Introduction to integrated methods in the vegetable garden
Chapter : Fertilization
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⇒ Brief description of some mineral fertilizers.
Mineral fertilizers are sold either in a simple form or in a compound form comprising several major nutrients. For amateur gardeners, some of the mineral fertilisers listed below are available in garden centre shops, agricultural cooperatives or on websites.
Ammonium sulphate (NH4)2SO4 (or ammonia sulphate)
Among the simple mineral fertilizers, ammonium sulphate, which is 21% ammonium nitrogen and 24% sulphur, is mainly used in field crops and grasslands for plants with high sulphur requirements such as citrus, garlic, crucifers and potatoes. It is found in nature as mascagnite in the vicinity of fumaroles in Italy at the edge of Vesuvius and Mount Etna, in Spain, and all over the world. This fertiliser is a pH corrector because of its acidifying action, which allows the release of elements blocked in the soil, such as phosphorus. This fertiliser is therefore recommended for soils with a high pH above 8. In addition, the gradual availability of nitrogen limits losses through leaching. This fertilizer is very well suited for a single nitrogen application on soils where other major elements predominate.
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Calcium cyanamide, which contains 18 to 21% nitrogen and 60 to 70% lime, is sometimes used in market gardening by incorporation into the surface layer of the soil, which accelerates the transformation of this fertiliser into urea and ammonium. Its use is delicate, as it is also a phytosanitary product and requires spreading before the crops are planted to avoid stressing them. On contact with the soil solution, calcium cyanamide breaks down into ammonia and is then mineralised into nitrate by the soil microflora. This fertilizer has a good action against weeds and disease spores on a thickness of 2 to 4 cm. The application of calcium cyanamide is therefore interesting because of its phytosanitary action, especially against soil insect pests, and as a nitrogen fertilizer. It must be taken into account that it is a calcareous fertilizer due to the significant addition of lime (about 50%) and is therefore not recommended for calcareous soils. It can be used to sanitise composts by adding clay in order to produce CAC at the same time.
Calcium cyanamide is also known in gardening for rebalancing overly acidic soils, maintaining lawns and reducing the proliferation of slugs and various types of tipulas harmful to crops. Bags of 10 or 25 kg of calcium cyanamide are sold in garden centres and on some websites, sometimes with doses and application advice for each vegetable. Calcium cyanamide kills all organisms, whether pathogenic or beneficial, but does not produce toxic residues and is not persistent. It should be used at the last moment after other methods such as rotations have been tried.
Cyanamide; recommended doses and conditions of use.
Source: The properties of calcium cyanamide ; Société Royale Horticole de Gembloux
Pearl urea contains 46% nitrogen, making it the most concentrated solid nitrogen fertilizer. After spreading, urea is rapidly transformed by the soil microflora into ammoniacal nitrogen (NH₄⁺ cation, also known as ammonium) with a more or less significant loss in the form of ammonia (NH₃) through volatilisation, the ammonium is then transformed into nitrate. Ammonium has the particularity of being fixed on CAC. When spread, industrial urea has the same disadvantages as manure; the loss of nitrogen by volatilization of ammonia can be significant, especially if the urea is not sufficiently buried (but also depending on the characteristics of the soil, local climatic conditions, etc.). The price of urea is lower than that of ammonium nitrate.
In France, it is difficult for individuals to obtain small quantities of pearl urea from garden centres. There is no other solution than to contact an agricultural cooperative or to buy it on an internet site, often in the form of a 50 kg bag, enough to provide a supply of urea for a few decades. Another solution is to share the contents of a bag with friends and neighbours. Urea is very soluble, so it binds the moisture in the air to form a solid block. After extracting the required dose, an open bag should be quickly taped shut after expelling the remaining air in the bag.
The use of nitrogen fertilizers in the ammonia form are all sensitive to ammonia volatilization which represents a financial loss for the farmer and the amateur gardener with an uncertainty on the quantity of nitrogen brought to the plants which is not compatible with the rules of rational fertilization. Synthetic fertilizers based on ammonium nitrate (ammonitrate) are not very sensitive to volatilization despite the fact that half of the nitrogen they contain is in the form of ammonium ion. For this reason, they are more reliable for the environment than urea fertilisers and allow for greater precision in fertilisation as required by integrated agriculture. Their production at the factory results in a smaller carbon footprint. Ammonium nitrate is less acidifying than urea.
Farmers have access to two main types of ammonium nitrate; 33.5% ammonium nitrate (AN) contains nitrogen in the form of ammonium ion (NH₄⁺) and nitrate ion (NO₃-) in equal parts. Ammonitrate 27% (CAN) additionally contains dolomite or calcium carbonate. Since 27 June 2010, the sale of mineral fertilizers containing 16% or more nitrogen in the form of ammonitrate is strictly limited to professionals (farmers, horticulturists, foresters). Of course, this restriction, which I consider excessive, has been imposed for safety reasons (sensitivity to explosion) since the AZD accident in Toulouse on 21-9-2001, and obliges amateur gardeners to replace this excellent fertilizer with pearl urea. In reality, the danger of an accident is quite low; 33% ammonium nitrate passes the classic explosive tests satisfactorily (1).
