Is the advice to “use potash and Epsom salts to grow sweeter citrus fruit” based on scientific fact, or is it just gardening folklore?
Adding potash (potassium) is only helpful when citrus trees are suffering from potassium deficiency, which is rare in healthy soils, and can be diagnosed by observing the leaves and fruit.
Too much potassium, on the other hand, will result in excess potassium levels, which will degrade the quality of oranges, by causing the fruit to become large but coarse, more acidic and less juicy, with a thick and rough rind.
An excess of potassium delays fruit maturity, and also increases regreening, a phenomenon caused by heat, where ripe oranges start turning green again, which affects late season varieties of oranges such as Valencia. This happens because citrus require cold to inhibit the formation of green chlorophyll pigments in their fruit.
Using potassium fertilisers (such as potassium sulphate/sulphate of potash) and magnesium fertilisers (such as Epsom salts/magnesium sulphate) in the soil at the same time is a really bad idea, as the nutrients potassium and magnesium compete for absorption by the plant.
Too much potassium can increase magnesium deficiency, and adding magnesium to a citrus tree which is suffering from potassium deficiency will only exacerbate the problem!
To understand how such gardening folklore comes about, and for a more technical explanation, we need to look at the science, particularly the role of potassium in plants. When we do, we realise that the erroneous advice is based on a misunderstanding of plant nutrition, the fallacy being “if a little is good, then much more is a whole lot better”, which is definitely not the case!
Balanced Fertilisers and Ideal Nutrient Ratios
Over-fertilising the soil is as bad as under-fertilising, because more fertiliser is not better!
The three main macronutrients (nutrients which plants use the most) are Nitrogen (N), Phosphorus (P) and Potassium (K).
The secondary macronutrients, (nutrients which are required in lower quantities, but are still very important), are Magnesium (Mg), Calcium (Ca) and Sulphur (S).
Adding any one of these nutrients in large amounts, when it’s not needed, can throw the other soil nutrients out of balance, because several of these nutrients compete with each other over uptake by the plant. It’s important to maintain appropriate nutrient ratios to avoiding creating nutrient deficiencies.
- Excess potassium competes with calcium and magnesium absorption.
- Excess iron can result in manganese deficiency.
- Excess sulphur may decrease the uptake of nitrate.
The reason why balanced fertilisers are formulated with specific ratios of different nutrients is so they don’t create nutrient imbalances in the soil, or disrupt the balance of growth between leaves, roots and fruit.
For more information of fertilisers, see article – The Organic Gardener’s Guide to Fertilizers and How to Use Them
Why Do Nutrient Elements Compete With Each Other for Plant Uptake?
To explain why some nutrients compete with each other for plant uptake, we need to understand some basic chemistry.
Atoms are composed of a core of protons and neutrons, with electrons orbiting around them.
Protons carry a positive electrical charge (+), electrons carry a negative electrical charge (-), and neutrons are neutral, with no electrical charge.
An atom usually has the same number of protons and electrons, so the charges cancel each other out, making the atom’s net electrical charge neutral.
In chemistry, when atoms or molecule gains electrons (which are negatively charged), this gives them a net negative electrical charge. When they lose one or more of their electrons, they take on a net positive electrical charge.
An atom or molecule with an electrical charge is defined as an ion, and negatively charged ions are referred to as anions, while positively charged ions are referred to as cations
Many plant nutrients are positively charged cations. Magnesium (Mg2+), calcium (Ca2+) and potassium (K+) are the most important plant nutrients that exist in soil solutions as cations. The reason why they compete for plant uptake is because they’re very similar in their properties.
If you’re interested in the chemistry details, here’s a further technical explanation, otherwise skip to the next section – “Functions of Potassium in Plants”.
In the periodic table of elements shown below, which is a table of all the chemical elements that exist, I have highlighted the elements magnesium, potassium and calcium in yellow.
It is important to note that all three of these elements (K, Mg and Ca) are located very close to each other, and reside in the first two columns of the periodic table of elements.
The vertical columns in the periodic table of elements are called groups, and there are 18 of them.
When elements are contained in the same group, they tend to have similar properties. This is because they generally have the same electron configurations in their valence shell (outer shell of electrons that are involved in forming chemical bonds), and therefore have a shared chemistry.
Each of these different groups have their own names.
- Group 1 elements (such as potassium K) are called Earth metals and readily give up one electron to become +1 cations with a single positive charge (K+).
