To develop, crops need water, light, carbon, oxygen and mineral elements.Air provides oxygen and carbon dioxide, the source of carbon that the plant binds by means of photosynthesis.The soil is used as a reserve for water and mineral elements to feed plants; it is a true bioreactor harbouring a complex ecosystem. The soil recycles organic matter into mineral elements that can be used again by plants thus offering farmers a plentiful yield!
To get back to basics: What nourishes plants?
On a plot of land, 1 m² of cereals requires for its growth:
- 660 g of oxygen, 630 g of carbon, 90 g of hydrogen +
- 20 g of nitrogen (N), 8 g de phosphorus (P2O5), 25g de potassium (K2O) +
- 8 g of calcium (CaO), 6 g of sulphur (SO3), 4 g of magnesium (MgO) +
- Trace elements: 0.15 g of iron, 0.05 g of manganese, 0.05 g of zinc +
- 0.01 g of copper, 0.006 g of boron, 0.001 g de molybdenum +
- Several million bacteria and fungi, earth worms, crustaceans, etc.
The plant’s needs change during its lifetime. At each stage of its development it must find the required elements in an assimilable form in the soil solution (water + mineral elements). Fertilizers provide the soil nutrients.
For greater detail…
What do crops need?
Soil is a complex medium that consists of:
- organic matter,
- fine and very reactive elements (clays),
- coarse siliceous or calcareous elements,
- composites based on iron, aluminium, calcium,
water and air making up 50% of its volume
In farming, the soil plays an essential function for the nutrition of crops because: It holds the soil solution, it binds certain nutrients and it harbours microorganisms that will play their role in the transformation of elements that are not assimilable with elements that are not directly assimilable by the plants…
Every soil is a unique biotope with its specific physical, biological and chemical characteristics. The availability of nutrients depends on this.
Nitrogen (N): an amazingly indispensable element
Le potassium joue un rôle primordial dans la formation et le stockage des sucres. Il aide également la plante à résister au froid, à la sécheresse et aux maladies. Le potassium de la solution du sol est retenu par l’humus ou l’argile ; celui contenu dans les minéraux ne sera libéré que très lentement. Comme pour le phosphore, le cycle du potassium est dépendant des caractéristiques physiques et chimiques du sol cependant il reste toujours bio disponible. Toutes les cultures n’ont pas les mêmes besoins en potassium : les pommes de terre, les légumes en général et les betteraves sont plus exigeants que les céréales par exemple. Généralement l’apport en potassium est réalisé avant la plantation.Nitrogen is an element essential for photosynthesis enabling the transformation of mineral matter into plant tissue. Nitrogen is present in the air but plants, with the exception of leguminous plants (lucern, clover, peas, etc.), cannot absorb it in a gaseous state. In the soil, nitrogen is in an organic or mineral form (ammonium NH4+, nitrate NO3-). To be usable by plants (residues from previous harvests, organic fertilizers, etc.) must transformed into nitrates by the microorganisms present in the soil; this phenomenon is known as mineralization. It is mainly nitrates that provide plants their nitrogen-containing nutrition.The nitrogen cycle depends on weather conditions and on the soil’s microbiology. Nitrates are poorly held by the soil so they must be added or mineralization must be favoured when the plants are ready to absorb them in order to avoid their being leached into groundwater. Nitrogen is necessary for growth and quality that affects the protein level of plants.
Phosphorus (P): the quality of age prematurely!
Phosphorus is necessary for plant growth. It is present in soils in the form of phosphate: dissolved in water, bound on soil particles or in minerals or even in an organic form.
As and when the roots draw off the phosphate dissolved in the water, the bound molecules are gradually released. The phosphorus in an organic form is slowly mineralized. But these exchanges are very slow. The phosphorus cycle depends to a great degree on the soil’s physical and chemical characteristics.
