Phosphorus In Soil: How Does It Work?

How does soil phosphorus help plants? Is all phosphorus available for plant uptake? Here, we explain the soil science behind this nutrient!

phosphorus in soil

Contents

Phosphorus in soil is second only to nitrogen in the amount needed by plants. Phosphorus is one of the most complex nutrients I have discussed thus far. From how it interacts with soil particles to how it pollutes the environment, the impact of phosphorus is huge. 

More detail can be found throughout the article, but even organic phosphorus is generally difficult for plants to reach in the soil where they are planted. Soil pH and composition play a role in phosphorus availability for plants and in plant uptake of organic phosphorus.

There are many different phosphorus-based amendments you can add to your garden. Solubility, the interaction of nutrients in soil particles, and phosphorus availability have an impact on soil pH. 

Regardless of your scale of crop production, understanding the soil phosphorus cycle helps make more informed decisions about how to use the nutrient on your crops.

This article will attempt to go into detail about the various aspects of phosphorus in your soil and its importance to your plants. Though this is an outline of the topic, after you read this, you’ll know more about how you can increase phosphorus availability to your plants. 

The Role of Phosphorus in Plants

Farmer's hands in rubber blue gloves hold chemical fertilizer next to bell pepper bushes in the garden. Chemical fertilizer consists of many small rounded granules in white, gray and brown. Pepper bushes have oval glossy green leaves with pointed tips and ripe, oblong red fruits with a glossy skin.
Phosphorus is vital for plant growth, with roots absorbing dihydrogen and hydrogen phosphate.

The primary forms of phosphorus uptake by plant roots are H2PO4(Dihydrogen phosphate) and HPO42- (hydrogen phosphate). There are various forms of organic phosphorus taken up in fewer quantities. The total phosphorus needed by plants is second to only nitrogen. 

In plant tissues, phosphorus concentration ranges from an average of 0.1-0.5%. Different parts of plants contain more phosphorus than others. Molecules like ATP (adenosine triphosphate) and others utilize phosphorus. ATP and phosphorus are critical molecules in photosynthetic processes. 

Genetic materials like DNA and RNA also contain phosphorus. Obviously, both of these are critical for plant growth. 

Flowering

Close-up of a hand in a blue glove holding granular fertilizer over a paper bag of fertilizer near a flowering narcissus plant in a garden. Fertilizers are small roundish granules of snow-white color. Narcissus is a perennial bulbous plant with strap-like leaves and a central peduncle bearing clusters of fragrant tubular flowers with six petals in pale yellow, yellow and orange.
Most gardens don’t need high-phosphorus flower “booster” fertilizers as plant hormones, not phosphorus, regulate blooming.

There is a general misconception is that suggests flower “booster” fertilizer will make your flowers bloom or significantly improve blooms. This is not really the case. While phosphorus plays a role in supporting blooming, it is not why plants bloom. 

Plants have hormones as humans do, and various factors influence the production of these hormones. There are many plant hormones that regulate blooming, amongst other things.

When it comes to blooming fertilizers, your garden probably does not need that level of added phosphorus. These fertilizers have almost 2:1 phosphorus to nitrogen ratio, which is not necessary for most gardens. 

In all likelihood, your home garden does not need that much phosphorus and is not in extreme depletion. The only way to really know is to do a soil test and to understand the needs of your garden.

YouTube video
Do a soil test as seen here in this video to understand the nutrient makeup of your soil.

Too much phosphorus will bypass plant uptake and leach into surrounding soil and waterways via runoff water. 

Spotting a Phosphorus Deficiency 

Top view, close-up of a phosphorus nutrient deficient cabbage plant. Kapcsta has a rounded head consisting of densely gathered large rounded blue-green leaves with white veins and slightly wavy edges. The outer leaves are purplish-pink due to a lack of phosphorus.
Purple-colored leaves and inhibited shoot growth are signs of phosphorus deficiency in plants.

The most easily recognizable symptom of a phosphorus deficiency is purple-colored leaves. One easily identifiable symptom is purple and really dark green leaves in early growth stages.

The discoloration generally occurs on older plant growth later in the season. Phosphorus deficiencies can also manifest in inhibited shoot growth. This leads to a poor root-to-shoot ratio. 

How Phosphorus Cycles in Soil

Now that we’ve discussed phosphorus availability and different situations that can arise related to plant uptake of phosphorus, let’s discuss the phosphorus cycle that occurs in soil.  

Unavailable Phosphorus

Top view, close-up of a woman's hand spreading fertilizer granules on dark brown soil. The granules are small, round, blue-green in color.
Soil components, such as iron and aluminum oxide particles and soil microbes, can limit phosphorus availability.

There are several ways phosphorus loss occurs in most soils. One way phosphorus levels are depleted for plants relates to iron and aluminum oxide particles.

Phosphate molecules stick to these iron and aluminum particles in the soil. The phosphorus is so tightly held that the plant roots cannot take them in. 

With that in mind, broadcasting a phosphorus fertilizer on soil may not have the desired effect. You would think that all the fertilizer applied to the soil will directly lead to plant uptake, but that is not the case.

