Plant nutrition

After writing an essay on apple polyploidy for our web site I was invited to contribute any others I thought important to our area of growing fruit trees, so I decided to do a piece on plant nutrition. I have an extensive collection of plants acquired over many years but no formal background in plant biology, and it quickly became apparent that it’s a large and complicated subject (eg I now know about C3, C4 and CAM* plants). So to make it easier I decided to break it into 3 parts, namely:

The elements plants need and what each element does for the plant
Deficiencies and toxicities, and
Soil amendments (as there’s no point putting on fertilisers if they don’t stay where the plant can access them, and in a form they can use)

Part 1 – The elements plants need

All plants require 17 elements to complete their life cycle, and an additional 3 elements have been identified as beneficial for some under certain conditions.

They are broken into the following groups:

The essential non-mineral elements

Carbon (C), hydrogen (H) and oxygen (O)

Macronutrients

Nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg), calcium (Ca) and sulphur (S)

Micronutrients

Iron(Fe), manganese(Mn), zinc(Zn), copper (Cu), boron (B), chlorine (Cl), nickel(Ni) and molybdenum (Mo).

Elements sometimes beneficial

Silicon (Si), sodium (Na) and selenium (Se)

With the exception of C, H, and O, which plants obtain from air and water, plants under natural conditions principally derive the remaining 14 elements from the soil or through fertilizers, manures, and amendments (with exceptions such as parasitic and carnivorous plants, rain, dust storms and sea sprays).

What does each element do for a plant

Carbon (C)

Carbon forms the backbone of most plant biomolecules, including proteins, starches and cellulose. C is fixed through photosynthesis. This process converts carbon dioxide from the air into starches and carbohydrates (sucrose and glucose) and these photosynthetic products provide both the energy and the C-skeletons for ammonium assimilation during amino acid biosynthesis.

Hydrogen (H)

Hydrogen also is necessary for photosynthesis which builds sugars and hence contributes to plant growth. It is obtained almost entirely from water. The amount of H in the soil affects pH and the availability of other elements.

Oxygen (O)

All plant cells need oxygen to live, because without O they can’t perform aerobic respiration (respiration is the process of breaking down food to get energy). In any given 24-hour period, a healthy, growing plant will give off a lot more O (thru photosynthesis) than it consumes, and consume a lot more carbon dioxide than it gives off. Oxygen is a component of many organic and inorganic molecules within the plant, and is acquired in many forms. So the cells in the green parts (leaves and stems) of the plant, where most photosynthesis takes place, can get all the O they need from the O produced by photosynthesis. The cells down in the roots where photosynthesis is not taking place get their O from air in the spaces between soil particles.

Macronutrients

Nitrogen (N)

Nitrogen is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide (i.e., photosynthesis). It is also a major component of amino acids, the building blocks of proteins. Without proteins, plants wither and die. Some proteins act as structural units in plant cells while others act as enzymes, making possible many of the biochemical reactions on which life is based. N is a component of energy-transfer compounds, such as ATP (adenosine triphosphate). ATP allows cells to conserve and use the energy released in metabolism. N is also a significant component of nucleic acids such as deoxyribonucleic acid (DNA).

Phosphorus (P)

Phosphorus is an essential nutrient, both as a component of several key plant structural compounds, and for its role as a catalyst in numerous key biochemical reactions in plants. It is noted especially for its role in capturing and converting the sun’s energy into useful plant compounds. Within a plant, it is present mainly as a component of nucleic acids (DNA and ribonucleic acid (RNA)), as well as a constituent of fatty acid phospholipids that are important in membrane development and function. It is present in both organic and inorganic forms, both of which are readily translocated within the plant. All energy transfers in the cell are critically dependent on P. As with all living things, P is part of ATP.

Potassium (K)

Potassium is considered second only to N in terms of its importance to plant growth. Plants require K for vital intracellular functions which indirectly support plant growth, including nutrient transport and photosynthesis especially under low light intensity. It also acts as a catalyst by regulating at least 60 enzymatic activities involved in growth processes within the plant cell, helps facilitate the translocation of sugars and nutrients, and aids the formation of complex substances. K regulates the opening and closing of stomata (small pores mainly on the under-surface of plant leaves) allowing carbon dioxide, O and water exchange with the atmosphere, thereby having major control over internal plant moisture – important under drought conditions.

The K ion is highly mobile and can aid in balancing the anion (negative) charges within the plant. It helps in fruit coloration, shape and also increases Brix. Hence, quality fruits are produced in K-rich soils. K deficiency may result in higher risk of pathogenic infections, wilting, chlorosis, brown spotting, and increased risk of damage from frost and heat. Unlike other major elements, K does not become a part of the chemical structure of plants. It seems to be of particular importance in leaves and at growing points.

Magnesium (Mg)

Magnesium is the central atom of the chlorophyll molecule which confers the green hue in leaves. It also plays an important role in activating enzymes involved in respiration, photosynthesis, nucleic acid synthesis, and aids in phosphate metabolism, serving as a carrier of phosphate compounds through the plant. Mg facilitates translocation of carbohydrates (sugars and starches) and enhances the production of oils and fats.

Calcium (Ca)

Together with Mg, calcium combines with pectin to form a pectate salt that forms the middle lamella that cements together the cell walls of two adjoining plant cells. It is also crucial in activating certain enzymes, to send signals that coordinate certain cellular activities, and to regulate transport of other nutrients into the plant. Ca is key to normal root system development, increases resistance to outside attack, and also increases the feed value of livestock forage crops.