Potato, vegetable and beet crops are those that require phosphorus the most. Quite often, short-term bioavailability is restrictive in the soil, as the phosphorus in the soil ages very quickly. Acidic soils rich in iron and unbound aluminium bind the soluble phosphorus very quickly like soils rich in Calcium and/or Magnesium.
Potassium (K): always ready!
Potassium plays a major role in the formation and storage of sugars. It also helps plants to be resistant to low temperatures, drought and diseases.
Potassium in the soil solution is held by the humus or clay; potassium contained in minerals will only be released very slowly.
In the same way as for phosphorus, the potassium cycle depends on the physical and chemical characteristics of the soil although it always remains bioavailable. Not all crops have the same potassium requirements: potatoes, vegetables in general and beet crops are more demanding than, for instance, cereals. In general the potassium contribution is achieved before planting.
The other elements: calcium, magnesium, sulphur, trace elements
The function of calcium and magnesium is mainly to improve soil structure; they are contributed in the form of soil amendments.
Sulphur is necessary for the synthesis of proteins; it is added by certain fertilizers in the form of sulphates. These are known as cruciferae (cabbage colza, etc.) and garlic, onions and leeks that have considerable sulphur requirements. It can be introduced in other less oxidized forms that are subject to Marketing Authorizations like Agrifix or Secofit. Additional agronomic properties shall then be demonstrated.
The trace elements (copper, manganese, zinc, boron, molybdenum, iron, etc.) participate in very small quantities in plant nutrition. However, a deficiency of any one of these elements can interfere with growth. The said deficiencies can be caused either by a content insufficiency or by the unavailability of the element.
Sulphur in good shape
SO2, H2S, SO4, S2O2, or even S…Sulphur is present in numerous forms in Nature! It is present in the air in the form of sulphur dioxide (SO2) and hydrogen sulphide (H2S), in rainwater and above all in an organic form in crop residues and animal waste, as well as in the soil.
In the soil, some sulphur can be found in the form of sulphur (S-), elemental sulphur (S) and sulphate, etc. Sulphur is everywhere. We are following up on our first circular on sulphur in agriculture.
Why should you correct a sulphur deficiency?
Oh yes, we almost forgot: Sulphur is indispensable for protein synthesis!Plants therefore have a basic need for it throughout their cycle including for the constitution of reserves for seeds!
But it is in its form containing a sulphate ion (SO42-) that plants assimilate it.
How do you correct a sulphur deficiency?
The spreading of fertilizers containing sulphur releasing the above form “with immediate effect” should be favoured. Extensive crops – colza, straw cereals, beet, potatoes but also vegetable plants – respond “well” to such contributions. “Sulphate of ammonia”, “potassium”, “superphosphates”, “solutions with nitrogen and sulphur content” or indeed “kieserites” of numerous sulphur-containing fertilizers are currently marketed. But “Careful! As a supplement to Cetiom, the contribution of sulphur in its S form is costly and its direct nutritional action is poor”. In cases of deficiency, Arvalis also advises against the application of wettable sulphur of the “Thiovit” type, “with its slightly longer time of action on ascertained deficiencies.”
Manure together with other organic amendments are another possible form of sulphur contribution. This type of fertilizer enables longer assimilation of sulphur. Depending on its animal origin, manures can contain between 1 and 3 kg/tonnes of sulphur, poultry waste offering the most plentiful sulphur content (nitrogen also).
For cereals, “The addition of sulphur is not beneficial on plots of land that have been spread with manure regularly for more than 20 years, except when the Spring follows very wet winters in high risk soils”, is the advice of Arvalis. “it is not always present in sufficient quantities for very productive crops, is the warning, on the other hand, put out by the “fertilizer” department of Ontario Canada’s ministry of agriculture. However, the effect of the contribution of commercial sources of sulphur on crops is less obvious when manure is spread regularly”.
The SO4 forms are very sensitive to leaching and losses are significant when rainfall between November and February exceeds 350 mm.