Plant nutrition - the essentials

In order to grow and flourish, plants need some 20 mineral nutrients taken from nature. Discover more here.

Background to plant nutrition

In addition to carbon, hydrogen and oxygen, the building blocks of life itself provided by water and carbon dioxide, most plants require some seventeen mineral nutrients for normal growth and function. These are traditionally and conveniently divided into a number of generally accepted groups, ie. major, secondary and micronutrients together with the so called functional elements. Micronutrients are also known as minor nutrients or trace elements and are laid out on the familiar Periodic Table below.

Nutrient grouping is based on the relative amounts required by plants, ranging from 1 unit of molybdenum to 150 thousand units of nitrogen. The table below gives typical dry leaf tissue concentrations (ppm) for various nutrients.

Of course the nutrient quantities needed by plants have no bearing on their relative importance. All are equally vital and any deficiency can have equally serious consequences.

 Major and secondary nutrients

There are three major nutrients that are essential to plant nutrition: Nitrogen (N), Phosphorous (P) and Potassium (K). They are described as major (or sometimes macro) nutrients because they are taken up by the plant in relatively large quantities.

By convention, in the UK and many countries, instead of elemental P and K these elements are referred to as their oxides. Instead of phosphorus we refer to phosphorus pentoxide (P2O5) and instead of potassium we refer to potassium oxide (K2O). The benefit of using oxides is that the numbers become a little more manageable.

Secondary nutrients are required in lower quantities than the major nutrients, but the quantities are still significant, and certainly greater than the micronutrients.

There are three secondary nutrients: Calcium (Ca), Magnesium (Mg) and Sulphur (S).

Nitrogen (N)

Nitrogen is required to make protein in plants and is associated with chlorophyll; hence plants with plenty of nitrogen have leaves which are a dark green colour, whilst plants which are deficient in nitrogen look pale green or yellow. Plants need lots of nitrogen when they are growing actively and making lots of new tissues (vegetative growth).

Whilst total nitrogen is always described as elemental nitrogen (N), there are in fact several forms of nitrogen since the element is presented in different oxidation states as it moves through the dynamic stages of the nitrogen cycle (where atmospheric nitrogen is cycled through various organic and mineral forms before ultimately returning to the atmosphere). Four forms of nitrogen are of importance in the nutrition of plants.

Nitrate nitrogen (NO3-N) is the most readily available form of nitrogen for uptake and utilisation by plants. It is also the form of nitrogen that is easily leached out of soils or growing media by rainfall or irrigation.

Ammoniacal nitrogen (NH4-N) can be taken up by plants directly, but since it is rapidly oxidized by bacteria to the nitrate form (the nitrification process), it is usually in this nitrate form that it is taken up by plants. Ammoniacal nitrogen does not leach from soils.

Ureic nitrogen (Ureic-N) has to be converted to ammoniacal nitrogen (and on the nitrate nitrogen) by bacterial activity before it can be utilised by the plant. There can be some losses in this process, e.g. volatilisation to the atmosphere, depending on the environmental conditions. The speed of this conversion is also dependent upon environmental conditions. For this reason urea is sometimes referred to as a "slow release" form of nitrogen.

Nitrite nitrogen (NO2-N) is toxic to plants. This is not normally a problem because nitrite is quickly converted to nitrate by bacteria. However, there can be environmental conditions (e.g. anaerobic conditions found in cold waterlogged soils) when the bacteria are prevented from completing their work and nitrite levels increase.

In Solufeed fertilizers nitrogen can be supplied from many different sources including ammonium sulphate, ammonium nitrate, urea, potassium nitrate and ammonium phosphates.

Phosphorus (P)

Plants capture the energy of sunlight. It is captured as chemical energy, and phosphorus plays a vital role in moving that energy into the cell to be used in all the chemical processes of the plant.

Any deficiency in phosphorus leads to stunted growth. Phosphorus is required as new cells are formed, and therefore there is strong demand for this element in the early stages of a plants life as it develops roots and shoots.

Phosphorus is often present in soils in an insoluble, unavailable form. Many factors including temperature and pH affect the availability of phosphorus to the plant. Temporary phosphorus deficiency, showing as a purpling of the leaf edges, can often occur during cold weather in the early spring, but passes as the soils warm up and availability increases.

Phosphorus interacts with other nutritional elements, with excessive levels depressing the uptake of iron, manganese, zinc and copper.

In Solufeed fertilizers phosphorus is usually supplied as ammonium phosphate or potassium phosphate.

Potassium (K)

Essential for the water regulation within the plant (turgor pressure) as well as the movement of carbohydrates and the creation of cellulose. Potassium is involved in the opening and closing of the stomata - the openings in the leaf by which air and water vapour enter or leave the leaf structure. Sufficient potassium is essential for flowering and assists with creating sweet, firm fruit and helps ensure plants have a good shelf life. Potassium is soluble in soils but moves relatively slowly by diffusion and so can be in local short supply around the roots of plants when uptake exceeds the speed of replacement.

Deficiency symptoms appear first on the margins of the oldest leaves as a pale green or yellow colour, progressing to give a completely burnt appearance to the leaf margin, known as "scorch".

Whilst nitrogen promotes soft lush growth, potassium balances this effect to produce firm compact growth, and the two elements are needed in similar levels of concentration in leaves.

In Solufeed fertilizers, potassium is usually supplied as potassium nitrate or potassium sulphate.

Calcium (Ca)

Sometimes described as the oxide CaO.

If plant cells are thought of as bricks, then calcium can be considered the cement that holds them together; it is an essential nutrient to build the structure of the plant, and like potassium it is essential for the production of crops with a long shelf life.

