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What is Phosphorus Fertilizer?

By J.S. Metzker Erdemir
Updated May 16, 2024
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All plants require phosphorus to grow. Phosphorus fertilizer can be any fertilizer that contains phosphorus, or a fertilizer that has an especially high phosphorus content. Many US cities have banned or limited the use of high phosphorus fertilizer on residential lawns and gardens because its overuse is seriously damaging to lakes and rivers.

Most prepared synthetic fertilizers sold in garden stores are made up of a mixture of nitrogen (N), phosphorus (P), and potassium (K). The amount of each element in NPK fertilizers is indicated by numbers that refer to the percentages of the elements contained in the mix by weight, so a 4-3-3 NPK fertilizer is 4 percent nitrogen, 3 percent phosphorus, and 3 percent potassium. Different balances of elements are required by different plants, so NPK fertilizers are often marketed with labels like grass fertilizer, tomato fertilizer, or rhododendron fertilizer. In fact, it is the balance of nutrients that is important, not the specific plant type written on the bag.

Synthetic phosphorus fertilizer is generally made by chemically processing rock phosphate from the ground. Organic phosphorus fertilizer can be found in composted animal manure or in ground-up animal remains left over from food production and usually sold as bone meal. Unprocessed rock phosphate is also used for organic fertilizer, but it can take several years to break down enough for plants to be able to access the nutrients.

Phosphorus is essential to plant growth because it plays a role in photosynthesis, cell division, and the plant's ability to use sugars and starches. Depending on soil conditions like pH, temperature, and other available nutrients, plants are able to make use of synthetic or organic phosphorus to varying degrees. Apparent phosphorus deficiencies such as reddening of the leaves can indicate other deficiencies that are preventing the plant from using the phosphorus. Often, there is adequate phosphorus in the soil, but the plants are unable to use it because the soil pH is too high or the nitrogen content is too low.

Many gardeners believe that phosphorus makes roots grow. This is true to an extent, but without other nutrients, large doses of high phosphorus fertilizer do not have a significant effect on roots. Most of the phosphorus used residentially ends up getting leached out of the soil by rain and irrigation before the plants can use it. Overuse of phosphorus fertilizer is a serious threat to water quality because it enters waterways through the storm drains. Phosphorus can cause blooms of blue green algae and other unwanted aquatic plants that deplete the oxygen in water and compete with other wildlife.

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Discussion Comments
By seag47 — On Jun 14, 2012

@wavy58 – Cow manure is also a good natural phosphorus fertilizer. My house is surrounded on three sides by cows, so I have plenty of it.

I like to wait until the manure has started to decompose. It begins to turn to dust and blend in with the ground. Soil mixed with decomposing manure is one of the best fertilizers I have ever used.

I use a shovel to scoop it up and I put it around my flower bed. I have some of the healthiest flowers in the community, and I think that I owe this to the manure.

Some people are too grossed out by it to use it, but if you can handle the thought, you will reap the rewards. Also, since cow manure is already on the surrounding land, I'm not putting any added phosphorus into the ecosystem.

By StarJo — On Jun 13, 2012

I have found nitrogen phosphorus fertilizer to be very helpful to my azalea bushes. They were headed for death before I used it. All the leaves had turned a yellow-green and they had no blooms.

My dad told me to dig a shallow trench around the base of the bush and sprinkle the fertilizer in it. He said that I should then water the trench to let it soak into the ground and feed the roots.

About a week later, the azalea leaves had turned back to a deeper shade of green. Two weeks later, I even saw some pink buds emerging. I will continue to use this fertilizer whenever they start to look sickly.

By cloudel — On Jun 13, 2012

I used a phosphorus organic fertilizer for my tulip bulbs. I had heard that bone meal was a great tulip fertilizer, so I bought a bag of it when I bought my bulbs several years ago.

The bad thing about bone meal is that it attracts predators. They know that it is animal remains, and the day after I had sprinkled out the bone meal, most of it was gone.

I saw my dog eating some of it, but I know she had help from other wild animals. At least some of it had already soaked into the ground, because I had watered the soil immediately after applying it. My tulips were huge and healthy the next spring, so maybe the bone meal did help, after all.

By wavy58 — On Jun 12, 2012

I worry about damaging the environment by using phosphorus fertilizer. It just doesn't seem right to put something that doesn't occur in nature on plants to help them grow.

So, I think that the best fertilizer is one made from composted plant material. I have a heap of grass clippings, leaves from my yard, and fruit rinds in my backyard that I keep covered with a tarp so that it decomposes properly, and it makes my plants flourish.

I might consider using bone meal or manure, but I wouldn't buy chemical fertilizer. There are just too many unknown issues it could cause.

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