These oxides, in addition to soil microbes like highly weathered soils or kaolin clay particles, have a profound effect on phosphorus availability. 

pH

Close-up of Soil pH Meter in the soil. The soil is dark brown, loose. Soil pH Meter is a device with a long thin metal probe with an oval black body with a built-in green screen that displays the pH level of the soil.
Soil pH greatly affects phosphorus availability, as both high and low pH levels can hinder plant uptake.

As with all nutrients, their availability is heavily impacted by soil pH. With phosphorus, both calcareous soils (high pH) and acidic soils or sandy soils (low pH) make it difficult for plants to take up phosphorus in the soil. These types of organic soils cause the phosphate to adsorb (stick to) the oxide particles more readily. 

Temperature and Moisture

Close-up of a gardener's hands testing the soil in a field. The soil is moist and dark brown in color.
Soil phosphorus availability is affected by cool, dry climates, making it harder for plants to take up the nutrient.

Cool, dry climates impact how available for plant uptake soil phosphorus is. In early spring, when root growth is limited because of temperatures, plants may struggle more to get phosphorus. 

As the soil warms, it will be easier, and phosphorus loss will be lessened. However, when moisture levels are low, it’s more difficult for plants to acquire phosphorus. This is because there are fewer solid mineral forms to bond to the soluble phosphorus. 

Compaction

Close-up of female hands checking the soil in a sunny garden. The soil is dark brown, compacted, consists of large sticky formations. The woman is wearing a purple sweatshirt.
Compacted soil or highly weathered soil can hinder root growth and limit nutrient uptake.

Soil compaction can also be problematic for phosphorus availability. Highly weathered soils or compacted soil acts as a physical barrier that prevents root growth, inhibiting the locating and absorbing of nutrients. 

A lack of soil aeration is a problem for other reasons as well, as plants cannot access minerals in the soil, water, or other nutrients. Utilizing cover crops and reducing tillage help prevent compaction. 

Methods to Increase Phosphorus Uptake

If you’re interested in promoting organic phosphorus uptake in the soil particles of your garden, here are a few ways you can do that. Remember that getting a good profile of your soil solution via a soil test or a soil test kit is absolutely essential to determine which of these works best for you.

Mycorrhizal Fungi

Close-up of soil with Mycorrhizal Fungi. The dark brown soil is covered with fungal white mold resembling cobwebs.
Mycorrhizal fungi help plants acquire phosphorus and form relationships with most plants.

Never underestimate the power of fungi. Mycorrhizal fungi play a critical role in soil particles acquiring phosphorus. Fungi have hyphae, which are spindly, microscopic threads that look for nutrients and connect plants. These fungi build relationships with most plants and will help them by providing nutrients while receiving other resources from the soil solution itself.  

The network of hyphae can stretch long distances. These networks are broken due to tillage, making it all the more important to reduce tillage. These fungi do a better job of finding and acquiring phosphorus compared to plants. 

Not all plants can form relationships with mycorrhizal fungi – for instance, any plant in the Brassica family doesn’t form relationships with fungi. Issues arise when applying too much phosphorus-focused fertilizer.

When there is an ample supply for the plant, there is no need for this relationship. This makes it all the more important to apply the appropriate amount of phosphorus. 

Organic Matter

Compost and composted soil. Organic waste in the form of skins from potatoes, zucchini, apples, bananas and egg shells. The soil is dark brown and slightly moist.
Adding organic matter to soil increases the availability of phosphorus.

Increasing organic matter in the soil solution is important for many reasons, and making phosphorus more available is just one of them. Iron and aluminum oxide particles have surfaces those organic matter particles can stick to. 

These larger organic particles take up surface area that would normally have phosphorus nutrients stuck to them. This means that the more phosphorus is in the soil solution, the more available to plants it is. 

There are several ways to increase organic matter in the soil solution. One of my favorite ways to do this is by leaving roots in the soil after the growing season has ended. At the end of the season or harvest time, cutting the plant at the base and leaving the root will help add a lot of carbon to the soil. 

Leaving all the plant material on the field also helps but isn’t as effective as improving organic matter. Another method for increasing organic matter in the soil solution is by including compost. This will add organic matter and organic phosphorus the plants can use as it breaks down. 

Nutrient Pollution 

Poisonous blue-green algae bloom in the river. The algae began to bloom and polluted the green river due to toxic phosphorus waste that enters the waterways.
Phosphorus can lead to nutrient pollution in waterways, causing algal blooms.

Phosphorus could not be discussed without mentioning the nutrient pollution problem it can create. One waterway impacted by too much phosphorus is the Chesapeake Bay, located in Maryland. While the body of water is in one area, the watershed stretches through many states in the Mid- Atlantic. 

This fragile ecosystem has a watershed that is continually plagued by nutrient pollution from agricultural runoff. This leads to algal blooms, which, when decayed, block oxygen in the water. Many plants and aquatic wildlife die because of this. Excess phosphorus along with nitrogen are the primary and secondary minerals causing this pollution. 