- Group 2 elements (such as magnesium Mg and calcium Ca) are called alkaline Earth metals and readily give up two electrons to become +2 cations with a double positive charge (Mg2+ and Ca2+).
Being chemically similar, these elements are taken up into plants through the same processes, and the plant doesn’t discriminate one from the other, so an excess of one will reduce the uptake of the other simply by being more abundant in the soil.
Now that we’ve established how and why plant nutrients compete with each other, our next step is to understand the role of potassium in plants and how this has been misinterpreted.
Functions of Potassium in Plants
When citrus trees produce fruit, they use large amounts of potassium from the tree compared to other nutrients. Potassium is a mobile nutrient and can be translocated (relocated) to where the tree needs it. During fruiting, potassium is moved from the leaves to the developing fruit and seeds.
What is the role of potassium in plants?
The short answer is that it’s very complicated, because potassium is not an integral part of any chemical compound that can be isolated from plants, but it is involved in the formation and utilization of electrical and osmotic gradients in cells and tissues, the chemical coordination and activation of enzymes and nucleic acids, as well as an agent of stress signalling and tolerance.
Through these three key functions, potassium supports several important physiological functions in plants, such as:
- Photosynthesis, and the formation of sugars and starch.
- Phloem transport, and the translocation of sugars and starch within the plant.
- Enzyme activation and stimulation, which enable production of proteins, starch and adenosine triphosphate (ATP). The chemical compound ATP is the energy-carrying molecule found in the cells of all living organisms and can regulate the rate of photosynthesis.
- Cellular electrochemistry, where bioelectrochemical signals, known as action potentials, which are caused by the depolarization of cellular membrane potentials. resemble nerve impulses, and allow plants to rapidly respond to external stimuli such as changes in light intensity, osmotic pressure, temperature, water availability, mechanical stimulation, cutting, wounding, insect attack – and alter their internal conditions.
Enzymes are substances that acts as catalysts in living organisms, they regulate the rate at which chemical reactions proceed, usually starting them or speeding them up, without themselves being altered in the process.
Since potassium influences many enzymatic reactions, it’s associated with almost every major plant function. The macronutrient potassium is important in the formation and functioning of proteins, fats, carbohydrates and chlorophyll, and in maintaining the balance of salts and water in plant cells. It’s also associated with the movement of water, nutrients and carbohydrates in plant tissue, and is involved in moving sugars from the site of where they are produced by photosynthesis to other sites in plants for storage.
Potassium also helps regulate the the opening and closing of the stomata (the pores on the underside of leaves), which regulate the carbon dioxide supply to plants, as well as the exchange of water vapour, oxygen. By closing, the stomata reduce water loss and wilting, and maintain turgor (pressure exerted by fluids in plant calls which keep them rigid).
Additionally, potassium works with phosphorus to stimulate and maintain rapid root growth, which improves drought resistance in plants. It helps reduce the effects of adverse weather conditions such as cold, drought, and flooding on citrus trees, and also improves plant health, resistance to diseases, and tolerance to pest attack from insects and nematodes.
Many of these functions of potassium, in combination, play an important role in fruit formation, and the development of fruit size, flavour, and colour. That said, it would be a logical association fallacy to think that adding more potassium will make oranges even bigger than they naturally get, even more flavoursome than their optimum state when properly ripened, and more colourful than their regular orange colour. We can see that such reasoning would be a gross oversimplification, considering the science of potassium in plants we’ve just discussed.
Symptoms of Potassium Deficiency in Citrus
Potassium deficiency in citrus is not a common problem, because soils can hold a reserve of potassium, and since this nutrient doesn’t leach away easily, the levels of potassium build up over time when a balanced fertiliser (that contains nitrogen, phosphorus and potassium) is used regularly.
Soils that have very high concentrations of calcium (Ca) and magnesium (Mg) or heavy applications of nitrogen-rich (N) fertilisers will have a decreases capacity to supply of potassium to plants. Acid sandy soils, which are more prone considerable nutrient leaching, may be more prone to potassium deficiencies. Soils in highly weathered tropical lands particularly show potassium deficiencies, but citrus are not grown extensively in these regions anyway.
Soils containing clay and organic matter are better able to hold potassium than sandy soils, and generally contain higher potassium levels. Since most potassium is found near the surface of the soil, drought conditions can dry surface soil and reduce the uptake of potassium temporarily, until the soil is watered.
