Potassium sulfate characteristics


The right time of fertilization is more important than the dosage that is carried out. A crop goes through several stages of growth, where the end result is the harvest and collection of fruits. However, to get an adequate yield and optimum quality from such a crop, there are a few things that every farmer should know. One of them is the contribution of potassium which, as we are going to see now, can be made with potassium sulfate .

A common practice of using potassium (and in this specific case, potassium sulfate ) is to use it to ripen and improve the quality of the fruits.

Potassium is found in the soil from silicates, whose origin we have to assign to magmatic rocks and minerals such as micas and feldspars. However, in order not to go into this whole topic that doesn’t really interest that much, we basically mentioned how we find potassium in the soil, from a yield point of view.

  • Directly assimilable potassium: it  is fixed on the surface of the clays and in the clay-humic complex, which we have discussed more than once in Gardenprue.
  • Interlaminar potassium:  this form is found between the different sheets of clay and it takes a lot for the root to assimilate the potassium that can be found in it. Yet it is there.
  • Potassium in mineral fraction: it  is a fraction of the total that we can find in the soil that is not available for the plant. From phenomena such as weathering or the degradative action of bacteria, it can be released very slowly and be absorbed by the plant.

To say, everything fits, that directly assimilable potassium only forms 0.1 or 0.15% of the total of this element that we can find in the soil.


The role of potassium on crops is, because it is generic, of a physical-chemical type.

It is responsible for stimulating the water content of the cells, so this fact is closely related to the fattening and quality of the fruit. It also exerts a controlling effect on the permeability of said cell membranes. Less water loss translates into better preservation of fruit and vegetables.

It also contributes to providing resistance to plants against cold, as it regulates the saline concentration of cellular juices.

Potassium and nitrogen are good friends, so a fertilizer plan rich in potassium will promote nitrogen absorption.

Effective potassium = effective nitrogen

Likewise, the same thing happens with drought. As potassium is responsible for reducing the transpiration rate of the plant, in times of drought it will have more resistance and will be able to stoically endure in situations of environmental stress.

And, of course, potassium plays a very important role in the production of sugars. That which makes the custard apple so sweet or the watermelon so juicy. By the way, if after what we have said you want to plant watermelons, you just have to follow the recommendations of its cultivation . 🙂


When melon and watermelon ripening season arrives, a common tactic used by many farmers is to use potassium sulfate to increase conductivity and achieve higher sugar production.

Potassium sulfate contributes almost 900 mS / cm for each half gram solubilized in 1 liter of water. Or what is the same, half a kilo for every 1000 liters of water.

[alert style = »red»] 1 gram / liter of potassium sulfate provides 1.8 mS / cm of conductivity [/ alert]By raising the conductivity with this fertilizer, the plant’s resistance to absorbing water is also increased, thus reducing the probability that the fruits will crack.

Although this cracking is not directly linked to the ability to absorb water (since in normal conditions the walls of the fruits can support it), it depends on the conditions of over- ripening of the fruits , which often occurs expecting a good economic result in their sale. .

Scientific studies have supported that introducing a little ammonium (monoammonium phosphate or ammonium sulfate) reduces the water absorption capacity, so it can also help stimulate the ripening processes and reduce cracking.

However, it can be counterproductive to introduce nitrogen in this phase, so if we had to choose between these two fertilizers, we would tend more towards monoammonium phosphate (12-61-0), which contains less nitrogen (12%) and provides phosphorus than activates ripening. 

The fact of stressing the plant by increasing the conductivity increases the production of sugars and increases the º Brix. The latter, by reducing the entry of water into the plant, can also be achieved by cutting back on watering. However, it can be dangerous in times of high temperature and great sunshine.

Play with moderate watering, the application of its high potassium and a little ammonia is the perfect combination to get very sweet fruits.


It is important to know the characteristics that this fertilizer offers us, especially when establishing a fertilizer plan and potassium fertilization.

Normally, the richness of potassium sulfate is 50%, and this is related to the fertilizer units . As you can see in the previous link, to establish the amount of potassium fertilizer that is contributed to a crop (according to UF) and potassium sulfate, it is done through the following relationship.

