Phosphoric Acid Uses in Agriculture: Doses and Prices

The main phosphoric acid uses in agriculture are as the raw material for phosphate fertilizers (MAP, DAP, triple superphosphate and liquid 10-34-0) and, in smaller amounts, as a liquid phosphorus source and pH reducer in drip fertigation. In the United States, more than 95% of mined phosphate rock goes into wet-process phosphoric acid and superphosphoric acid, which then become granular and liquid fertilizers and animal feed supplements, according to the USGS Mineral Commodity Summaries 2026.

This guide explains each use in plain terms: how the acid is made, which products come from it, how growers inject it through drip lines, how much phosphorus a crop removes, and what rules and safety steps apply. It also gives a dose and cost calculator for a liquid acid product, a dated chart of phosphate fertilizer prices, and a chart of the countries that mine the rock behind it.

Phosphoric acid uses in agriculture at a glance

Phosphoric acid (H3PO4) is a strong mineral acid that carries phosphorus in the orthophosphate form plants take up. On the farm, you rarely meet the pure acid. You meet the products made from it. The table sums up the main uses and where each one fits.

The analysis numbers are N-P2O5-K2O percentages, the way fertilizer labels state them. The grades come from the National Corn Handbook bulletin on phosphorus fertilizers (Iowa State University Extension, NCH-13, revised 1991), which is still a clear guide to how the products are made. Always check the label on the product you buy.

How phosphoric acid is made

There are two ways to make the acid. The bulletin explains that “furnace acid” is made by heating phosphate rock to form elemental phosphorus and then turning that into acid. It is pure but costly, so it goes mainly into food and industrial uses. “Wet-process acid” is made by treating phosphate rock with sulfuric acid and filtering off the gypsum that forms. This is the route for fertilizer.

Wet-process acid is green or black, depending on how much carbon is left in it. The National Corn Handbook bulletin NCH-13 says its metal and organic impurities do not reduce how well the phosphorus feeds crops, though they can affect how liquid products store. It also notes that fertilizers made from furnace acid work no better in the field than those made from cheaper wet-process acid.

The by-product matters too. The US EPA page on fertilizer production wastes describes phosphogypsum as the main waste of the wet-acid process. It is the calcium sulfate left after the acid is filtered out, and it contains small amounts of natural radioactive elements from the rock. That is why it is stored in managed stacks.

Merchant acid and superphosphoric acid

Plants sell wet-process acid as “merchant-grade” acid, which fertilizer blenders and some growers buy. When water is driven off, two or more acid molecules join to form superphosphoric (polyphosphoric) acid. The bulletin gives the usual P2O5 ranges below. Superphosphoric acid is the base for liquid ammonium polyphosphate.

Chart

P2O5 content of phosphate materials

Typical analysis range, % P2O5 by weight

Fertilizers made from phosphoric acid

Most of the acid is turned into dry or liquid fertilizers that are easier and safer to ship, store and spread. The US figures show how central this route is. The USGS reports that in 2025 about 25% of US wet-process acid was exported, as DAP, MAP, merchant acid and other phosphate products.

Monoammonium phosphate (MAP)

MAP is made by reacting one part (mole) of ammonia with one part of phosphoric acid. NCH-13 lists common grades of 11-52-0 and 11-53-0. MAP is a good choice near the seed because it gives off less ammonia than DAP. It is also used in suspensions.

Diammonium phosphate (DAP)

DAP uses two parts of ammonia per part of acid and is usually 18-46-0. The bulletin calls DAP the leading phosphate material worldwide. On high-pH soils, or placed in direct contact with seed, the ammonia it releases can harm seedlings, so follow placement guidance from your local extension service.

Triple superphosphate (TSP)

TSP is made by treating phosphate rock with phosphoric acid instead of sulfuric acid. A USDA technical review of triple superphosphate describes the reaction of fluorapatite with phosphoric acid in water to form monocalcium phosphate. NCH-13 gives TSP an analysis of 0-44-0 to 0-46-0, the highest of the straight P fertilizers.

Ammonium polyphosphate (10-34-0)

The most common liquid P fertilizer is 10-34-0. It is made by reacting superphosphoric acid with ammonia and adding water. NCH-13 says it typically holds about 65 to 70% of its phosphorus in polyphosphate form. Growers use it as a starter fertilizer and as a base for liquid blends.

Single superphosphate

Single (normal) superphosphate does not use phosphoric acid. It is rock treated with sulfuric acid, which leaves about 12% sulfur in the product. NCH-13 notes its main drawback is low analysis, which raises transport cost per unit of P2O5.

Using phosphoric acid in fertigation and drip lines

This is the main direct farm use of the acid itself. In drip and micro-sprinkler systems, liquid acid is injected into the water. It does two jobs at once: it supplies phosphorus to the root zone, and it lowers water pH so that calcium and magnesium salts do not clog the emitters.

A University of Florida guide on adjusting fertigation water pH says acid for drip lines can be made from sulfuric acid or phosphoric acid, and that phosphoric acid is used especially when a liquid P fertilizer such as 0-54-0 goes through the drip system. The same guide notes that water from the Floridan Aquifer often has a pH of about 8.1, high enough to cause clogging, and that most crops grow best between pH 5.5 and 6.5.

