Examples of rational fertilisation for some vegetable plants.

The model of reasoned fertilisation described below is based on the principle that, apart from nitrogen, the quantity of all the elements absorbed by the plants (including trace elements) is generally sufficiently provided by the incorporation of bottom dressing comprising mull-type compost of the order of 80 to 100 kg/are supplemented by 2 kg/are of Binor 12.12.17 blue fertiliser (which contains magnesia and boron). This complete fertilisation is not carried out if a laboratory analysis shows a surplus of phosphorus and potassium. The addition of compost is designed to compensate for the natural loss of humus (approx. 40 kg for 30 cm depth of worked soil) and the loss of humus produced by crops during the year. Additional nitrogen is usually required during the crop cycle in the form of mineral fertilisers. Nervous organic fertilisers (such as blood powder), which can produce an imbalance in fertilisation, are not used (as they do not allow for a single supply of nitrogen), especially as their cost is excessive.

Before planting, one can incorporate an organic fertilizer such as castor oil cake (N=5,P=2,K=1) which has the advantage of not acidifying the soil (Organic fertilizer - NF U 42-001 - usable in organic farming in application of RCE N°834/2007). Caution: castor oil cake is very toxic to humans and mammals because of the ricin it contains. Plants are not affected by this poison (a). Castor oil is an interesting asset in the garden as it scares away moles, mice, field mice and voles. It also has a repellent effect on white grubs, moles and slugs. However, it is not known whether it is toxic to earthworms (there are not enough scientific studies on the degradation of ricin and its metabolites in the soil). There are preparations of detoxified castor oil cake used as animal feed. It is advisable to spread castor oil cake more than one month before planting. Because of its toxicity, gloves are essential.

The quantity of organic and mineral fertiliser is incorporated into the soil of each bed according to the nature of the crop it will support (e.g. leeks require more bottom dressing), i.e. for mineral fertiliser, about 2.5 times less than the dose recommended by the manufacturer (recommended dose without the addition of organic fertiliser). This fertilisation model takes into account the observations made in the previous article. This volume of compost is also intended to compensate for natural humus losses (see article: Estimation and correction of humus losses). Nutrient reserves of the main major elements are monitored by laboratory analysis every 3 to 5 years, except for nitrogen, which evolves more rapidly and is measured in the soil during the growing season with inexpensive laboratory equipment. A mineral correction in phosphorus and/or potassium during the growing season is sometimes undertaken for certain very greedy vegetables.

Depending on the crop species and the measurements taken during the growing season, the nitrogen requirement is supplemented with pearl urea, Binor sulphonitrate and potassium nitrate. Urea and sulphonitrate are broken down within a week in summer. The volume of irrigation water is controlled to prevent the nitrate reserve in the soil from leaching out, but this cannot be avoided in the event of heavy rainfall, which requires frequent analysis of the nitrate content of the soil to determine when it should be replenished.

As for pearl urea, which is not always easy to obtain for the amateur gardener, it should be noted that this mineral fertilizer has the same disadvantages as organic fertilizers. Urea is subject to a greater or lesser loss of ammonia depending on the way it is used and the properties of the soil. It should not be surface applied. It is incorporated either by hoeing in 5 to 8 cm or by watering abundantly after dissolving in a watering can. It is preferable to replace it with ammonitrate when available or sulphonitrate.

Duration of urea hydrolysis and nitrification as a function of outdoor temperature

Source : Vilsmeier and Amberger (1980-1984) ; COMIFER – Calcul de la fertilisation azotée ; guide méthodologique pour l’établissement des prescriptions locales -Edition 2013.

a) The median lethal dose (LD50) of ricin is about 22 micrograms per kilogram of body weight if exposure is from injection or inhalation (2 milligrams for an average adult). Oral exposure to ricin is much less toxic, as some of the poison is inactivated in the stomach. An estimated lethal oral dose in humans is about 1 milligram per kilogram. Five to 20 castor beans are considered fatal to humans.

