If you have been following this series, you have already seen two different ways in which fertilization money is lost. With nitrogen, the problem is that it goes away because it evaporates, leaches, or escapes. With phosphorus, it is the opposite; it stays in the soil but is trapped in forms that the root cannot take. For potassium and secondary nutrients, the problem is of another nature, and it is the most difficult to see at first glance, and that is balance.
To avoid confusion with this balance, let me explain; Potassium (K), calcium (Ca), and magnesium (Mg) are cations, positively charged particles that travel dissolved in the soil solution and adhere to the negative charges of clays and organic matter. The roots absorb them through the same doors, and that is where the competition begins when one of these cations is in excess, occupying those doors and blocking the entry of the others. That is why you can have plenty of magnesium in the soil analysis and still see a magnesium deficiency in your leaves, because you applied so much potassium that magnesium could not enter.
It is a twist that completely changes the way of fertilizing with these nutrients; absolute quantities matter less than the relationships between them. It is not about applying more potassium but maintaining the balance that allows each nutrient to fulfill its function, and that balance is precisely what determines the quality of your fruit in terms of its size, sweetness, firmness, and how long it lasts after harvest.
The nutrient that your crop extracts in the largest amount
There is a general idea in the field that nitrogen is "the most important" because it gives the green and growth, but if you look at what a fruit crop actually takes from the soil, you will be surprised in many cases, as potassium is the nutrient that is extracted the most, even more than nitrogen.
The numbers confirm this, as in Hass avocado, a production of 20 tons per hectare removes about 94 kg of K₂O from the soil, compared to 52 of nitrogen and only 21 of phosphorus. Potassium almost doubles nitrogen, and it is no coincidence because potassium is the nutrient of quality and fruit filling. Why does a crop need so much potassium? Because it performs functions that no other nutrient can replace. Potassium regulates the opening and closing of the stomata, those microscopic pores through which the plant breathes and transpires, and when there is drought, it is potassium that allows the plant to close those pores and conserve water; when moisture is available, it helps to absorb it, which is why a well-nourished crop in potassium tolerates drought and heat stress better.
But its most visible role is in the fruit, as potassium directs the transport of sugars from the leaves, where they are produced by photosynthesis, to the fruits, where they accumulate; from there depends the size of the fruit, its sugar content, its color, and its shelf life after harvest. A potassium deficiency translates directly into smaller, less sweet fruits that deteriorate faster, and as if that were not enough, potassium strengthens cell walls, making the crop more resistant to diseases and pests. It is not an exaggeration to call it the nutrient of quality if a good part of what the market pays you for caliber, sweetness, presentation, and durability goes through it.
The balance that almost no one measures between K, Ca, and Mg.
Here we come to the question that many producers ask when they see a deficiency they do not understand; if the analysis says I have enough magnesium, why do my leaves show symptoms of magnesium deficiency?
The answer lies in the antagonism between cations. Potassium, calcium, and magnesium do not act independently; they compete with each other to be absorbed. An excess of one reduces the entry of the others; the best-documented case is that of potassium over magnesium; when you apply high doses of potassium, the plant absorbs so much potassium that magnesium gets displaced, and the classic chlorosis appears, yellowing between the veins of the old leaves, with the veins still green, even though there is magnesium in the soil. By applying more potassium to improve quality, you may be creating a magnesium deficiency that reduces it.
And it works in all directions; an excess of calcium and magnesium can induce potassium deficiency; an excess of potassium or sodium can induce magnesium deficiency; liming with low magnesium limes can unbalance the relationship and leave the crop short of magnesium. Fertilization with ammonium sources also competes because ammonium (NH₄⁺) is another cation that fights for the same doors.
To manage this, technicians look at two things in the soil analysis:
- The first is the cation exchange capacity (CEC): how many cations your soil can retain in total, a "pantry" that is large in clayey soils and with organic matter, and small in sandy soils.
As we have already addressed in our blog The organic matter: the asset that pays part of your fertilization without you knowing, which you can read here, organic matter is what increases the cation exchange capacity of your soil, that is, its ability to retain and deliver potassium, calcium, and magnesium in a balanced way.
- The second is the relationships between cations: the K/Mg ratio, the Ca/Mg ratio, and base saturation. As a general reference, the K/Mg ratio is usually managed in a balanced range so that potassium does not suppress magnesium.
Here it is worth being honest because there is a lot of misinformation; for decades, the idea of a universal "ideal ratio" of cations was promoted, for example, a fixed proportion of calcium, magnesium, and potassium equal for all soils and crops. The most rigorous research has nuanced that idea since there is no magic number that works for everything, and forcing an exact proportion can lead to spending on amendments that the crop does not need. What is real and well established is antagonism, so the goal is not to pursue a perfect figure but to ensure that each cation is at an adequate level without one being so high that it suppresses the others. The relationships are useful warning signals, not rigid recipes, and that is why two lots with the same fertilization can behave completely differently depending on their CEC, texture, and liming history.