Potassium deficiency is likely to occur on calcareous soils (soil containing high amounts of limestone, which is calcium carbonate, chemical symbol CaCO3) because of elemental antagonism, since calcium ions (Ca2+) compete with potassium ions (K+) for plant uptake.
When high applications of nitrogen fertilisers are used, especially with synthetic chemical fertilisers, potassium deficiencies can also occur, and excess nitrogen also increases leafy green growth at the expense of fruit production.
How do we identify potassium deficiency?
- Mild potassium deficiency doesn’t affect the yield of citrus, though the fruit may be smaller in size.
- Severe potassium deficiency does reduce the yield of citrus, because of heavy flower and fruit drop.
The symptoms of potassium deficiency vary in citrus, and as a result are often not easy to recognise, and can be mistaken for other nutrient deficiencies.
- Moderately low potassium levels cause a general reduction in tree growth, without producing visual symptoms of deficiency in fruit and leaves. This may show as slow vegetative (leafy green) growth, small leaves, fine branches, and a thin canopy.
- The effects of low potassium levels will affect fruit quality and yield before leaf deficiency symptoms are visible. Symptoms include smaller fruit with smoother, thinner rinds which may be prone to splitting or dropping, and decreased fruit yields, less fruit will be produced.
- Potassium deficiency symptoms in leaves begins with an irregular, blotchy or mottled chlorosis (yellowing) between leaf veins, with most of the yellowing starting near the apex (tip) half of the leaf, and leaf edges, and eventually becoming more irregular in pattern, bronze or brown in colour, with the leaf edges eventually becoming brown.
This occurs on older leaves first, as potassium is a very mobile nutrient, and plants can mobilise it to younger leaves when it is in short supply, but deficiency symptoms can also appear in younger leaves.
When potassium deficiency symptoms appear, production will have already been seriously impacted.
How to Correct Plant Potassium Deficiency
A safe and organically certified way to correct potassium deficiency in soils is by applying potassium sulphate (sulphate of potash, chemical formula K2SO4) at the following rates:
Heavy soils – 10g (two level teaspoons) per square metre (0.35 oz/yd²)
Sandy soils – 20g (four level teaspoons) per square metre (0.70 oz/yd²)
To apply potassium sulphate as a liquid, add 20g (four level teaspoons or 0.70 oz) into a 9 litre (2 gallon) watering can and apply over an area of 2 square metres (2 yd²)
Where potassium is applied at low rates, one application per year is usually sufficient, particularly
When applying potassium at higher application rates, such as in sandy soils, split applications are used, where only part of the total amount is applied, and the rest is applied at a later date, to improve utilisation and minimise leaching losses.
Please ignore advice aimed at farmers which recommends spreading a large single application of potassium sulphate (1-2 kg per tree) in a band around the drip line for severe deficiencies. This is only done because it’s cheaper to apply fertiliser once rather than spend the time to add smaller amounts each year. Such a large application is meant to last several seasons (years) as potassium does not leach readily from most soils.
Another good organic option is to use seaweed extract, if it has been extracted with potassium hydroxide (the production method is called alkaline hydrolysis) it will contain decent levels of potassium. In Australia, the product seaweed concentrate Seasol contains 1.5% w/v of potassium. Mix 40-100ml of concentrate into a 9L watering can, and apply over an area of 2.5 square metres. Apply at the start of spring before flowering, and if necessary apply every 2-4 weeks while flowering and fruiting takes place. If soils are healthy, a s ingle application will be sufficient.
It’s important to keep in mind that continued application of potassium fertiliser over many years can cause potassium to build up to unnecessarily high levels. Good quality sources of potassium, such as potassium sulphate are expensive, and using more than is needed is both wasteful, and potentially harmful to plants.
Why Potassium Chloride is Bad for the Soil and is Best Avoided
Do not use potassium chloride (muriate of potash, chemical formula KCl) under any circumstances, as it’s cheap and nasty, and the only reason it’s used on farms in commercial agriculture is to cut costs! It increases soil salinity, it contains chloride (Cl– the negative ion of chlorine) which when in excess will cause chlorine toxicity and burn the foliage, and it’s harmful to soil bacteria and as a consequence damages soil health, though the manufacturers will vehemently deny that last point!
Research indicates that use of potassium chloride leads to accumulation of its salts in the soil, which can cause a physiological disturbance in soil organisms, and can become a biocide in the soil, a compound that kills living things. With the use of potassium chloride, microbial activity decreases, and the increased chloride (Cl) content of the soil promotes the immobilization of nitrogen (N), preventing the mineralisation of nitrogen into its inorganic or mineral form ammonium (NH4+) which is available to plants.
