When it comes to lawns, nitrogen hogs all the attention. But grass needs 16 essential nutrients to grow healthy and vibrant. One of the major three is Phosphorus (the P in NPK).
What is Phosphorus and Why is it Important for Lawns?
Phosphorus (also commonly referred to as phosphate) is the P in NPK for fertilizers and is the second number listed in its analysis. In a 3-1-2 fertilizer, there’s 1 part phosphorus to 3 parts nitrogen and 2 parts potassium. Most lawns already have enough phosphorus just from the native soil, especially if they’re long established.
Phosphorus is all about supporting a plant’s photosynthesis, allowing it to take sunlight and turn it into energy to grow and reproduce. In gardening, it helps with fruit and seed production, but grass doesn’t need much help there. For grass, it’s more important for healthy roots – although as you’ll read below, too much will cause root development to the detriment of a healthy relationship with fungi.
Traditionally, new lawn formulas have used higher amounts of phosphorus to help kick start seedlings, as the seedlings’ shorter roots mean it has a harder time accessing nutrients.
When grass decomposes (such as when you leave grass cuttings on your lawn!), the phosphorus returns to the soil.
Between regular fertilizer applications, little soil erosion, and decomposing plant matter, most lawns will never have a phosphorus deficiency. You’re much more likely to have too much phosphorus, as once it’s added, it’ll stay in the soil. Over the years, it will build up to problematic levels.
Some soils are naturally lower in phosphorus levels, and a newly established lawn (like for a newly built house) or a lawn grown in sandy soil is more at risk of phosphorus deficiency just because they’re low in nutrients overall.
Soil tests are so vitally important before applying fertilizers! They tell you what you’re lacking and what you already have too much of (even before problems start), and that allows you to avoid a lot of these problems. Test your soil every 3 to 4 years unless there’s a problem.
If you have sufficient levels of phosphorus, use fertilizers with low amounts of phosphorus, if any. Compost is an excellent fertilizer, as nutrient levels are much lower and last longer than synthetic lawn fertilizers but are still enough for most lawns. Most lawns need a lot less NPK than you’d think looking at the average lawn fertilizer bag.
If you have high phosphorus levels, then use fertilizers without phosphorus.
Can Your Lawn Have Too Much Phosphorus?
Yes, your lawn can have too much phosphorus. Even if you’re not adding huge amounts and only adding what’s in the regular fertilizer, phosphorus builds up in the soil. Once it builds up enough or if there’s runoff, it leaches into nearby sources of water. Algae take advantage of the additional phosphorus and explodes in growth. The thick green algae just looks and smells horrendous. Once the algae dies, it robs oxygen from the water, killing the fish and all other aquatic life. Some types of algae are poisonous to humans and the wildlife.
Too much phosphorus is bad for lawns too.
Excessive phosphorus reduces your lawn’s ability to take up iron and zinc. If you think you have an iron deficiency, take a soil test before you add Ironite as you may find your soil’s iron stores perfectly fine, there’s just too much phosphorus. Phosphate also combines with iron to create insoluble iron phosphate, which cannot be used by the plant or even broken down. Shallow-rooted turfgrass like St Augustine are especially susceptible to iron and zinc deficiencies.
Too much phosphorus also inhibits mycorrhizal relationships between fungus and grass roots. Beneficial fungi (mycorrhizae) are much better at absorbing water and mineral nutrients than plant roots, especially in clay soil or alkaline soil, and so they pass on those nutrients to your plants in exchange for sugars.
But when phosphate levels are too high, they can’t do that. Instead, your grass is forced to use its energy to grow deeper roots. That may sound good as deeper roots usually means better drought resistance (unless you have a shallow topsoil layer), but that’s better achieved through deep waterings.
Mycorrhizae also aid plants in disease, drought, and pest resistance. Without these mycorrhizal relationships, your lawn is much more prone to insect, disease, and drought.
What Are Signs Of High Phosphorus Levels In Grass?
The best way to tell if your soil has high phosphorus levels in your grass is by a soil test, which can give you an accurate view of your soil’s nutrient levels.
