What are Macronutrients?
Macronutrients are the six essential plant mineral nutrients which are required by crops in annual quantities of 10s or even 100s of kg/ha, compared with the micronutrients which crops require in grammes or a few kg/ha. Excluding nitrogen, two of the so-called primary macronutrients are Phosphorus (P) and Potassium (K) with the other three Sulphur (S), Calcium (Ca) and Magnesium (Mg) referred to as secondary nutrients. This separation into primary and secondary macronutrients in no way relates to their agronomic importance, all are equally essential, but to a distinction which was required for simple definitions in fertiliser regulations.
Functions and deficiency of:
Phosphate
Phosphate is a component of several essential cell components and specifically helps to promote root development and early flowering and ripening. Its role in root development is important for establishment, leading to a focus on early applications. Phosphate plays a role in the energy storage and transfer within a plant and is therefore important in the production of oil in oilseed rape.
A deficiency of phosphate can reduce both the above and below ground biomass, with a reduction in rooting having knock-on implications for the uptake of other nutrients from the soil. Although the nutrient is relatively immobile in soil, it is mobile within the plant, meaning deficiency symptoms show up on older leaves first. Symptoms appear as a reddish discolouration of leaves, due to a build-up of anthocyanins in leaves. For this reason, phosphate deficiency during the colder months can be difficult to identify in the field, as anthocyanins tend to be related to an increase in stress, which may be due to a number of reasons.
Potash
Potassium fulfils many vital functions in a wide variety of processes in plants, animals and man. It is required by plants for many uses, with the greatest quantity being required as an osmoticum in the ’sap’ in green tissue cells, where it maintains tissue turgidity. As green tissues senesce and the sap dries, the associated potassium is no longer required by the plant and is returned to the soil with the dead leaves and crop residues. Thus K is naturally recycled widely and in large quantities. Good soil reserves are an essential requirement for adequate nutrient supply of K to plants which commonly contain more potassium than any other nutrient including nitrogen.
Sulphur
The role of sulphur in all plants is in support of nitrogen in protein production which is hugely important for high yields and quality; it is an essential constituent of all plant and animal proteins. For breadmaking wheats good protein production is even more important because it benefits loaf volume. In Brassicas, (vegetables, oilseed rape (OSR), kale etc) sulphur is also in the glucosinolate compounds, which give them their hot taste. Sulphur also reduces the likelihood of immature rapeseed which faces penalty pricing in the market. This explains the greater requirement and importance of sulphur applications to Brassicas. Sulphur supply is also important for legumes such as beans, peas and clovers, to support the nitrogen fixed from the atmosphere.
Deficiency symptoms in Cereals, Grass and Brassicas show up in the younger leaves first. Symptoms are a pale yellow appearance (chlorosis) and later on as stunting. Much later OSR flowers will have pale yellow, or almost white, petals. Symptoms in some crops are easily missed and may not be noticed at all, especially cereals and grass.
Calcium
Calcium is responsible for proper plant cell division and strengthening cell walls giving them rigidity and strength. It also improves the absorption of other nutrients by roots and their translocation within the plant. It activates many plant growth-regulating enzyme systems, helps convert nitrate-nitrogen into forms needed for protein formation and contributes to improved disease resistance. It also plays a role in regulating various cell and plant functions.
Calcium deficiency causes deterioration and disintegration of cell walls and the collapse of tissues. Plant cells become leaky under calcium insufficiency, resulting in the loss of cell compounds and eventually death of the cell and plant tissue.
Magnesium
The main roles of magnesium in plants are in the formation of chlorophyll and of enzyme activators. Photosynthesis, protein formation and energy transfer all depend, in part, on an adequate supply of magnesium. It is taken up by plants as the ion Mg2+ and is mobile once in the plant, so can move from older to younger tissues. Deficiency therefore tends to be seen first in the older leaves when the concentration in the dry matter falls below 0.2% Mg.
An early symptom of deficiency is the loss of a healthy green colour between leaf veins, followed by yellowing (chlorosis), which starts at the leaf tips and margins and spreads until the entire leaf is affected. Symptoms can be confused with nitrogen or manganese deficiency but often are more mottled with darker and lighter green in cereals. Some plants, eg. strawberries, can develop orange or reddish colouring of leaves. Many cereal crops develop short-lived magnesium deficiency symptoms in early spring, but these often disappear and are not always followed by any effect on yield.