The reader of these lines may be tempted to reduce the volume of compost proposed. This is the volume recommended on some bags of approved organic fertiliser sold in garden centres, often enriched with nitrogen by incorporating manure such as chicken droppings. A reduction in the amount of organic fertiliser applied without being compensated by an increase in the amount of compound mineral fertiliser may result in a significant drop in yields and the appearance of diseases due to the depletion of major elements in the soil, particularly phosphorus and potassium. In organic farming, yield losses have been observed in field cereals when phosphorus exports are not or insufficiently compensated (1).

In the event of a sudden drop in available nitrogen and if the soil's potash reserves are not high, potassium nitrate is suitable for an instant and precise supply of nitrogen without passing through the soil microflora. Potassium nitrate is consumed immediately by the leaves and roots. Solid potassium nitrate (potassium nitrate or saltpetre) is available on some websites or in liquid form in garden centres or agricultural cooperatives.

For each vegetable crop, the mineral requirements vary according to local climatic conditions, the nature of the soil, the growing season, the way in which the crop is protected from the weather, etc. It is also necessary to take into account the new varieties that appear every year and which do not necessarily have the same mineral requirements as older varieties. The figures given below concerning mainly the nitrogen supply are therefore purely indicative and concern only field crops.

Garlic

Humus-poor soil often results in the production of tiny heads. For winter garlic, potassium must be added at the end of the winter, otherwise there is a risk of stunted cloves that will have difficulty keeping.
In March and after the end of the heavy frosts, we start the fractional contributions with small doses of nitrogen and potassium according to the 1/1 ratio (mixture of potassium nitrate + sulphonitrate every 15 days). The dose for each application is about 0.100 kg/are.

Be careful, too much nitrogen produces more foliage and reduces the volume of the heads. 2 to 3 weeks before harvest, some growers tie the foliage so that the rising sap (from the roots) accumulates in the heads, which will then increase in volume. Winter garlic is pulled up in May at the first signs of wilting to prevent the bulb from becoming mouldy.

Aubergine

Beetroot

In the Mediterranean region, the Egyptian red beet, Sultan race, resists well to the seed set during the summer period. I advise sowing in spring, as this vegetable plant fears high temperatures which stop its growth. It is an early variety producing large, fragrant roots without cavities. It keeps well in the fridge in plastic packaging (several weeks). Cooking for large roots; 30 mm in the pressure cooker.

Cardoon

Prefers humus-rich silico-clay soil. Excessive limestone hardens the cardoon. Requires potassium to be introduced at the beginning of the crop (organic or mineral). Nitrogen is then added in the form of urea pearl 3 times during the growing season. Dose 0,200 Kg/are.

This vegetable plant of Mediterranean origin is less sensitive to seed set if sowing is done in early spring after the last frosts. Its growth slows down in the hot season and becomes more vigorous at the end of the summer.

Carrot

About 100 days after the semi, potassium is applied (unless the soil already contains sufficient potassium) to promote root growth, for example in the form of potassium nitrate. Dose 0.010 kg/are (about 3 g potassium nitrate in a bag dissolved in a 12 l watering can for 5 m²). This dose is repeated every 2 to 3 weeks after checking the condition of the roots.

Too much nitrogen increases the production of split roots. Carrots are sensitive to potassium, but too much quickly produces an induced magnesium deficiency. A brown colour of the roots indicates a boron deficiency. Too much straw manure results in forked roots.

Celery root and celeriac.

Chlorosis of old leaves is a disease that is often found in celery. It starts at the top of the leaves and spreads between the veins. Later, brown necrotic spots appear and the leaves eventually fall off. The disease is most often caused by a magnesium deficiency, but it can also be induced by an excess of calcium or even potassium. For some varieties, it is excess sulphur that causes leaf chlorosis. A soil laboratory analysis is the only way to know the origin of this disease. If there is a magnesium deficiency, this can be corrected by spraying with a 1 to 1.5% solution of magnesium sulphate (this product is also known in sports circles for its muscle-relaxing properties), which can be purchased from websites, agricultural COPs and some garden centres.