Other signs are iron and zinc deficiencies caused by excessive phosphorus. A soil test will also rule out other causes, like alkaline soil or insufficient iron and zinc stores in the soil.
Symptoms of iron deficiency include:
- The top of the blades turn yellow (chlorosis). From a distance, this looks like your yard has a yellowish tinge to it. (Nitrogen deficiency also turns grass yellow, but it starts from the bottom with the old growth.)
- The yellow will overtake the green or even turn white as it progresses down the grassblade.
- Reduced stem growth and leaf size (really difficult to tell).
- Yellowing (chlorosis) between the veins, starting with old leaves or a general reddening of leaves.
You may not see any sign of zinc deficiency, as grasses are more tolerant of low zinc soils than other plants.
How Do You Fix High Phosphorus Levels In Grass?
- Stop using phosphorus on your lawn. This includes compost. If you need to amend with other nutrients (which you know thanks to your soil test), then choose a fertilizer or amendment that adds that specific nutrient without phosphorus.
- Remove grass clippings from the lawn. Phosphorus readily returns to the soil when plant matter decomposes, so removing grass clippings instead of allowing them to decay will help remove phosphorus from the soil.
- If your grass shows iron or zinc deficiencies, then use a foliar spray. Iron foliar sprays are quite effective in the short-term and bypass the phosphorus that limits their uptake. Be careful when spraying iron as it can leave rust stains on sidewalks and other structures. Foliar sprays are short-term solutions that will help your grass while your soil recovers.
- Use patience. Phosphorus stays in the soil for a long time, but over time, it’s converted into stable, insoluble forms that won’t affect soil health (except in acidic soil). How long it takes depends on how high the levels are and other conditions, but [soils tested at 150 to 200 parts per million can take 3 to 5 years to rectify.
What Are Some Of The Best No Phosphorus Fertilizers?
Feather meal contains 15% nitrogen which releases over 4 to 6 months, making it a very effective slow release fertilizer. It has no phosphorus or potassium.
More fertilizers are being manufactured these days without phosphorus in nitrogen or nitrogen-potassium combinations.
Lesco Professional Turf Fertilizer 15-0-15 for Bermuda, Centipede, and St Augustine
You can find Lesco Profrofessional Turf Fertilizer at the home depot in-store or online here.
Gordon’s Liquid Lawn and Pasture Fertilizer 20-0-0 (With Copper, Manganese, Zinc)
You can find Liquid Lawn and Pasture Fertilizer on TractorSupply by clicking here.
What are Some Signs of Phosphorus Deficiency in Grass?
Nutrient deficiencies are difficult to identify in plants, and phosphorus deficiency especially so. If you suspect phosphorus deficiency (or any deficiency), the best action to take is to get a soil test done. If you live in a region that has banned phosphorus in lawn fertilizers, then you’ll need a soil test showing phosphorus deficiency.
A soil test will also test your pH levels. Phosphorus combines with other chemicals in alkaline (7.3 pH and above) and very acidic soils (below 5.5 pH), making the phosphorus unavailable for plants to absorb. Adding more phosphorus won’t treat the underlying problem.
Phosphorus deficiency symptoms include:
- Reduced vigor or slow growth, even though the grass may look darker green.
- Leaves may turn darker green then purple. (Other factors can cause red/purple leaves, so this isn’t conclusive.)
- Grass may grow thin and less dense.
RELATED: Read our 7 Ways to Thicken a Thin Lawn
How Do You Fix Phosphorus Deficiency in Your Lawn?
Fixing a phosphorus deficiency in your lawn is tricky. Your lawn has likely grown thin and less dense, which means phosphorus is much more likely to runoff and leach away, rather than stay in your soil.
- Test your soil. Phosphorus deficiency is particularly tricky to identify correctly, and knowing your phosphorus levels will help you accurately apply the right amount of phosphorus. The soil test will also rule out other causes, like that your soil is alkaline or acidic.