The "secondary" nutrients that are not secondary
Calcium, magnesium, and sulfur are called secondary nutrients, and that name is misleading; they are secondary in the amount the plant needs, less than N, P, and K, not in their importance. The lack of any of them is as harmful to your crop as that of a primary macronutrient. In fact, in some plants, the concentration of magnesium in the tissue is comparable to that of phosphorus. It is worth knowing what each one does because their deficiencies are frequent and often confused.
Calcium (Ca) is structure. It is the cement of cell walls: it gives firmness to tissues and participates in the formation of new cells. That is why its deficiency attacks precisely the growing parts, the new shoots, the tips, the fruits in formation, and manifests in problems that cost direct money, such as blossom end rot (the dark spot at the base of tomatoes or peppers), tip burn, deformed fruits, and flower drop. Calcium moves little within the plant, so a fruit that was left without calcium at its formation moment does not recover it later.
Magnesium (Mg) is the heart of chlorophyll. It is literally the central atom of the molecule that gives the green color and captures sunlight energy for photosynthesis. Between 15% and 20% of the magnesium in the plant is in its green parts. Without enough magnesium, photosynthesis is reduced, and with it, sugar production and yield. Since magnesium does move within the plant, its deficiency appears first in the old leaves, yellowing between the veins, just the opposite pattern of calcium.
Sulfur (S) is part of proteins and amino acids and is necessary for the plant to utilize nitrogen well and form chlorophyll. Its deficiency resembles that of nitrogen, producing pale leaves that are often confused; the difference is that sulfur deficiency appears first in the young leaves. And here is a fact that few producers know: sulfur deficiency is increasing worldwide. For three cumulative reasons: higher yields extract more sulfur from the soil; modern fertilizers are purer and no longer provide the "bonus" sulfur that old formulas brought; and sulfur deposition from the atmosphere has decreased. The result is that many soils that previously had enough sulfur no longer do, and the deficiency goes unnoticed because it is confused with nitrogen deficiency.
Are you applying potassium without knowing if your soil has the balance to take advantage of it? Harvis crosses the exchange capacity, the relationships between cations, and the history of each lot to tell you if your problem is one of quantity or balance — and where a deficiency you see is actually an excess of another nutrient. Talk to us here →
Management and balance strategies
Unlike phosphorus, which is applied in modest amounts because it accumulates, potassium is applied in high amounts and should be split. There are two reasons: first, because the crop extracts it in large volumes, especially during the fruit filling stage. Second, because potassium, although retained in clayey soils, leaches easily in sandy soils and those with low CEC, where it is, along with sodium, the cation that is most lost through leaching under heavy rains. Splitting the dose and timing it with the crop's demand prevents loss and improves utilization.
Choosing the right source is a decision that affects quality. The three main sources of potassium behave differently:
Potassium chloride (KCl), or muriate of potash, is the most used and economical, with about 60% K₂O. Its problem is twofold; it has a high salinity index (116 compared to 74 for potassium nitrate) and contributes a lot of chlorine, about 40 kg of chloride for every 100 kg applied. In crops sensitive to chlorine, including avocado, potato, tobacco, and strawberry, KCl can cause toxicity and affect both yield and quality. For those crops, it is not the best option.
Potassium sulfate (K₂SO₄) has about 50% K₂O and also provides 18% sulfur, with a low salinity index. It is the preferred source for chlorine-sensitive and high-quality crops, and it kills two birds with one stone: it delivers potassium and corrects sulfur, which, as we saw, is often deficient. Potassium nitrate (KNO₃) provides potassium and nitrate nitrogen, with the lowest salinity index, ideal for fertigation and growth stages.
For secondary nutrients, amendments serve a dual purpose. Dolomitic lime (calcium and magnesium carbonate) provides calcium and magnesium and raises pH, useful in acidic soils that are also low in magnesium. Agricultural gypsum (calcium sulfate) provides calcium and sulfur without altering pH, is up to 200 times more soluble than lime, and has an extra benefit in acidic soils: it reduces aluminum toxicity and improves deep rooting. The choice depends on what your soil needs; if you lack magnesium and the pH is low, dolomite; if you lack calcium and sulfur but the pH is already fine, gypsum.
Reference doses for high-density technified coffee, Cenicafé recommends about 260 kg/ha/year of K₂O, 50 of MgO, and 50 of sulfur; potassium is, after nitrogen, the nutrient that is applied the most. In Hass avocado, field doses of potassium range from 200 to 350 kg/ha/year of K₂O, adjusted according to fruit load and soil analysis. Vegetables are among the crops with the highest potassium demand, especially from fruit set until just before harvest, and benefit from regular applications of potassium sulfate or nitrate due to their sensitivity to salinity and chlorine. In all cases, the exact dose should come from a soil analysis, not from an inherited formula.