An effective product is "Algoflash anti-browning conifer care" (conifers are also very sensitive to magnesium deficiency). Some growers use this solution every year as a preventive measure. It is also possible to incorporate a bottom dressing before planting, such as Binore blue fertilizer, which contains magnesium.

Leaf chlorosis can be confused with early wilting caused by a heatwave. Celery will produce new leaves as soon as the heatwave is over, provided it is well watered. Head size will often be smaller.

Fractional applications of non-calcium-bearing nitrogen (such as potassium nitrate) before root balling (celery head formation) increase yields. The total dose is about 0.130 kg/are divided in 2 times. After head formation, nitrogen has an adverse effect. If too much nitrogen is used, rust-coloured spots appear on the roots. Potassium chloride (2 kg/are), when available, would also give good results.

Celery is very sensitive to boron deficiency, which is not easy to manage (it manifests itself mainly in reduced growth). The level of boron deficiency in the soil must be determined before cultivation by a laboratory analysis, as excess boron is also toxic. Boron deficiency during the crop cycle can be corrected with foliar fertiliser. Binore blue fertiliser, which contains boron, is very suitable for this type of crop.

Cucumber

Cucumbers are also sensitive to iron, magnesium and manganese deficiencies. It does not tolerate molybdenum deficiency at all. Cucumbers should therefore be grown in slightly alkaline soil, which favours the absorption of molybdenum.

Within 15 days of the first leaves appearing, the cucumber must develop a large root clump with a radius of 50-60 cm, without which it will not be possible to produce much fruit over a long period. This period of root formation is crucial and requires additional nitrogen fertilisation in the root zone after checking the soil for nitrates with a test strip.

Molybdenum can be supplied in a complete liquid fertiliser for pot plants or garden plants (including several trace elements; available in garden centres and on the Internet, e.g. by clicking here) or in a liquid fertiliser including boron, available from agricultural suppliers or on the Internet. Molybdenum should be applied systematically in cucumber crops as a preventive measure.

Cucumbers require a lot of fertiliser if they are to produce fruit for 4-6 months, so regular nitrogen applications are needed every two weeks (e.g. Binor sulphonitrate, potash or lime nitrate, soluble calcium nitrate), i.e. about ten applications during the growing season. Approximate dose of nitrogen for each application: 0.100 to 0.150 kg/are. Potassium and phosphorus applied as a bottom dressing before planting should be sufficient. A high potassium application does not improve yields.

Courgette

Shallots and other edible onions

Excess nitrogen favours the development of the foliage to the detriment of the bulbs which become susceptible to mould. During the growing season, apply only 1 to 2 times mineral nitrogen for a total of 0.4 to 0.55 kg/are.

Shallots should be harvested in July before they are completely wilted and after checking the condition of the bulbs. If one waits too long, mould will set in the bulbs. The shallots are placed on a rack in the open air, protected from moisture and sunlight, until the foliage is completely wilted. The tops are then collected and placed in a compost bin. In this way, any risk of soil-borne diseases is greatly reduced, and the shallots will be well nourished in a soil that is still well supplied with humus from previous years and maintained by rotation. This cultivation technique should be adopted for all consumable bulbs such as Mulhouse onions.

Bean

Apart from the fact that it is susceptible to viruses brought by aphids, the bean is known for the peculiarity of its roots where nodosities shelter nitrogen fixing bacteria. However, a nitrogen deficiency can occur at the beginning of a spring semi, as the soil is not sufficiently warmed up for the nitrogen fixing bacteria to develop in the nodules. Beans are very sensitive to zinc deficiency. It is sensitive to copper and molybdenum deficiency. It does not like calcareous soils prone to manganese deficiency. All these deficiencies can be avoided by adding well-decomposed compost before planting and by not planting during the cold season. Potassium is a determining element in bean cultivation. Unless the soil is already well supplied with potassium, it must be added during the crop, e.g. in the form of potassium nitrate.

