How Phosphorous Affects Your Lawn and Environment
- "The Lawn Care Nut"July 22, 2026
You all who follow my teachings and strategies know that the two elements in fertilizer that I talk about most often are nitrogen and iron. That’s because these are the two that are going to give your lawn the deepest, darkest blue-green color response. And that is one reason why I’ve always liked using Milorganite because it contains 2.5% chelated iron along with 6% slow-release nitrogen to not only green up the grass but helps the turf pull in that iron to green up even more.
But there is another element in Milorganite that is equally important. In fact, it’s a macronutrient, so one could argue that in bulk, it’s needed much more than iron. That element is phosphorus.
It’s the second number on the bag of Milorganite, which is four. That 4 stands for 4% and lets you know that in every bag of Milorganite, 4% of it is phosphorus.
What Does Phosphorus Do For Grass Plants?
When I was taught about fertilizers for lawns and the big 3 macronutrients (the 3 numbers on the bag) I learned “Up” “Down” and “All Around” as a way to know what each of these does for plants. Nitrogen turns plants green and gives them top growth, phosphorus builds strong roots and potassium helps with all-around health and balance.
That’s a pretty good way to describe it, for sure, but it’s pretty elementary.
I wanted to see if there was a better way to explain it, so I asked ChatGPT to explain it to me as if I were a high school student. Here’s what I got:
"Explain to me what phosphorus does for plants and explain it to me at a high school level."
Phosphorus is the nutrient that helps plants convert sunlight into usable energy, build strong roots, and produce flowers, fruits, and seeds, making it essential for healthy growth and reproduction.
Phosphorus is one of the three primary nutrients plants need to grow, along with nitrogen (N) and potassium (K). If nitrogen helps plants grow leaves and stems, phosphorus is the nutrient that helps plants capture, store, and use energy.
Think of phosphorus as a plant's "battery"
Plants make sugar through photosynthesis using sunlight, water, and carbon dioxide. Phosphorus is a key part of the molecules (called ATP) that store and move that energy throughout the plant.
Without enough phosphorus, it's like a phone with a weak battery—it has the ability to work, but it doesn't have enough energy to do it efficiently.
What phosphorus does
Phosphorus plays several important roles:
???? Develops strong roots – It encourages roots to grow deeper and become more extensive, helping plants find water and nutrients.
???? Supports flowering and fruit production – Plants need phosphorus to produce flowers, seeds, fruits, and vegetables.
???? Helps seeds germinate – Young seedlings rely on phosphorus to establish healthy roots and begin growing.
⚡ Transfers energy – It helps move energy produced during photosynthesis to where it's needed for growth.
,???? Builds DNA and cell membranes – Phosphorus is a basic building block of every plant cell, so it's essential for creating new tissues.
What happens if a plant doesn't get enough phosphorus?
Plants with a phosphorus deficiency often:
- Grow more slowly than normal.
- Develop weak or shallow root systems.
- Produce fewer flowers, fruits, or seeds.
- Have older leaves that may appear dark green with reddish or purplish coloring (especially in cool weather).
Can you have too much phosphorus?
Yes. While phosphorus doesn't usually burn plants the way excessive nitrogen can, too much phosphorus in the soil can create problems.
- It can interfere with the plant's ability to absorb important micronutrients like iron and zinc, leading to nutrient deficiencies.
- Excess phosphorus that washes off lawns or fields into lakes and rivers acts like fertilizer for algae. This can cause algal blooms, which reduce oxygen in the water and harm fish and other aquatic life.
My follow-up question to ChatGPT was:
"Explain the difference in phosphorus in a slow-release fertilizer like Milorganite's vs. a synthetic fertilizer?"
The biggest difference is not the phosphorus itself—phosphorus is the same chemical element regardless of where it comes from. The difference is how it becomes available to plants.
Think of it like food
Imagine your body needs energy.
- Synthetic phosphorus is like drinking a sports drink. The nutrients are immediately available.
- Milorganite's phosphorus is like eating oatmeal. Your body digests it gradually, providing nutrients over a longer period.
Both provide what you need—they just do it at different speeds.
Why does this matter?
Plants don't absorb all of their phosphorus at once. They take it up throughout the growing season as roots expand.