Runoff, erosion, and leaching are the ways nitrogen and phosphorus contribute to nutrient pollution. Runoff and erosion are the two main processes that result in excess phosphorus leaching into waterways. The leaching of phosphate minerals is less concerning compared to nitrogen. 

Nitrogen has a much higher leaching potential than phosphorus, meaning it easily moves through the soil profile as the soil surface erodes. Phosphorus forms are mostly held up with calcium, kaolin clay particles, iron, and aluminum particles, causing much less leaching.

There are ways to significantly reduce erosion and runoff. Reducing tillage, adding mulch, and planting cover crops are all ways to limit erosion and runoff. These practices help water trickle through the soil profile rather than flowing over the land. 

Phosphorus Amendments

It’s worth discussing the different phosphorus forms of amendments available and how they are typically used. In this section, we’ll discuss how this essential nutrient is used in different modes of crop production.

Inorganic

Close-up of a male hand holding diammonium phosphate fertilizer against a blurred brown background. The fertilizer is a small rounded granules of light brown color.
Inorganic forms of phosphorus fertilizers include rock phosphate, ammonium polyphosphate, diammonium phosphate, and others.

There’s a laundry list of inorganic forms of phosphorus fertilizers. Rock phosphate, superphosphate, concentrated superphosphate, ammonium polyphosphate, and diammonium phosphate are all forms of inorganic phosphate. 

Ammonium phosphates are some of only a few that are soluble. Rock phosphates are an example of insoluble inorganic phosphate compounds.

Whatever amendment is chosen, it’s possible that more will need to be applied to account for immobilization. Therefore, staying away from aluminum phosphates is advised, as adding aluminum can be bad for the soil.   

Organic

Close-up of a woman's hand holding a fertilizer of pressed granulated chicken manure against the background of loose soil. A garden shovel is inserted into the soil. There is also a bag of fertilizer in the background.
Organic phosphorus amendments include compost and manure.

There are two main forms of organic phosphorus fertilizer. The organic matter in compost breaks down over time, releasing nutrients. Manure is another popular amendment. Not all manure is the same depending on the animal and its diet; the nutrients vary. 

Chicken manure commonly used in agriculture contains a higher amount of phosphorus. For large-scale crop production, chicken manure can be problematic for nutrient pollution as the quantity of excess phosphorus increases. 

One of the most common store-bought amendments is bone meal. This organic phosphate has high amounts of phosphorus (and calcium) and can be a good option to use as well. Like other organic sources, this will release nutrients over time. Since less is released all at once, more may need to be applied to get the desired effect compared to inorganic amendments.  

Future Issues in Phosphorus Fertilizers

Close-up of a farmer's hand holding a granular fertilizer in his hand over a young tobacco plant in a sunny garden. The fertilizer consists of many rounded blue granules. The plant has a rosette of oval, elongated green leaves.
Phosphorus depletion may lead to costly fertilizer use in the future.

There is one thing related to phosphorus that will continue to be an issue as years go on. Mining for phosphate minerals is limited to specific places in the world.

As we continue depleting phosphorus, using phosphorus fertilizer in the future will be costly. All is not lost, though. Understanding and applying the fertilizer in the most effective ways possible can help make phosphorus use more efficient.  

While running out of phosphorus may seem a crazy thing to think about, it is a possibility for the future. As you scale up in farming, the costs for this nutrient could become quite expensive. 

Frequently Asked Questions

Q: What is the role of phosphorus in soil?

A: Phosphorus is an important nutrient for plant growth, specifically in large quantities. Phosphorus is often fixed by various soil particles, limiting its availability. 

Q: What is the main source of phosphorus in soil?

A: Phosphorus in soils can be found in organic compounds and inorganic phosphate. Plants can take in the inorganic phosphate forms and will take up small organic forms of phosphorus. 

Q: Is high phosphorus in soil good?

A: High phosphorus content in the soil solution can potentially harm the environment via nutrient pollution. If the phosphorus is held onto iron and aluminum oxides, it is less of a concern for leaching as it likely will leave the soil profile. Soil erosion and runoff are greater concerns with phosphorus. This stresses the importance of keeping the soil covered and reducing tillage to limit the amount of runoff.

Q: What happens if there is not enough phosphorus in soil?

A: If there is not enough available phosphorus in the soil, plants will become nutrient deficient. This phosphorus deficiency shows as dark purple or green colored leaves in addition to stunted shoot growth. 

Q: What pH is Phosphorus most available?

A: Phosphorus is most available at a soil pH of 6.5, which is the optimum soil pH for the availability of all nutrients. 

Q: What happens if plants get too much phosphorus?

A:  Too much phosphorus will kill your plant. However, it is unlikely to do this in most settings. As I have mentioned previously, in many soils, phosphorus uptake is lessened when the nutrient makes contact with a limiting nutrient.

Q: What is a good natural source of phosphorus?

A: Compost and manure are both good organic sources of organic phosphorus in basic soils. Chicken manure specifically has high amounts of phosphorus. This can be problematic for large-scale farmers, but home gardeners can use it to supplement phosphorus. Conduct a soil test to understand your soil profile better and apply the correct amount. 

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