If Your Soil is Phosphorus Deficient:
- If your soil is compacted, core aerate your lawn to decrease runoff. This allows water to filter deeper into the soil. Otherwise, any rainfall or irrigation will just carry off any amendments instead of sinking into the soil.
- Choose the right phosphorus amendment. Your soil test may already include amendment recommendations. Your choice should reflect the level of phosphorus in your soil. Don’t automatically reach for the highest concentration like bone meal or bat guano or a high phosphorus fertilizer. You may be better off using a lower concentration or an amendment that releases more slowly over time to avoid runoff and reduce the risk of adding too much. Erring on the side of caution also saves you money.
- Measure your lawn to determine square footage. Figuring out the right amount of fertilizer depends on knowing exactly how large your lawn is. Without an accurate figure, you risk applying too much.
- Apply half as much phosphorus as the package recommends. Less is often more when it comes to amendments. Most phosphorus sources become available within 2 days of application. If it’s not enough, you can always do the next application within two weeks.
- Apply fertilizer or amendment when the soil is dry to moderately moist and lightly water it in.
- Avoid adding amendments right before intense rainfall and avoid overwatering.
- Leave grass clippings on your lawn to return the phosphorus back into the soil.
If Your Soil is too Alkaline or Acidic:
The best course of action is to choose a grass species that’s suited for your pH level. Grass species adapted for these pH levels use phosphorus more efficiently and so it doesn’t matter as much if the phosphorus is unavailable.
For alkaline soils, that’s Bermuda and Seashore paspalum in warm-season zones, Fairway wheatgrass and Fescues in cool-season, and Buffalo Grass in transition zones.
For acidic soils, that’s Centipede grass in warm-season zones, and Chewings or Red Fescue in cool-season zones.
If that’s not an option, then you will need to adjust your pH level. Adjusting pH levels is tricky, which is why it’s important to use a more precise soil test number than using a home kit. Adding too much of an amendment can swing your pH level too much.
What Are Some of the Best Phosphorus Fertilizers?
The best phosphorus fertilizer is the best fertilizer for your soil’s particular needs. Your soil test report should include recommendations. You can also contact your local university extension office for specific advice to your local and your test results.
While most composts will have some phosphorus, if you need more bang, then look for cow or chicken manure. Cow manure usually has 0.8-0.5-0.5 and chicken manure has 1-1.5-0.5. These might not look like a lot when you’re looking at the NPK ratings on synthetic fertilizer bags, but these low amounts are actually enough for many lawns.
Rock phosphate typically contains 30% phosphate that releases slowly over many years (at around 3% concentration). This is a choice best kept for acidic soils, as alkaline soils renders it insoluble (your grass can’t uptake it).
Fish meal is pretty fast-acting (although it needs a chance to break down) with a substantial amount of phosphorus and nitrogen (generally around 6% of each, plus calcium).
Phosphorus Lawn FAQs
Why Is Phosphorus Banned In Turf Fertilizers?
Phosphorus is being banned because excess phosphorus leaches into rivers, lakes, and streams leading to algae blooms. These algae blooms, beyond smelling terribly and limiting recreational use, cause low oxygen levels in the water which harms and kills fish and other aquatic life. Lawns are a big culprit, as many people apply phosphorus with their fertilizer even though most lawns already have enough. Few lawns ever have phosphorus deficiency. Don’t worry about your lawn – lawns that have only nitrogen and potassium fertilizers have seen no negative effects.
How Long Does It Take Phosphorus To Work?
How long it takes for phosphorus to work depends on the phosphorus source. With synthetic sources, it’s most available within a few days (except in fall, if you have cold winters, as the phosphorus will stay in the soil for months before uptake). Bone meal and fish meal release high amounts of fertilizer rapidly, while rock phosphate releases a low amount (3%) of phosphorus over many years.
When Should I Apply Phosphorus To My Lawn?
Only apply phosphorus to your lawn if a soil test shows deficient phosphorus levels. Otherwise, a no-phosphorus or a minimal phosphorus product (like compost) is all that’s needed. In timing, research has shown that there’s little difference between applying in the spring and the fall. Only apply fertilizer when your grass is growing vigorously.
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.