Because Milorganite's phosphorus is released gradually by soil microorganisms:
- It more closely matches the plant's ongoing nutrient demand.
- There is less immediately soluble phosphorus in the soil solution after application.
- It contributes organic matter that supports healthy soil biology, which helps cycle nutrients naturally.
The above is the guaranteed analysis from a common synthetically derived and blended fertilizer containing phosphorus.
Where Does Phosphorus Come From?
There are a few different sources of phosphorus on this planet. The one that I first think of is called “bone meal” and my Grandma swore by it in her garden. Every new planting got a dusting of bone meal in the hole before placing the plant. It comes from exactly what it states, “crushed bones.”
Another popular source of phosphorus is bird guano (poop). Not all guano is created equally, as early fertilizer prospectors learned when they claimed nearly 200 remote islands for the harvesting of bird guano for fertilizer. Here is an interesting article from the Smithsonian.
There is also some phosphorus in cow and chicken manure which is used mostly in organic farming, as well as composting.
Rock Phosphate Mining
Today, most of the phosphorus used in fertilizers for crops and lawns comes from rock phosphate that is mined in Florida. In fact, 75% of the phosphate used in the United States, as well as about 25% of the phosphate used around the world (source) is mined here in Florida, about 15 miles from where I live in Manatee County.
Above you can see soil test data showing phosphorus levels across the state of Florida. Note the panhandle where phosphorus levels are very low compared to the rest of the state.
In order for plants to utilize the phosphorus mined, the rock phosphate must go through a few different processes from washing to separating (beneficiation) to reacting it with sulfuric acid to produce phosphoric acid.
The most common fertilizer product used in lawn fertilizers is diammonium phosphate (DAP), made by reacting ammonia with phosphoric acid. This yields a source of phosphorus that is water soluble and available for plant uptake right away.
There can be a downside here though, and that being that the phos is so readily available, it can be easily washed out of soil or leached through. In Florida for example, we get heavy afternoon rainstorms during summer and these can wash phosphorus off target, into streets, to storm drains, and out into the rivers and the gulf or Atlantic feeding algal blooms and worsening red tide. This is such a concern here that many counties have enacted fertilizer ordinances that restrict the application of nitrogen and phosphorus fertilizers through our rainy season June - September.
Phosphorus in Milorganite
The phosphorus in Milorganite is plant available, but since it’s from a natural source, it’s slow release. The phosphorus is literally bound into a homogeneous prill that will only release nutrients slowly as heat, friction, and microbes break them down. This is one reason why you don’t have to “water in” your Milorganite after you apply it. The prills are not water-soluble. Instead, they are slow-release, which reduces the chances for leaching. You can read more about the P in Milorganite here, including a study conducted by the University of Florida.
Above: side-by-side of a synthetically derived fertilizer with blended nutrients vs. the homogenous prills of Milorganite. With Milorganite, each prill contains 100% of the analysis and it’s all slow release.
How You Can Help Avoid Runoff into Waterways When Fertilizing:
- Avoid fertilizing within 10-25 feet of any waterways.
- Avoid fertilizing if heavy rain is in the forecast.
- Never dispose of fertilizer in waterways, wetlands, or other bodies of water.
- After fertilizing, sweep excess fertilizer from solid surfaces, such as driveways, decks, patios, streets, and sidewalks, back onto the lawn.
- Create a buffer zone of grasses and natural vegetation along the shoreline to help prevent soil erosion and retain nutrients that might otherwise enter a waterway.
How To Know If You Need Phosphorus
The best way to know if phosphorus will be beneficial to your lawn strategy is to conduct a soil test. Since phosphorus is mined here, I decided to look at soil test data from all around Florida to see what was revealed. And while I found that our averages in most areas were very high; 20% of the samples taken still showed low phosphorus.
Additionally, in the panhandle of Florida (Northwest Florida) the naturally existing phosphorus is quite low across most of the samples. This has to do with how Florida was formed and the study of “plate tectonics.” Lots of interesting reading here.
At the end of the day, plants need phosphorus. It’s one of the big 3 macronutrients and no matter who or what explains it, we know it’s all about those roots. The thicker the roots, the thicker the top and if you are going to apply phosphorus to help with this, Milorganite is and always has been a great choice.
I’ll see you in the lawn!
