The most widely used potassium fertilizer is muriate of potash (potassium chloride), which contains 60% potash. But it’s far from the only option. Potassium shows up in many synthetic and organic fertilizers, each with different strengths and tradeoffs depending on your soil, your plants, and how you garden.
On any fertilizer bag, the third number in the N-P-K ratio tells you the potassium content, expressed as potash. A bag labeled 18-4-10, for example, is 10% potash by weight. If that bag weighs 100 pounds, it contains 10 pounds of potash.
Muriate of Potash (MOP)
Potassium chloride, sold as muriate of potash or MOP, is the workhorse of potassium fertilizers. Its N-P-K ratio is 0-0-60, meaning it delivers potassium and nothing else. It’s cheap, widely available, and the default choice for most agricultural and lawn applications.
The main downside is chloride. MOP has a high salt index of 116, which means it can stress plants and soil microbes if overapplied. Chloride-sensitive crops like tomatoes, peppers, strawberries, beans, and many fruit trees don’t do well with repeated MOP applications. For these plants, sulfate of potash is the better pick.
Sulfate of Potash (SOP)
Potassium sulfate, or sulfate of potash, carries an N-P-K of 0-0-50. It delivers slightly less potassium per pound than MOP but brings sulfur along with it, which many soils need. Its salt index is only 46, less than half that of MOP, so it’s much gentler on roots and soil life.
SOP costs more than MOP, which is why large-scale grain farmers typically stick with MOP. But for vegetable gardens, orchards, and any chloride-sensitive crop, the extra cost is worth it. If your soil test shows low sulfur alongside low potassium, SOP solves both problems at once.
Potassium Nitrate
Potassium nitrate provides about 44% potash plus a dose of nitrogen. It’s a dual-purpose fertilizer, but the cost makes it impractical for field crops. You’ll mostly see it used on high-value crops, in greenhouses, and in hydroponic systems where precise nutrient control matters. It dissolves cleanly in water, which makes it ideal for fertigation (feeding through irrigation lines).
Organic Sources of Potassium
If you prefer organic fertilizers, several options supply meaningful potassium, though at lower concentrations than synthetic products.
- Kelp meal: 4% to 13% potassium, plus a broad range of trace minerals. It breaks down slowly, feeding soil biology as it releases nutrients.
- Wood ash: 3% to 7% potassium. It also raises soil pH, so use it only if your soil is acidic. Avoid it on blueberries, azaleas, and other acid-loving plants.
- Greensand: About 5% potassium. It releases very slowly over months or years, making it a long-term soil amendment rather than a quick fix.
Organic sources work best when you’re maintaining adequate potassium levels over time. If a soil test reveals a serious deficiency, you’ll likely need a concentrated product like SOP or MOP to correct it, then switch to organic sources for maintenance.
Why Plants Need Potassium
Potassium activates more than 60 enzymes involved in basic plant metabolism. Its most visible role is controlling the opening and closing of stomata, the tiny pores on leaf surfaces that regulate water loss and gas exchange. When potassium is abundant, stomata function efficiently, and the plant can photosynthesize and manage water stress more effectively.
Potassium also drives water movement through plant tissues. It builds the internal pressure (turgor) that keeps cells firm and stems upright. Plants low in potassium wilt more easily, grow more slowly, and produce smaller fruit. During drought, potassium is the first nutrient the plant draws on to adjust its internal water balance.
How to Spot a Deficiency
Potassium deficiency shows up on the oldest leaves first because the plant pulls potassium from older tissue and shuttles it to new growth. The classic symptom is yellowing along the leaf margins, starting at the tip and creeping down the edges toward the base. In severe cases, those yellow margins turn brown and crispy as the tissue dies. The newest leaves may still look healthy while the lower canopy deteriorates, which can make the problem easy to miss at first glance. Affected plants often appear stunted overall.
A soil test is the only reliable way to confirm what you’re seeing. Magnesium deficiency can look similar, and applying the wrong nutrient wastes time and money.
How Much to Apply
For lawns, the maximum safe single application is about 1 pound of potash per 1,000 square feet. Going heavier risks burning the grass, especially in warm weather. Water the lawn after application to wash the granules off the leaf blades and into the soil.
For gardens and field crops, rates depend entirely on your soil test results. Soil type matters too. Sandy soils with low organic matter can lose potassium to leaching, so splitting your total amount into two or more applications works better than a single heavy dose.
When to Apply
Fall is an effective time to apply potassium for most situations. For corn and soybeans, field research has shown fall application performs just as well as spring application. Winter wheat needs all its potassium at fall planting. For lawns, a fall potassium application helps build winter hardiness before cold weather arrives.
The one exception is sandy soil with low organic matter. On those soils, fall-applied potassium can leach below the root zone over winter, so a spring application is the safer bet.
Watch the Balance With Other Nutrients
Potassium, calcium, and magnesium compete for uptake through plant roots. Research on sugarcane found that increasing potassium application caused consistent drops in leaf calcium and magnesium, even when those nutrients were present in the soil at adequate levels. Potassium suppressed magnesium uptake more than calcium. This effect, called cationic antagonism, means that heavy potassium applications can create secondary deficiencies of magnesium or calcium over time.
This is another reason soil testing matters. If your soil is already high in potassium, adding more won’t help your plants and could actually starve them of other nutrients they need. A balanced approach, guided by actual soil data, gives you better results than guessing.