Lettuce

In the Provence-Alpes-Côte-d’Azur region, during the summer season, the cultivation of lettuce is problematic because of its very short crop cycle, which quickly leads to a rise in seeds. To avoid this inconvenience, it is necessary to adopt the cultivation method used by market gardeners, which is to renew the plantations every week. For the summer season, heat-resistant varieties such as lazy blond lettuce should be chosen. The harvest period lasts only a few days for each planting. A home gardener must calculate his consumption of lettuce over a week in order to plan the number of plants, taking into account possible losses due to slug attacks, botrytis, hail, etc. The periodic renewal of plants allows for production until the frosts. It should be noted that in large-scale market gardening, lettuces are often transplanted onto a plastic cover to reduce weed growth. This process avoids mechanical or chemical weeding, and saves time and money for the market gardener.

Melon

Melons are grown on particularly well aerated soil in order to obtain deep roots. A very deep ploughing is even recommended, of the order of 40 to 50 cm or more. It is a plant that needs frequent watering and is very sensitive to iron, magnesium and molybdenum deficiency. It is also sensitive to phosphorus deficiency, resulting in a drop in production of around 50%. For this reason, some vegetable growers apply soluble phosphate during the crop cycle. Poor nitrogen supply results in flower drop and loss of production. An excess of nitrogen also produces a loss of yield.

In the Provence-Alpes-Côte-d’Azur region, open field cultivation by market gardeners and farmers is noted in the spring by the use of plastic tunnels and plastic mulches. This technique has the disadvantage of reducing the possibility of making additional nutrient inputs. For this reason, and also because it is very suitable for melon cultivation, fertilisation is mainly carried out before planting. Several studies have shown that a large amount of compost before planting is very favourable to the melon crop, provided that the compost is ploughed in 4 to 6 months before planting to avoid the transmission of telluric diseases. A good result can also be obtained with a mineral fertilizer applied in the spring before planting. In the middle of the crop cycle, a small dose of nitrogen is useful, the dose being 0.060 to 100 kg/are.

Turnip

Parsley

Parsley seeds are frequently taken by ants and this is often the main cause of a failed sowing. The seedling should therefore be protected around the edges with an ant-repellent insecticide containing, for example, a pyrethroid.

Leek

Leek cultivation is fairly easy provided that the soil is rich in nitrogen, phosphorus and potassium, in the order of 1.5 kg/are to 0.6 kg/are for these two major elements. If the soil is sufficiently rich in potassium and phosphorus, 60% of the nitrogen (urea or ammonium nitrate) is supplied in 2 to 4 applications after transplanting. Total dose: about 0.250 kg/are.

Pear

Also known as chard, pear is fairly easy to grow and is not very sensitive to deficiencies in iron, magnesium, etc. However, it consumes a lot of nitrogen and especially potassium. An application of potassium nitrate during the summer season is often very beneficial. As long as the soil is well supplied with potassium and phosphorus following an organic and mineral fertiliser, only the nitrogen is applied in 3 times every 2 weeks over a period extending until the first frosts in autumn. Approximately 0.3 kg/are.

Pea

It has a fairly short vegetative cycle and is heat averse. Like the bean, its roots have nodules filled with bacteria that fix nitrogen. In case of deficiency, and although peas are not very demanding in terms of phosphorus, the latter is applied before sowing. In general, peas do not require much iron and magnesium. A soil that is well supplied with potassium and phosphorus following a compost application in the autumn does not need any additional application of these elements during the pea crop cycle.

Pepper

Potatoes:

The dry climate of the Provence-Alpes-Côte d’Azur region is favourable to the cultivation of potatoes, which fear excess humidity favouring mildew attacks. It is easier in the Provence-Alpes-Côte d’Azur region to control the amount of water by irrigation just necessary to avoid mildew. Even in the Provence-Alpes-Côte d’Azur region, potatoes can fall victim to late blight caused by poorly controlled irrigation on heavy, compact soils.

