When we talk about lawn fertilizer, we obsess over nitrogen. Nitrogen is the stuff that makes our grass grow fast and green. But lawns need over 16 soil nutrients, and one of the biggest three is Potassium (the K in NPK).
What is Potassium and Why is it Important for Lawns?
Potassium is one of the three big fertilizer nutrients, the K in NPK. It helps plants withstand temperature extremes, diseases, and drought, and aids root development.
It manages water movement between plant cells and throughout the plant (causing plants to become turgid or flaccid), and it’s partially responsible for opening and closing the stomata. When stomata are open, water can evaporate through them, and when they’re closed, the water remains locked inside. This process is extremely important for plants to stay alive during extended dry periods.
Potassium helps plants withstand cold weather. For that reason, many people in cool-season regions give their lawns a potassium boost in the fall. If your soil is already rich in potassium, though, or you’re already fertilizing in the fall anyway, that’s unnecessary. Your grass will take up what it needs.
One common myth is that potassium will allow plants that aren’t hardy to a region to survive the winter [PDF], but the science doesn’t back that up. Potassium can’t give a plant characteristics that it doesn’t already have. It won’t make your St Augustine lawn cold-hardy or drought resistant. But if your soil doesn’t have enough potassium in it, then your lawn will struggle during heat and cold.
If you’re using an NPK fertilizer, then it already has potassium in it.
Can Your Lawn Have Too Much Potassium?
Yes, your lawn can have too much potassium. Most home gardens will have enough potassium already and regular fertilizer applications will replenish stores. Too much potassium restricts how much magnesium a plant can absorb. This makes it look like your lawn has a magnesium deficiency when it’s actually suffering from potassium toxicity.
As with all nutrients, it’s important to get a soil test first! There’s no point in amending a nutrient that’s already there in sufficient quantities, and in fact, giving too much of a nutrient harms your lawn in other ways.
The great thing about organic fertilizers like compost is that they do not contain large amounts of nutrients, but contain a lot of different nutrients (like magnesium) that become available at low, manageable amounts over a long period of time.
Grass doesn’t really need as high concentrations of NPK that commercial fertilizers seem to argue. It’d be really hard to overdose your lawn with compost, and your lawn will be healthier for it.
What are Some Signs of Potassium Deficiency in Grass?
While most yards will have plenty of potassium, too wet, heavy clay, acidic and sandy soils are particularly prone to potassium deficiencies. Acidic pH levels restrict how much potassium plants can absorb. Sandy soils leach nutrients and nutrients need to be replenished more often. Sandy soils also tend to be highly acidic. Some turfgrasses can also be prone to potassium deficiency.
If you suspect a nutrient deficiency, the best thing you can do is send for a soil test. The soil test will give you the complete data of what’s actually available and what’s deficient in the soil. Nutrient deficiencies can be hard to diagnose by themselves and incorrectly amending with nutrients can end up causing other deficiencies – like the above potassium toxicity looking like a magnesium deficiency.
Potassium deficiency can be especially hard to diagnose as symptoms mostly appear when the grass is undergoing environmental stress (hot and cold weather, drought).
Signs of potassium deficiency include:
- The grass blades turn yellow and brown (chlorosis) starting on the tips and proceeding toward the leaf base. The grass blade may even die.
- Grass blades look wilted even after watering.
- Grass doesn’t grow as quickly as it should, with shorter, finer blades.
- Grass has less resilience to environmental stresses like hot and cold temperatures. If other people in your area with the same grass have lawns that bounce back and yours doesn’t, check the potassium levels.
How Do You Fix Potassium Deficiency in Your Lawn?
You can fix potassium deficiency in your lawn by adding a source of potassium. You have many options depending on your situation. If your soil test shows low but not deficient levels of potassium, then focus on a long-term strategy. If your soil test shows deficient levels, then use a short-term solution like liquid fertilizer, which will start fixing the problem immediately and give you time to sustainably restore levels in the long-term.
- Seaweed products, like kelp meal. Kelp actually contains all 16 elements needed for plant growth (most in micro doses that don’t disrupt the balance of nutrients) and includes the hormone cytokinin, which helps grass grow vigorous roots. It generally contains 1% nitrogen and phosphorus and 5% potassium, although products will vary. If you live by the coast and can legally forage kelp washed up on shore, kelp can be really inexpensive. Otherwise, your best bet is concentrated liquid kelp, which is expensive per square foot.
- Wood ash. The original source of fertilizer potassium! But be careful – wood ash makes soil more alkaline, so if you already have slightly acidic, neutral, or alkaline soil, give this one a miss. If you have really acidic soil (which can restrict potassium uptake!), then this can really help. Only use a reputable source. It contains 2% phosphorus, 6% potassium, and 20% calcium.