Nitrate fertilisers are considered to be a booster and allow direct and highly targeted nitrogen inputs. Sodium nitrate from the Atacama Desert, also known as Chilean saltpetre, contains 16% nitrogen and 2.5% sodium, some boron and other trace elements. Sodium nitrate is now synthesised. Its constant and intensive use has the effect of destroying CAc, with sodium replacing the calcium that binds humus to clay. It is preferable to use potassium nitrate (also known as « nitrate de potasse » or saltpetre), which does not have this disadvantage and contains 13% nitrogen and 44% potassium.
Lime magnesia nitrate titrates to 15% nitrogen and contains 14% lime and 8% magnesia soluble in water.
Soluble calcium nitrate contains 15.5% nitrogen, of which 14.4% is NO₃ and 1.1% is NH₄ and 26.5% is CaO. It is an excellent cover fertilizer for vegetable and fruit crops.
There are simple potassium fertilisers to meet specific needs, especially when laboratory analyses reveal a deficiency in this element. Potassium sulphate, which contains 50% potassium oxide, potassium chloride (60%), potassium nitrate, already mentioned above, and patentkali, which is a double potassium sulphate containing 30% potassium oxide, 43% SO3 and 10% magnesium oxide, are some examples of potassium fertilizers.
Industrial phosphate fertilisers are all derived from natural phosphates, with the exception of slag from the dephosphorylation of steelworks. Natural phosphate reserves are estimated to last about 300 years at the current rate of exploitation (2). The industry uses several processes to make phosphoric anhydride (P₂O₅) available to plants. Single, concentrated or triple superphosphates are produced by treating rock phosphate with sulphuric acid or phosphoric acid or a mixture of both. Triple superphosphate contains at least twice as much phosphorus as rock phosphate (38-42% phosphoric acid). Superphosphates are partly soluble in ammonium citrate, whereas rock phosphates are insoluble. Alumino-calcic phosphate (phospal) is made from ore extracted from a mine in Senegal and 75% of this phosphate is soluble in ammonium citrate.
Superphosphates also contain trace elements from the rock phosphate used in their manufacture. Superphosphates are suitable for all crops and can be found in compound fertilizers. They are most effective in soils with a good CAC content and a neutral or slightly alkaline pH. For very calcareous or very acidic soils with a lack of phosphorus, phosphate fertilisers should be applied around the young roots, i.e. in the first ten centimetres of the soil. Phosphate fertiliser should not be applied to the surface after sowing, as the phosphates are not very mobile and will not sink sufficiently into the soil.
As far as sulphur is concerned, the requirements expressed in sulphuric anhydride (SO₃) are 175 to 200 kg/ha for crucifers and liliaceae, 100 kg/ha for legumes and 30 to 75 kg/ha for other crops. It is necessary to take into account the restitutions by the remains of crops, rainwater which brings 20 to 50 kg of SO₃ per hectare and per year. For a specific use one can spread calcium sulphate, phosphogypsum which contains 47% SO₃, or a foliar fertilizer, or the blue compound fertilizer or Binor sulphonitrate which contain sulphur.
Reasoned fertilisation implies providing each crop with a fertiliser containing the major fertilising elements: nitrogen, phosphorus and potash. This is the purpose of compound fertilisers whose ratio between the different elements is studied in order to correspond to the global needs of most vegetable plants. The way plants react to fertiliser depends on the ratio between the three elements N-P-K; N= nitrogen, P= phosphorus K= potassium.
Under the name of compound fertilizers, there are different mixtures of 2 or 3 elements to meet the needs of the farmer and the amateur gardener. The proportion of these elements is indicated on the packaging. For example, Binor's universal blue fertiliser (one of the best known in garden centres): 12,7,17, the ratio between the different elements is as follows: nitrogen =12, phosphorus =7, potassium =17.
The label sometimes indicates the proportion of another element that is essential for certain crops, such as magnesium and sulphur.
Compound fertilisers are not a must for vegetable crops if a laboratory analysis shows that one or more elements are predominant. In this case, the missing element(s) can simply be added with an industrial fertiliser, a correction that is usually impossible with an organic fertiliser.
For trace elements, in general, soils are sufficiently rich. However, if a laboratory analysis shows a deficiency, this can be corrected with a specific fertiliser containing most of the trace elements that plants need.
Fertilisers are commercially available for specific crops to correct a specific problem or to feed plants at an intense stage of their crop cycle. These fertilizers are often sold to be absorbed through the leaves (hence the name foliar fertilizers). For example, foliar applications containing phosphorus on potato plantations are very effective in increasing the number and size of tubers, especially when they also contain potassium, calcium, zinc and manganese, which are essential for this crop. Cucumbers often need a foliar fertilizer containing molybdenum.
1) Société chimique de france ; Sensibilité des nitrates à la détonation
2) USGS US Geological Survey 2016