Potatoes like a light and especially sandy soil; ideal conditions that can be found in particular on the island of Noirmoutier, renowned for the early cultivation of the "bonnotte", picked by hand before maturity (90 days after planting). In this Breton island, the varieties "Sirtima", "Iodéa" and "Lady Christ" are also grown. In order to obtain a taste quality comparable to the various potato varieties cultivated in Noirmoutier, the structure of a heavy soil must be improved by the addition of amendments rich in sand, but not only. The soil must be well supplied with humus, ensuring a good reserve of assimilable phosphorus and rich in trace elements.

Potatoes are very demanding in terms of phosphorus, which must be applied before planting or ridging; for example, solabiol rock phosphate containing 29% phosphoric anhydride for shrubs and vegetables if the reserve of assimilable phosphorus in the soil is not sufficient. An application of sulphate of potash improves the resistance of the plant, which will produce larger and more abundant tubers. All tuber varieties are sensitive to iron, zinc and manganese deficiency and very sensitive to boron deficiency. Deficiencies of these trace elements are often the cause of poor yields and diseases that are exacerbated by lack of rotation. A good quantity of compost introduced in the autumn would reduce rhizoctonia attacks.

In conventional field crops, the inputs are about 100 units of nitrogen, 150 units of phosphate and 150 units of potash just before planting in the form of 11-15-15, i.e. 10 qx/ha. At the beginning of the crop, ammoniacal nitrogen applications such as urea are very suitable. The dose is about 0.150 kg/are. If nitrogen is essential to ensure tuber growth, an excess of nitrogen favours the development of vegetation to the detriment of tuberisation. An excess of nitrogen also produces an increase in reducing sugars in the tubers, resulting in browning when fried. It is therefore necessary to carry out a nitratest before deciding on a nitrogen application.

The tubers are placed in a furrow that is ploughed with a hoe or a tool with a ploughshare to a depth of 3 to 5 cm (in large-scale cultivation a planter is used). Each furrow is then covered by a light ridging so that the tubers are at a depth of 12 to 15 cm. One to two ridging operations are necessary during the growing season. The last ridging is done at the latest when the vegetation reaches 15 to 20 cm in height.

Radish

Tomato

Phase 1: From planting to the setting of the first fruits, tomatoes consume few nutrients. This is also the period of root formation. A supply of soluble phosphorus (in the form of foliar fertiliser) produces well-filled roots. Tomato stems are fragile because they contain little dry matter. Their cuticle is fragile, which can favour the first attacks of bioaggressors. For this reason, some growers spray the plants with Bordeaux mixture as a preventive measure before transferring them to the field.

Phase 2: This lasts 9 weeks and corresponds to the formation and harvesting of 80% of the fruit, with the consumption of a lot of nitrogen and phosphorus. A large part of this phosphorus is found in the fruit. The consumption of potassium is very high. Calcium and magnesium uptake is low.

The concentration of nitrogenous matter is high in the first fruits and then gradually decreases. Foliage with light green shades indicates a nitrogen deficiency.

Tomatoes respond well to sulphur applications. Successive applications of Binor sulphonitrate and potassium nitrate are very useful.

During this phase, the stems of the tomatoes become stronger and more resistant, starting with the oldest stems; a development that can be seen when pruning the suckers when they have become too large. Total nitrogen dose: about 0.150 kg/are.

Phase 3: This phase, which lasts about 5 weeks, is characterised by an increase in the consumption of nitrogen, magnesium and calcium and a reduction in the consumption of potassium. The stems continue to grow stronger. Ammoniacal nitrogen applications are useful. Nitrogen dose, about 0.200 kg/are.

Phase 4: The plant is exhausted and enters its "third age" with a decrease in fruit production. In the open field, fungal diseases begin to set in. Nitrogen consumption is practically zero. On the other hand, calcium consumption is very high. Phosphorus consumption is normal, but potassium and magnesium consumption is very low.

fund fertilizer: Because of the high consumption of phosphorus and potassium during certain periods of the vegetative cycle, a mineral fertiliser rich in these elements complements the compost applied in the autumn in spring.

1) Bilant de 12 ans de suivi de la fertilité du sol ; ARVALIS-infos.fr :