Sulphur (S)

Sulphur is an essential element in forming proteins, enzymes, vitamins, and chlorophyll in plants. It is essential for chloroplast growth and function, and also plays a crucial role in nodule development and efficient N fixation in legumes. Protein synthesis requires large amounts of S, especially in the formation of oils within seeds. There is a significant balance between N and S; without enough S, plants cannot efficiently use N and other nutrients to reach their full potential. S is an important factor in determining the nutritional quality of foods.

Micronutrients

Iron (Fe)

Iron is necessary for plant photosynthesis by participating in electron transport, and is an important cofactor for many other enzyme systems. It is not a structural part of chlorophyll but very much essential for its synthesis. Copper deficiency can be responsible for promoting an Fe deficiency.

Manganese (Mn)

Manganese is necessary for photosynthesis, including the building of chloroplasts. Deficiency may result in coloration abnormalities, such as discoloured spots on the foliage. It is used in plants as a major contributor to various biological systems including photosynthesis, respiration, and N assimilation. Mn is also involved in pollen germination, pollen tube growth, root cell elongation and resistance to root pathogens. It can replace Mg in activating several enzyme systems, and also activates indoleacetic acid (IAA) oxidases influencing the plant hormone auxin.

Zinc (Zn)

Zinc is required in a large number of enzymes and plays an essential role in DNA transcription. A typical symptom of Zn deficiency is the stunted growth of leaves (commonly known as little leaf) caused by the oxidative degradation of the growth hormone auxin. Zn is a component of at least four plant enzyme systems and is specific for carbonic anhydrase. It activates various types of enzymes that influence carbohydrate metabolism (not the principal reason for Zn deficiency symptoms) and protein synthesis.

Boron (B)

Boron has many functions within a plant: it affects flowering and fruiting, pollen germination, cell division, active salt absorption, and the metabolism of amino acids, proteins and carbohydrates. Many of these functions involve moving highly polar sugars through cell membranes by reducing their polarity and hence the energy needed for transport. If sugars cannot pass to the fastest growing plant parts rapidly enough, those parts die. It is also essential for the proper formation and strength of cell walls; inadequate B results in short thick cells producing stunted or distorted fruiting bodies and roots. B deficiency causes a reduction of exudates and sugars from plant roots, which can reduce the attraction and colonization of mycorrhizal fungi. The Ca to B ratio must be maintained in a narrow range for normal plant growth.

Copper (Cu)

Copper is important for electron transport in chloroplast photosynthesis, hence chlorosis is a deficiency symptom. It activates enzymes involved in lignin synthesis, is essential in several enzyme systems including plant respiration, and assists in metabolism of carbohydrates and proteins. It participates in nitrogen fixation, is necessary for oxidase enzymes that reduce molecular O, and is also required for seed and fruit formation. Deficiency reduces pollen viability.

Chlorine (Cl)

Chlorine, in the ionised form chloride, is necessary for osmosis and ionic balance, and also plays a role in the evolution of oxygen in photosystem II of the photosynthetic process. It raises cell osmotic pressure, affects stomatal regulation, and increases plant tissue hydration.

Nickel (Ni)

In higher plants, nickel is absorbed by plants in the form of the Ni²⁺ ion. Ni is essential for activation of urease, an enzyme involved with nitrogen metabolism that is required to process urea. Without Ni, toxic levels of urea accumulate, leading to the formation of necrotic lesions. In lower plants, Ni activates several enzymes involved in a variety of processes, and can substitute for Zn and Fe as a cofactor in some enzymes.

Molybdenum (Mo)

Mo is a cofactor to enzymes involved in nitrogen metabolism and in amino acid synthesis, and is a component of two major plant enzyme systems: (a) nitrogenase, which converts N gas to ammonia, and (b) nitrate reductase, which converts nitrate to nitrite; this is subsequently converted to ammonium by a different enzyme. Mo requirement is reduced by increased availability and utilization of ammonium as a nitrogen source.

Silicon (Si)

Silicon is not considered an essential element for plant growth and development as it’s abundant in the environment and hence if needed is available. It has been shown to strengthen cell walls and improve plant strength, root mass and density, health and immunity, and above-ground biomass and crop yields.

Sodium (Na)

Sodium is involved in osmotic (water movement) and ionic balance in plants, and the regeneration of phosphoenolpyruvate in C4 and CAM plants. Na can potentially replace K’s regulation of stomatal opening and closing.

Selenium (Se)

Although not essential, under certain conditions selenium has been shown to stimulate plant growth, improve tolerance to oxidative stress, and increase resistance to pathogens and herbivory. It is an essential element in animal and human nutrition, and Se deficiencies are known to occur when food or animal feed is grown on Se-deficient soils. Use of fertilizers containing Se can increase concentrations in these foods, and hence improve animal and human health.

* C3, C4 and CAM are the three different processes that plants use to fix C during the process of photosynthesis. Fixing C is the way plants remove it from atmospheric carbon dioxide and turn it into organic molecules like carbohydrates. The C3 pathway gets its name from the first molecule produced in the cycle (a 3-carbon molecule) called 3-phosphoglyceric acid. About 85% of plants use this process. The C4 process is named for the 4-carbon intermediate molecules that are produced, malic acid or aspartic acid. Around 3% of plants use this process. Plants that use crassulacean acid metabolism, also known as CAM plants, are succulents that are efficient at storing water due to the dry and arid climates they live in. CAM plants store sunlight energy and keep their stoma closed during the day to prevent water loss. The stoma then open at night to take in carbon dioxide from the atmosphere. The carbon dioxide is converted to malate which then enters the Calvin Cycle photosynthetic process.

Richard Pittaway