100 FU of potassium for a culture = 200 kg of water-soluble potassium sulfate, K2O (50%).

As for sulfur (in the form of SO3), it contributes between 18% and 18.5%.


  • The sulfur contained in potassium sulfate plays an important role in the physiology of plants, activating the production of proteins, enzymes and vitamins.
  • By reducing the pH of the root environment, in alkaline soils it improves the availability of phosphorus and iron, especially, although it also influences the assimilation of other micronutrients.
  • It is interesting to manage a product highly concentrated in potassium without the need to add nitrogen (as potassium nitrate would)
  • It can be used for organic farming.

[alert style = »red»] Not all potassium sulfates on the market have an ecological registration , but there are some that do. [/ alert]


First, we start from the basis that potassium sulfate has been chosen because our intention, apart from contributing potassium to the soil (and to the plant), also wants to reduce the pH of an alkaline soil .

If we also have a soil with moderate salt content and we have to provide some fertilizer based on this element, we would use potassium sulfate.

There are crops that respond very well to this fertilizer, due to the mixture of potassium (47 to 52%, normally 50%) and sulfur (17%), such as citrus, fruit trees, brassicas , legumes, vine, banana, etc.

In sprinkler irrigation , you can add between 1 and 2.5 g / l of water, or 2-9 kg / 100 L in drip.

Some recommendations, as we can extract from some products based on potassium sulfate:

  • Citrus:  6 kg / ha and day.
  • Fruit trees:  250-500 kg / ha and year.
  • Vegetables:  300-1,000 kg / ha.
  •  Pepper:  7 kg / ha and day, in the fattening and ripening phase (during the last 2-3 weeks).
  • Olive tree  0.75-2 kg / tree and day
  • Pineapple:  8-9 kg / ha and day, during the flowering of the suckers.
  • Banana:  5 kg / ha and day.

Of course, all this is by way of recommendation. The ideal is to establish a suitable fertilizer plan based on a soil analysis.


We must also take into account the potassium levels that, naturally or through previous contributions, the soil where we are going to cultivate.

If the levels of potassium in the soil  are normal or adequate, we simply have to add potassium according to the absorption that the plant makes of this element. That is, to maintain the fertility of the soil at natural and adequate levels.

If the soil has poor potassium levels , the objective is to saturate the interlaminar spaces of the clays (which we have discussed before) and the surface areas. If we have a sandy soil , the contributions of potassium sulfate will be greater with respect to another texture, since there will be more washing.

If we have a soil with a high potassium content, we must evaluate, through soil analysis, possible problems related to the accumulation of salts and magnesium deficiencies in the plant.

This does not mean that there is no magnesium in the soil, but that the plant is not able to assimilate it due to the K / Mg antagonism .

Within the application of potassium sulfate, we will have different forms of presentation, adapted to fertigation (fast soluble fertilizer in water) and in granules, for bottom fertilizer, slowly soluble.

Potassium sulfate powder

Quickly soluble in fertigation tanks, for application in sprinkling, dripping and injected into irrigation pipes.

Granulated potassium sulfate

The potassium sulfate ball can be unprotected or protected (waxy layer) to control solubility. It is generally applied directly to the ground as a background fertilizer or around trees, and is slowly diluted by environmental humidity or rain.


Although according to the technician who recommends the fertilizer plan and all the variables that can influence the amount with which a soil is fertilized (especially, measured in the soil analysis), an example of the potassium fertilizer units that some need vegetables and greens would be the following: ( Extracted from the practical guide to rational fertilization of crops).