The phenomenon of chlorine toxicity is well documented. When the chloride concentration in a plant’s leaves exceeds its tolerance, the plant cells die (necrosis) and leaf burn develops. If leaf damage is extensive, leaf fall can occur. In citrus, chlorine toxicity injury occurs first at the leaf tips, and then progresses down along the edges of the leaf as severity increases. When there has been extensive defoliation (leaf loss), dieback of branches can occur, and even tree death. The older leaves usually show the symptoms of chlorine toxicity first, and the symptoms look similar to those caused by drought and fertilizer salt burn.
Salinity is defined as the amount of salt in the soil or water. The main salt in most saline soil is common salt, or sodium chloride (NaCl). Other salts contributing to salinity are calcium, magnesium and potassium chlorides, and sodium sulphates.
All fertilisers have a salt index, a numerical value which indicates the fertilisers contribution to soil salinity. Potassium chloride has a salt index of 114, while potassium sulphate has a much lower salt index of 46.
It’s important to be aware that potassium chloride is nothing more than culinary salt-substitute sold at the supermarket for people with low-sodium diets. If we salt the soil, we damage it!
What Causes Magnesium Deficiency in Plants?
There are two primary causes of magnesium deficiency in plants:
- An actual shortage of the nutrient magnesium in the soil
- An imposed or indirect deficiency created by imbalances of other nutrients in the soil or plant.
Actual magnesium deficiencies occur most commonly in sandy, light or acid soils, though they can occur clay soils used for intensive crop production. In all soil types, where rainfall or irrigation causes heavy leaching, magnesium can be washed out of the soil and lead to a deficiency. Under such conditions where nutrient leaching occurs, adding highly soluble Epsom salts (magnesium sulphate, chemical formula MgSO4) is ineffective for correcting magnesium deficiencies, and just contaminates the water table below where the water seeps into.
Indirect magnesium deficiencies are not due to a lack of magnesium, but are caused by imbalances of other nutrients in the soil. Even when soil levels of magnesium are adequate, excessive levels of potassium create an imbalance in plants by interfering with the root uptake of magnesium, as discussed earlier.
Other than decreasing potassium application, another way to address the issue of indirect magnesium deficiencies is by the addition of nitrogen fertilisers. It has been observed that trees high in nitrogen were found to be less susceptible to magnesium deficiency compared to those with reduced nitrogen levels. A balanced fertiliser contains the correct amount of nitrogen in the right proportion relative to the other nutrients, and if trees are fertilised in spring an autumn, they won’t ever be nitrogen-deficient.
Symptoms of Magnesium Deficiency in Citrus
When magnesium deficiency first appears in citrus, the yellowing of the leaf between the green veins begins at the tip and edges of the leaf, and moves down towards the leaf stem (petiole). With prolonged deficiency, these areas can turn completely yellow, leaving a dark green inverted V-shape at the base of the leaf.
In clay soils, or soils rich in organic matter, Epsom salts can be used to treat magnesium deficiencies, apply 20g (4 level teaspoons) of Epsom salts (magnesium sulphate) per square metre (0.70 oz/yd²), spreading it evenly around the drip line of the tree, then water in well. Another option is to dissolve 10g (2 level teaspoons) of Epsom salts into a litre of water (0.35 oz/0.25 ga) and apply it at a rate of 1 litre per square metre (0.25 ga/yd²) of soil with a watering can.
Dolomite lime (which is calcium magnesium carbonate) can also be used will correct mild magnesium deficiency symptoms in soils with low to neutral pH, because this soil amendment product is alkaline, and will raise the soil pH.
In calcareous soil (containing high amounts of limestone) a foliar application of Epsom salts may have to be used to correct a magnesium deficiency. For a foliar application, dissolve 10g (2 level teaspoons or 0.35 oz) of Epsom salts into a litre (0.25 ga) of water and spray over the leaves. Some sources discourage application onto foliage, as the amount of magnesium that can be absorbed through the leaves is quite limited, and a magnesium foliar spray may burn the leaves.
How Soil CEC Affects Application of Potassium and Magnesium
The amount of clay and organic matter in the soil will have an impact on the effectiveness of potassium or magnesium that is applied to the soil.
Clay minerals tend to have a net negative charge through to a very low net charge close to zero, and organic matter has a much higher negative charge that clay. The total amount of negative charge in a soil is called cation exchange capacity (CEC).