- Compost is a long-term strategy, as it generally contains 1% of nitrogen, phosphorus, and potassium, as well as other nutrients. It can renew the nutrients in your soil to keep up levels of potassium without risking burning your lawn or potassium toxicity. You can also add wood ash or kelp to your compost to increase the potassium content.
- This potassium sulfate is marketed as Sul-Po-Mag or K-Mag. It’s a long-term solution as it’s less soluble than muriate of potash, which means it stays in your soil for longer. It boasts 22% potassium, 18% magnesium, and 27% sulfur. Be very careful! Use a soil test first to determine whether you need magnesium and sulfur, as too much of either can cause problems.
- Also known as glauconite, greensand contains 8 – 10% potassium. It’s renowned as a great slow-release source of potassium that’s also organic. It can also hold water and nutrients, which can be a big help for sandy soils. Unfortunately, it is expensive.
Muriate of Potash vs Sulfate of Potash
Muriate of Potash (Potassium chloride) and Sulfate of Potash (Potassium sulfate) are both water-soluble salts that have a high potassium content. They’re both commonly used in fertilizers and potassium amendments.
Potassium sulfate is usually preferred of the two, although it’s more expensive, as potassium chloride (muriate) has a higher salt content and chloride in it, which can adversely affect plants. Potassium sulfate cannot be applied as a liquid fertilizer while potassium chloride can. Langbeinite is potassium sulfate.
How to Add Potassium to Your Lawn (Step-by Step)
1. Get a soil test done. A soil test will give you detailed information about what nutrients are currently available and which ones have been depleted. Potassium deficiency and low potassium can be tricky to diagnose without one, and it’ll give you a much better idea of how much you actually need to make up for and what kind of strategy you need. Are levels so low that you need something that acts quickly or can it wait until your next scheduled fertilizer application?
2. Choose a fertilizer or amendment based on the soil test. There’s very few fertilizers and no natural amendments that are potassium only, so with the soil test data, you can choose one that won’t skyrocket other nutrients. The right fertilizer or amendment may also fix other nutritional deficiencies at the same time. You also can choose the right amount of potassium to add and the right form for how quickly the potassium becomes available based on how low your levels are.
3. Follow the instructions on the fertilizer for how much to apply. If you haven’t already, measure out your lawn. Do not estimate! If you overestimate, you run the risk of burning your lawn (especially with synthetic fertilizers). Always use caution and err on the side of too little. Apply granular or solid fertilizers (like kelp meal and compost) with a broadcast spreader.
Does it Matter What Time of Year You Add Potassium to Your Lawn?
The common advice is to add potassium to your lawn in the fall to help your grass overwinter. Potassium is easier for plants to uptake when the soil is warmer. If you’re regularly fertilizing once or twice a year, that’s really the best time since most fertilizers – whether synthetic or natural like compost – will contain some potassium.
If your grass shows signs of potassium deficiency and soil shows low levels of potassium, then it’s best to apply potassium as soon as is reasonable.
Like all fertilizers and amendments, avoid adding them when grass is dormant, about to go dormant, or coming out of dormancy. You’ll end up stressing out your grass when it’s focused on protecting itself and could damage your lawn.
What are Some of the Best Potassium Fertilizers?
Potassium Lawn FAQs
What is Potash?
Potash is the common name given to a group of minerals and chemicals that contain potassium. These chemicals are most often used for fertilizer. It was originally produced by leaching and evaporating wood ash into potassium carbonate, but now most potash used is from mined potassium salts.
Different forms of potash have different concentrations of potassium, so to make things consistent, we measure potassium levels by potassium oxide (K20). Potassium oxide (K20) won’t tell you which form of potash is used, only the concentration.
What’s the Difference Between a Liquid Potassium Fertilizer and a Granular Potassium Fertilizer?
Liquid potassium fertilizer is more mobile in the soil, while granular potassium fertilizers (which contain high salt content) can burn your lawn if misapplied but continue to feed your lawn for longer. If your lawn is suffering from potassium deficiency, then using a liquid fertilizer will give it aid faster, but it won’t continue to feed your lawn.
If your soil has low potassium but isn’t deficient yet, going with a long-term solution will pay off. Long-term potassium sources without salts include dried kelp meal and hardwood ash.
Is Potassium More Important in Warm or Cold Climates?
Potassium is important in both warm and cold climates, as it regulates water in the plant and improves resilience to environmental stresses – whether that’s high heat or the cold. It also improves disease resistance, which is helpful everywhere.
Can Potassium Change the Color of Your Grass?
No, potassium cannot change the color of your grass. Potassium deficiency may cause the tips of your grass to turn yellow and brown, but other than that, it doesn’t affect the color.
If you are interested in the other big fertilizer nutrients in NPK check out:
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.