Outdoor crops

Artichoke (15-20 t / ha): 300-380 kg / ha

Apio (60-80 t / ha): 380-600 kg / ha

Berenjena (50-70 t / ha): 320-400 kg / ha

Broccoli (15-20 t / ha): 370-450 kg / ha

Zucchini (20-30 t / ha): 110-160 kg / ha

Onions (60-70 t / ha): 200-250 kg / ha

Col (45-55 t / ha): 290-320 kg / ha

Cauliflower (25-35 t / ha): 300-360 kg / ha

Spinach (22-28 t / ha): 180-220 kg / ha

Pea (3-5 t / ha): 90-140 kg / ha

Green beans (12-16 t / ha): 130-160 kg / ha

Lettuce (30-40 t / ha): 180-230 kg / ha

Melon (30-40 t / ha): 250-330 kg / ha

Pepino (25-35 t / ha): 120-160 kg / ha

Pepper (55-65 t / ha): 300-340 kg / ha

Leek (25-35 t / ha): 130-200 kg / ha

Rábao (22-28 t / ha): 90-110 kg / ha

Watermelon (45-55 t / ha): 180-220 kg / ha

Tomatoes (55-65 kg / ha): 300-330 kg / ha

Carrot (60-70 t / ha): 300-450 kg / ha


For all those who have a fertilizer spreader, say that not everything goes and everything can not be mixed with everything. In a basic table of incompatibilities between nutritional elements (fertilizers), we can find the following relationship:

COMPATIBILITIES:  potassium nitrate, ammonium nitrate, monopotassium phosphate, monoammonium phosphate, magnesium nitrate, magnesium sulfate.

INCOMPATIBILITIES:  calcium nitrate.

Your fgr in water:

Water solubility 111 g/L (20 °C)
120 g/L (25 °C)
240 g/L (100 °C)


These two formulas of potassium fertilizers are characterized, above all, by not providing nitrogen to the crop. That is, it is an alternative to the well-known  potassium nitrate that provides 13% nitrogen and 46% potassium.

In terms of richness, there are few different ones, since potassium sulfate contributes between 50 and 52% of potassium (and 46-47% of SO3) and potassium chloride has a potassium richness of 60%, somewhat more than the previous one. .

However, it must be taken into account that there are crops that are sensitive to the contribution of chlorines, but having a calculation of the contribution of chlorine to the soil and what the crops will absorb, it is a highly recommended fertilizer and we should not be afraid of it. .


Regardless of whether we contribute potassium sulfate, potassium nitrate, potassium chloride or organic matter (which usually has between 4 and 6% of this element) to the soil (and later to the plant), we must maintain minimum levels of this element to guarantee adequate production.

We can know this if we know the reference values ​​in a soil analysis .

In the exchange complex , the potassium must be between 2 and 6% of the total, leaving a soil in good condition as follows:

Exchange rates (meq / 100 gr):

  • Sodium: 0.5-3.0%
  • Potassium: 2.0-6.0%
  • Calcium: 40.0-80.0%
  • Magnesium: 10.0-30.0%

As you can see, in this case the main protagonists are calcium and magnesium.

Saturated extract

An acceptable value of potassium in the soil, in order not to have problems with the supply of this element to the plants and, on the contrary, so as not to impair the absorption of other elements if it is in excess, would be the following:

  • Potasio: 1,0-5,0 meq/L

In the event that the value is close to 1 and depending on the crop, organic matter (OM) will have to be applied that can be enriched with potassium chloride or potassium sulfate , in case you want to limit the application of chlorides.

If we have a high pH soil with obvious iron chlorosis problems in the middle of the season, acidifying the soil with potassium sulfate will greatly improve the future response of the crop.

On the other hand, if we have a soil that is already acidic, we will have to look for alternatives to potassium sulfate and choose fertilizers or compounds that do not reduce the pH. In this case, the contribution of organic matter (compost, liquid slurry, manure, etc.) will provide many benefits (improvement of the CEC of the soil, texture, sponginess, etc.) and will provide potassium.

Increase 200 UF / ha of potassium in soil with a low content of this element

It is usually done in crops that are demanding in this element, providing a background fertilizer rich in potassium. For example, in industrial tomato, potato, onion, etc.

For this, if we apply 400 kg / ha of potassium sulfate , we provide 200 kg of pure potassium and around 75 kg of sulfur

If we have an organic matter at 4% potassium, we need to apply 5000 kg / ha of this element. Perhaps the ideal mixture is to use a mixture of both components to take advantage of their full potential.

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