Calcium is not easily taken up by the plant - it is only readily absorbed by the newly developed tip of the growing root. For this reason it is important that sufficient calcium is available continuously to the plant right from the start of its life. Peat based compost usually contains calcium in the form of limestone which provides a long-lasting source of calcium to the growing plant.

In concentrated solution, calcium reacts with phosphorus to form an insoluble compound called calcium phosphate. Being insoluble, this falls out of solution as a muddy deposit in the fertilizer mixing tank. Since plants take up nutrients in solution, then even if this calcium phosphate could be provided to the plants, it could not be taken up by them. For this reason a concentrated solution of calcium has to be kept in its own tank, separate from the N:P:K fertilizer solution, and only mixed into the irrigation water at low concentration immediately prior to irrigating the crop. If the NPK fertilizer solution is sufficiently acidic, then the calcium and phosphorus will stay in solution in the stock tank. However, for this to work also requires a special sulphur-free acid fertilizers since the calcium in solution will also react with sulphur to form insoluble calcium sulphate (or gypsum).

Calcium deficiency shows up as reduction in the growth of the youngest leaves and growing points. These leaves become deformed and dead patches may form particularly round the edges. Calcium deficiency also shows up as blossom end rot in tomatoes or bitter pit in apples. The availability of calcium is affected by factors such as pH and interaction with nitrogen - high levels of ammoniacal nitrogen can depress uptake of calcium.

Solufeed fertilizers that contain calcium usually have calcium nitrate as the source.

Magnesium (Mg)

Usually described as the oxide MgO. An essential component of chlorophyll and in various essential enzyme systems.

Magnesium is included in peat composts as magnesium limestone, providing a long lasting source of the nutrient to the plant. A deficiency of magnesium shows up as inter-veinal yellowing and occurs all over the plant - young leaves and old leaves. Leaves may become hard and brittle.

Solufeed fertilizers supply magnesium as magnesium sulphate (in a variety of forms) and magnesium nitrate.

Sulphur (S)

Sometimes described as the oxides - either SO3 or SO4. An essential component of amino acids and hormones as well as plant oils.

 Micronutrients or trace elements

The working plant is very simply represented by the diagram below. Photosynthesis harnesses the sun's energy to produce sugars from carbon dioxide and water.

Sugar firstly provides the raw material for the onward biosynthesis of proteins, starch, lipids and all the other plant materials. And secondly is the fuel for respiration - driving the processes.

Hundreds of complex, interlinked metabolic pathways are involved in photosynthesis, respiration and biosynthesis. Micronutrients are intimately involved in them all, as organometallic proteins, enzyme co-factors and catalysts for example.

When the supply of one or more micronutrient is limited many metabolic processes can be detrimentally affected. This in turn leads to functional disturbances with or without the appearance of visual symptoms. Where there are symptoms, these can often be explained at the biochemical level.

Chlorosis or leaf yellowing is a symptom caused by many micronutrient deficiencies and indeed other disorders; it is a direct consequence of reduced chlorophyll production for whatever reason.

A brief description of some of the more important micronutrient functions follows.

Boron is required for protein synthesis, carbohydrate metabolism and water regulation. It is important in cell division so critical for normal development of the growing points (meristems) in shoots and roots. Deficiencies result in various growth deformities such as heart rot in beet. In the UK deficiencies mainly affect brassicae and root crops growing on sandy free draining soils, grasses are rarely affected. Treatment usually consists of foliar application of soluble boron compounds or the application of boron enriched fertilizers.

Copper is an essential part of several enzymes involved in respiration, chlorophyll production and carbohydrate and protein metabolism. It is important during plants' reproductive phases, correct growth of the pollen tube for example. Carrots are commonly affected especially those growing in organic peaty and chalky soils. Symptoms are ambiguous and subject to accurate diagnosis, correction is normally achieved by foliar application of copper compounds, notably CuEDTA.

Iron is essential for chlorophyll synthesis and for redox processes in respiration. Deficiencies in

arable or vegetable crops are rare in the UK but many fruit crops may be affected where growing on alkaline soils. Here the tell-tale of interveinal chlorosis where the veins remain starkly green is often seen. Happily prevention/treatment is simple using modern iron chelates.

Manganese is perhaps the most common micronutrient deficiency in the UK affecting virtually any crop in many situations. The element is involved in many enzyme systems such as those responsible for riboflavin, ascorbic acid and carotene production. Deficiencies are most often associated with high pH and organic soils and is exacerbated by high phosphate levels and poor soil consolidation. All crops can be affected and serious losses can result. Treatment by foliar application of manganese compounds is normal and use of manganese chelates offer certain practical advantages. Soil applications are generally ineffective.

Molybdenum is very rare and in reality only affects a few crops, notably cauliflowers. Here prophylactic molybdenum applications of molybdate compounds are usually used for prevention of the classic "whip tail" deficiency symptoms. The element is needed for nitrogen metabolism and hence protein synthesis.

Zinc is the most common micronutrient deficiency in the world but rarely affects plants in the UK. Most likely to occur in high pH soils with high phosphate levels and would be associated with manganese (and copper) deficiencies. Responses to zinc application in all ready high yielding cereal crops have been reported.

Undoubtedly because of natural selection in their ancestral wild plants, different crops have different micronutrient needs and differing sensitivity to deficiencies. While cauliflowers for example needs large amounts of boron to grow correctly, similar quantities would be toxic to potatoes or grasses. 

Causes of micronutrient deficiencies

Actual causes of micronutrient deficiencies are many and varied and often occur in combination. In nature and in extensively grown crops, manifestations of deficiencies are rare and even a short time ago were little more than curiosities.

Sandy soils lead to leaching of soluble micronutrients which is increased by low pH.

Alkalinity is the single most important factor and affects the availability of all micronutrients except molybdenum.

 

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