And as I have mentioned in previous blogs, it is worth insisting on the risks of excess because with potassium they are particularly deceptive. The first you already know, which is the induced antagonism, the magnesium or calcium deficiency that you cause by over-applying potassium. The second is luxury consumption, as the plant absorbs excess potassium when available, without that translating into more yield; it is money that enters the plant but not into your pocket. The third is salinity; sources with a high salinity index, applied excessively or in soils with little leaching, raise electrical conductivity and can burn roots and reduce water absorption. More potassium, again, is not better.
The potassium that is there, but your crop does not utilize
If one crosses the data from many lots, situations appear that are rarely seen by looking at a single soil analysis. Lots that show symptoms of magnesium deficiency are almost never the ones with the least magnesium in the soil. They are the ones that received the highest doses of potassium without adjusting the balance, lots where magnesium is present but blocked.
Two neighboring farms applying exactly the same complete formula. One harvests fruit of good caliber and color; the other sees chlorosis in the old leaves and smaller fruits. The difference is not in what they applied; it is in the relationship between cations and the exchange capacity of each soil. The first has a soil with good balance; the second has one where potassium dominates and displaces magnesium and calcium.
The data that only emerges when crossing several lots and cycles is that the problem is almost never "lack of potassium" or "lack of magnesium" in absolute terms. It is a problem of proportion, and some lots need less potassium and more magnesium to rebalance; others need to correct pH before any cation can function; others have such low CEC that potassium leaches away, and much more needs to be split. Distinguishing a quantity problem from a balance problem, crossing base saturation, K/Mg ratio, CEC, texture, and liming history by lot, is what separates a recommendation that improves quality from one that, unknowingly, deteriorates it.
The quality of your harvest is not defined by the nutrient you apply the most. It is defined by the one your crop could not absorb because another blocked the door. And that is not solved by applying more: it is solved by knowing, lot by lot, what is in excess and what is blocked.
Consulted references
- Sadeghian K.S. and González O.H. (2012). General fertilization alternatives for coffee plantations in the production stage. Technical Advances Cenicafé No. 424. https://biblioteca.cenicafe.org/jspui/bitstream/10778/1107/3/avt0424.pdf
- AGROSAVIA. Technology for Avocado Cultivation — Nutrition and Fertilization. https://repository.agrosavia.co/bitstreams/72f26968-4e57-4967-a089-fd8b844a16e7/download
- ICL Growing Solutions (2025). Innovative fertilization for avocado in Latin America. https://icl-growingsolutions.com/es-mx/agriculture/knowledge-hub/innovative-nutrition-plans-for-avocado-crops-in-latin-america/
- National University of Colombia (2012). Effect of changes in the relationships of interchangeable calcium, magnesium, and potassium. UN Repository. https://repositorio.unal.edu.co/bitstream/handle/unal/8983/16077856.2012.pdf
- InfoAgro. The Potassium-Calcium-Magnesium (K/Ca/Mg) relationship in plant nutrition. https://www.infoagro.com/documentos/larelacionpotasiocalciomagnesiokcamgnutricionvegetalplantas.asp
- Gaspar, A (2022). Base saturation and Cation Exchange Capacity. Corteva https://www.corteva.com/es/agronomia-y-servicios/informacion-agronomica/saturacion-de-bases-y-capacidad-de-intercambio-catonico.html
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- Bioactivador (2026). Potassium in Agriculture: Functions, Quality, and Fertilization. https://www.bioactivador.com.pe/potasio-agricultura-funciones-calidad-fertilizacion/
- Fontanilla J. (2013). Comparison of the main sources of potassium in fertigation (salinity index and EC). https://www.interempresas.net/Horticola/Articulos/115589-Comparativa-principales-fuentes-potasio-aplicadas-fertirrigacion-para-agricultura.html
- Tessenderlo Kerley, (2020) . Potassium sulfate for chloride-sensitive crops. https://www.tessenderlokerley.com/north-america/node/73
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- PT Horticulture (2023). The function of magnesium in plant cultivation. https://www.pthorticulture.com/es-us/centro-de-formacion/la-funcion-del-magnesio-en-el-cultivo-de-plantas
- Castellanos, J. Z. 2014. Soil Acidity and its Correction. Technical Sheets from Fertilab, Mexico. 4 p Extracted from https://www.intagri.com/articulos/suelos/manejo-y-correccion-de-acidez-de-suelo
- Intagri. Series of technical articles on soil management and plant nutrition. Institute for Technological Innovation in Agriculture. https://www.intagri.com/articulos/suelos