Soil nutrients
This paper aims to provide a brief summary of the nutrients present in soil, including their chemical symbol, those generally present in large or small amounts, an explanation of compounds and ions, root absorption of nutrients and also plant deficiency symptoms for the more common nutrients.
Soils contain many nutrients in large and small amounts. These are also referred to as chemical elements which have chemical symbols. They can be chemical elements which are in a solitary state but more commonly are found bound to other elements to form a compound.
Those chemicals/nutrients/elements present in larger amounts are referred to as macronutrients or macro elements and those in small or trace amounts as micronutrients or trace elements. Oxygen, carbon as carbon dioxide CO2, hydrogen as water vapour H2O and nitrogen occur in large amounts in the atmosphere and soil, and are the most abundant elements.
Compounds are combinations of elements bound together through chemical bonds. A well-known example is water, chemical symbol H2O, which tells us that the water compound is comprised of two hydrogen elements and one oxygen element or hydrogen atoms and an oxygen atom.
Nitrogen is present in the atmosphere and the air in soil pore spaces as N2, two N atoms bonded together. (note the number of N atoms is written in small print). N2 is not able to be used by plants but nitrogen is also present in the soil as ammonium NH4+ and nitrate NO3-. The compound of ammonium NH4+ is comprised of one N and 4 H and the + indicates that the compound is an ion with a positive electrical charge, having lost an electron and is called a cation. The compound nitrate NO3- has one nitrogen and 3 oxygen and has a negative charge, having gained an electron and is called an anion.
The positive charge of a cation enables the compound to chemically bond to the soil, especially with clay particles which are often negatively charged and organic matter which also has a negative charge. Sand is generally neutral and have little capacity to bond with nutrients and this is one of the reasons that sandy soils are often infertile, drying out quickly because of drainage is another. The negatively charged anions don’t always bond with the soil, they are in the soil moisture solution and are readily available to the plant through its roots but are also subject to leeching by water movement through the soil.
This bonding of cations to soil is called the soils Cation Exchange Capacity CEC and applies among others to calcium, magnesium and potassium. Potassium is present as K+.
Phosphorous P is present as the compound phosphate PO43- , one phosphorus, 4 oxygens with 3 negative charges, having lost 3 electrons. PO43- doesn’t readily bond with the soil but also is not water soluble which means it isn’t easily leached but it can move through the soil with smaller soil particles.
Fertilizer containers will often show a ratio of N:P:K, Nitrogen : Phosphorous : Potassium, as these are often required in the soil in greater amounts, or the container will show an analysis table of all the nutrients in the fertilizer. As a general rule nitrogen, as nitrate NO3-, is required for leaf growth and the production of chlorophyll which is where photosynthesis takes place. Phosphorus, naturally occurring as phosphate PO43-, is required for cell development, above ground structure and root growth. It is a component of the energy transfer biomolecule ATP, adenosine triphosphate. Potassium, present as a potassium ion K+. assists in the movement of water and sugars and helps to make fruit sweeter with more juice. It also increases cell wall thickness making the plant more resistant to fungal attack such as rots.
Nitrogen is most economically supplied by a continuous breakdown by bacteria of organic matter in the soil, from plant material or animals waste. If warmth, moisture and air are available in the soil, the breakdown of nitrate containing material by soil microorganisms will occur.
Superphosphate is made from accumulated seabird excreta, guano, which is mined, pulverized and treated with sulphuric acid to create a form of phosphate that is soluble in water or a very dilute acidic soil solution.
Potassium sulphate can be applied as a top dressing. Potassium is also present in bird guano.
The addition of organic matter to the soil through decomposing plant material and decomposing animal manures increases a soil’s CEC, as well as adding nutrients from the organic matter and increasing the soils water holding capacity by enabling formation of pore spaces to retain soil moisture. Organic matter will decompose down to make humus which contains humates that bond with the cations. Humus is at the stage where it won’t decompose anymore and is comprised of carbon which becomes part of the soil structure, thus the addition of carbon through plant material and animal manures increases the fertility of the soil.