Negatively charged soils, those with a high CEC, retain positively charged ions (cations) such as magnesium, calcium, potassium and ammonium. Soils with higher CEC values are able to retain more of these plant nutrients than those with lower CEC values. Since organic matter has a higher CEC value than clay minerals, it can hold much more nutrients and therefore increases a soil’s fertility.
In an ideal soil, the cation exchange sites should be occupied by the following cations, in these proportions:
- 2 – 4 % by potassium (K+)
- 10 – 20 % by magnesium (Mg2+)
- 60 – 80 % by calcium (Ca2+)
- less than 10 % by other cations that aren’t plant nutrients, such as hydrogen (H+) and aluminium (Al3+) ions
What determines whether plants get enough potassium to grow or not is not just the amount of potassium in a soil, but also on the amount of other cations in the soil. The amount of potassium available is dependent on the total cation exchange capacity (CEC) of the soil and the percent of those exchange sites occupied by potassium.
Before adding any fertilisers to the soil, it’s important to first ask ourselves whether it is necessary, if it will make any difference, and what damage can be caused to the soil or plants if we use too much.
It’s quite irresponsible to advise gardeners to simply apply potassium and Epsom salts to their orange trees on the promise of drastic crop improvement, without any regard to soil conditions and plant needs, without diagnosing nutrient deficiencies correctly first, or understanding the soil chemistry of how these two nutrients interact, and the simple fact that they actually antagonise each other.
- University of Minnesota Extension – Potassium for crop production
- University of Florida Extension – Potassium (K) for Citrus Trees, Publication #SL381, Date: 2019-09-18 by Mongi Zekri and Tom Obreza
- NSW Government, Department of Primary Industries – Citrus nutrition, Series: Agfact H2.3.11 Edition: Second edition Last updated: 31 Dec 2002
- University of Florida Extension – Macronutrient Deficiencies in Citrus: Nitrogen, Phosphorus, and Potassium, Publication #SL 201, Date: 2021-02-22 by Mongi Zekri and Thomas A. Obreza
- Epsom salts: miracle, myth…or marketing – MASTERGARDENER, SPRING 2007 by Linda Chalker-Scott, Ph.D., MasterGardener WSU editor, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University
- The Maine Organic Farmers and Gardeners Association – Resources, Soil, Potassium, Fall 2011, by Eric Sideman, Ph.D.
- University of Florida, Institute of Food and Agricultural Sciences Extension – A Guide to Citrus Nutritional Deficiency and Toxicity Identification, Publication #HS-797, Date: 2021-03-18 by Stephen H. Futch and David P. H. Tucker
- Jing Cui, Guillaume Tcherkez, Potassium dependency of enzymes in plant primary metabolism, Plant Physiology and Biochemistry,
Volume 166, 2021, Pages 522-530, ISSN 0981-9428, https://doi.org/10.1016/j.plaphy.2021.06.017.
- Dev T. Britto, Devrim Coskun, Herbert J. Kronzucker, Potassium physiology from Archean to Holocene: A higher-plant perspective, Journal of Plant Physiology, Volume 262, 2021, 153432, ISSN 0176-1617, https://doi.org/10.1016/j.jplph.2021.153432.
- NSW Government, Department of Primary Industries – Using saline water for irrigation
- Pereira, David Gabriel Campos et al. Potassium chloride: impacts on soil microbial activity and nitrogen mineralization. Ciência Rural [online]. 2019, v. 49, n. 5 [Accessed 13 July 2021] , e20180556. Available from: <https://doi.org/10.1590/0103-8478cr20180556>. Epub 18 Apr 2019. ISSN 1678-4596. https://doi.org/10.1590/0103-8478cr20180556.
- University of Florida, Institute of Food and Agricultural Sciences – Boron (B) and Chlorine (Cl) for Citrus Trees by Mongi Zekri and Tom Obreza, Publication #SL406 Date: 2019-09-18
Hi Angelo. Thanks for this interesting and detailed explanation. Do you have any suggestions on the opposite problem, that is magnesium levels that are too high? It makes my heavy soil tightly bound and wet, sticky and poorly drained in winter and spring (here in Melbourne). My trees and shrubs are also turning an unnaturally dark green, and my plants can be prone to fail due to waterlogging. Would gypsum help, and if so is it effective if used to top dress around established plants, rather than being dug in? I would appreciate any thoughts.