especially straws) for financial reasons to other farmers or breeders.

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

Vegetable plants have intense and instantaneous nutrient requirements during certain periods of their crop cycle. For example, tomatoes need more potassium during flowering and fruiting. Other crops are sensitive to phosphorus, such as seed vegetables, while others require more nitrogen, such as leaf vegetables. It is well known that if you want to encourage the rooting of young plants, the soil must be rich in assimilable phosphorus.

The aim of fertilisation for each vegetable crop is to avoid global and instantaneous deficiencies that could appear during their nutrition. Chemical fertilisers, especially foliar fertilisers, have the advantage of offering very precise compositions of mineral salts to meet the varied needs of plants and correct deficiencies that might appear 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, some vegetables, such as cucumbers, cannot tolerate a fertiliser that is too high in ammonium nitrogen. The form of nitrogen supplied during the growth of certain plants is therefore also of great importance, which is taken into account in integrated agriculture.

Nitrogen, potassium and phosphorus management during the crop cycle.

Plant nitrogen nutrition is one of the most important production factors. In case of nitrogen deficiency, losses can be as high as 90% of the expected production. In principle, the nitrogen requirement of a crop is defined as the amount of nitrogen to be supplied in relation to the slightly smaller amount that will actually be taken up by the crop, in order to achieve an optimal production target without limiting factors. For a home gardener, it is not easy to estimate these two quantities of nitrogen, but a good idea of the amount of nitrogen to be supplied can be obtained by measuring the soil nitrate with laboratory test strips.

As with other nutrients, nitrogen requirements vary throughout the crop cycle. Determining these needs allows us to locate the stages of high mobilisation. For example, in the spring, lettuce has a high nitrogen uptake for about 40 days after planting, and then it decreases until harvest. This nitrogen demand is lower in autumn (1). From semi to peak, the nitrogen demand of lettuce 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 is highest in the summer crop 70 days after sowing (which is usually August for outdoor crops) and then decreases very slightly. Over this period, the nitrogen demand of carrot increases from 0.5 kg/ha to more than 3 kg/ha.

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

As for potassium, for crops sensitive to a deficiency in this element and when the soil is sufficiently supplied with nitrogen and phosphorus, potassium is applied before the crop is planted, especially in autumn-winter for perennial crops. Potassium is applied alone or together with phosphorus in the form of a binary fertiliser if phosphorus is also deficient. After planting the crops, potassium can be applied as potassium sulphate between the rows by using a hook (a tool still called a "cultivator") or a rake. Potassium sulphate can be found in some garden centres or on the internet.

It is recommended to split the potassium fertilisation, for example for leguminous crops that follow another crop such as winter garlic, 2/3 in the autumn and 1/3 after the garlic has been harvested.

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

1) Ctifl - 31-7-2012 ; Elément de décision pour une fertilisation raisonnée en azote sur les cultures fruitières et légumières
2) Ranjard et all 2010 – signalé dans l’Atlas français des bactéries du sol.