The majority of plants absorb nutrients from the soil through their roots. Contact of the roots to the soil nutrients happens in 3 ways, interception, diffusion and mass flow. The roots intercept nutrients as they grow though the soil; the nutrients move though the soil solution by diffusion, that is from high to low concentration with the low concentration being caused by the root absorbing the nutrients; and mass flow of soil moisture containing nutrients towards the roots caused by absorption of soil water by the roots as a result of transpiration of water from the leaves necessitating the uptake of more water by the roots.
Increasing root surface area is an important adaptation by plants and this is performed by root hairs, tubular extensions of epidermal cells, with those plants naturally occurring in infertile soils having a high root mass to leaf ratio. Many Australian plants have proteoid roots also known as cluster roots to absorb more phosphates, these are arranged in a longitudinal pattern along the root and also excrete acidifying and chelating (bonding) compounds in order to dissolve the phosphates in water.
The surface area of roots and their ability to absorb nutrients can be greatly increased by plants having a symbiotic association with mycorrhizal fungi, which benefits both plant and fungi. The fungi growing on the outside of the plant root has mycelium which extends further into the soil, is even finer than the root hairs, and can increase penetration between the soil particles. The fungi in return receives plant sugars produced by photosynthesis through its attachment to the plant roots. This mycorrhizal association is of particular importance in absorbing phosphates.
Another symbiotic association occurs with nitrogen fixing bacteria in nodules on the roots of leguminous plants. The rhizobia bacteria are able to fix atmospheric nitrogen and convert it into the soluble form ammonium NH4+ available to the plant. Nitrogen content of the soil gradually increases as leaves and various parts of the plant die, are decomposed by other bacteria and fungi and the nutrients are released into the soil.
Some common plant nutrient deficiency symptoms:
Deficiencies are influenced also by water logging, soil pH and soil bacteria using all the available soil nitrogen to breakdown woody cellulose material. Another common reason for stunted growth is salinity in the soil.
Nitrogen — most common nutrient deficiency, stunted growth, yellowing of the leaves, older leaves lower down the plant go yellow first, new growth staying green the longest. Nitrogen is quite mobile within the plants, and in the soil, an is readily transferred from older leaves to new growing leaves.
Iron Fe — yellow leaves but with the veins in the leaf staying green and between the veins yellow, interveinal chlorosis.
Phosphorus — slow growth and pale-yellow leaves. Phosphorus deficiency is less common, except in Australia which has extremely old soils but can occur in other areas with high rainfall and clay soils. Although because phosphate is often stored in the growing points in stems and roots and as such grains and root-crops, harvesting crops will remove significant quantities of phosphates and fertilizing will eventually be required.
Potassium — yellow and/or purple leaves with brown leaf margins (edges) curling up, possibly circular brown spots between leaf veins, and reduced flowering.
Application of animal manures and organic mulches to the soil surface will generally correct the deficiencies. Manuring on the surface reduces the contact with soil thus allowing time for bacteria and other organisms time to break down any toxic substances before the manure is washed into the soil by rainfall.
References:
Chittenden, F.J., (1977). The Royal Horticultural Society. The Dictionary of Gardening (2nd Ed.) Oxford. Great Britain. University Press. Book.
Clarkson, D. T. (1981). Nutrient interception and transport by root systems. Physiological processes limiting plant productivity, 307–330.
Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.
Nutrient deficiencies. (2020). Royal Horticultural Society. Retrieved from https://www.rhs.org.uk/advice/profile?pid=456
Raven, P. H., Evert, R. F., & Eichhorn, S. E. (2005). Biology of plants. Macmillan. Book.
Zumdahl, S.S. and DeCoste, D.J. (2011) Introductory Chemistry: A Foundation (7th Ed.). Brooks/Cole, USA.: Cengage Learning. Book.