How to inject it, step by step

1. Test the water first. Get pH, alkalinity (bicarbonate), calcium and magnesium measured. The acid dose depends on alkalinity, not on pH alone.
2. Run a jar test. Add measured drops of diluted acid to a known volume of your water and record the pH. Scale that to your flow rate.
3. Choose the acid. Phosphoric acid also feeds the crop. In very hard water it can form calcium phosphate deposits, so some growers use sulfuric acid there instead.
4. Inject after the filter and backflow device. Use acid-resistant pumps, tubing and seals. Local rules often require a check valve on the injection line.
5. Check pH at the end of the line. Use a calibrated meter. Adjust the injection rate until the target holds.
6. Flush with clean water. End each run with plain water so acid and nutrients do not sit in the lines.

Never mix acid and chlorine products in the same tank. Mix order matters: add acid to water, never water to acid. Follow the product label, your equipment maker’s instructions and local rules.

Compatibility in liquid mixes

Phosphate can react with calcium in the tank or the line. Keep calcium nitrate and phosphate products in separate stock tanks, or inject them at different times. Pre-dilute the acid before you add micronutrients. If you see a white or cloudy deposit, stop and test the mix in a jar before the next run.

Starter, banded and foliar phosphorus

Phosphorus moves very little in soil. A root has to grow close to it to take it up. That is why placement matters as much as rate, and why many products made from phosphoric acid are placed in a band near the seed instead of spread over the whole surface.

Starter fertilizer

A starter is a small dose of nutrients placed with or near the seed at planting. Liquid 10-34-0 and MAP are common choices. Starters help most in cold, wet springs, in no-till fields where soil warms slowly, and on soils with low P tests. On soils with high P tests the yield gain is often small, so test before you pay for a starter program.

Keep the rate low when the product touches the seed. Salt and ammonia from the fertilizer can damage germinating seed. Seed-safe limits differ by crop, row spacing and soil, so use the limits your local extension service publishes for your area.

Banding

Banding puts the fertilizer in a strip, often 2 inches beside and 2 inches below the seed, or deeper in strip-till systems. A band keeps more phosphorus in contact with roots and less in contact with soil calcium, iron and aluminium, which can tie it up. Banding also leaves less P on the surface, where rain can wash it off.

Foliar sprays

Soluble phosphate sprays can top up a crop for a short time, for example during fast growth. A leaf can only take in a small amount per spray, so foliar P is a supplement, not a replacement for P in the soil. Concentrated acid or a wrong mix can scorch leaves. Test on a small area first and follow the label.

▶ How chemical applications can cause phytotoxicity in crops, the symptoms to look for and how to manage them. · 19:28

Greenhouse, hydroponic and high-value crops

Greenhouse and hydroponic growers use phosphoric acid in much the same way as drip growers outdoors, but with tighter control. Nutrient solutions are mixed in stock tanks, and an injector doses them into the water. The acid lowers pH and adds phosphate in one step.

Growers in these systems watch two numbers closely: pH and electrical conductivity (EC), a measure of total dissolved salts. Too high a pH lets iron and other micronutrients come out of solution. The University of Florida guide notes that ferrous iron, which most vegetable crops rely on, begins to oxidize when pH goes above 5.3. Too low a pH can harm roots. Most growers aim for the 5.5 to 6.5 range the guide gives, unless the crop prefers lower.

Because recirculating systems reuse water, phosphate can build up over time. Test the solution often and adjust the recipe instead of adding more acid by habit.

Feed phosphates for livestock

The USGS lists animal feed supplements among the main end uses of US wet-process phosphoric acid. Feed-grade calcium phosphates, made by purifying the acid and reacting it with lime, supply phosphorus to poultry, pigs and cattle. Feed products must meet stricter limits on fluorine and heavy metals than fertilizer, so they use cleaned acid. If you buy mineral mixes, check the phosphorus and calcium content on the feed tag.

Signs your crop is short of phosphorus

Phosphorus shortage is hard to see until it is severe. Common signs include:

• Slow early growth and small, stunted plants.
• Dark green leaves, sometimes with purple or red tints on older leaves or stems, most often in corn seedlings in cold soil.
• Delayed flowering and maturity.
• Weak root systems.

Cold soils can cause purple seedlings even when soil P is adequate, because roots take up less in cold weather. So confirm with a soil test and a plant tissue test before you add more phosphorus. A satellite map of crop vigour can point out the weak zones, but only a test tells you the cause.

How to choose the right phosphate product

Use these questions to pick between the products made from phosphoric acid:

• Do you irrigate by drip? Liquid phosphoric acid or a soluble phosphate is the practical choice. Best for: vegetables, orchards, vines, greenhouse crops.
• Do you also need nitrogen? MAP or DAP supply both. Best for: grain and oilseed crops where N and P go on together.
• Do you want P without N? TSP gives straight phosphorus. Best for: legumes, pastures and fields with enough nitrogen.
• Do you plant with a liquid starter kit? 10-34-0 suits in-furrow or 2×2 placement. Best for: corn and other row crops in cool soils.
• Do you need sulfur too? Ammonium phosphate-sulfate or single superphosphate adds it. Best for: canola and sulfur-poor soils.

Compare products on cost per pound of P2O5, not cost per ton. A cheaper product with a lower analysis can cost more per unit of phosphorus once you add freight and spreading.

Common mistakes to avoid

• Applying phosphorus every year without a recent soil test.
• Using the acid in hard water without a jar test, which can block emitters.
• Mixing phosphate and calcium products in the same tank.
• Leaving acid in drip lines after a run instead of flushing.
• Spreading P on frozen, snow-covered or saturated soil.
• Comparing prices per ton instead of per unit of P2O5.
• Skipping gloves and goggles for a “quick” job with concentrated acid.

How much phosphorus does a crop need?

The rate starts with a soil test, then what the crop will remove. University of Illinois Extension gives crop removal values for phosphorus: corn removes 0.37 lb of P2O5 per bushel, soybeans 0.75 lb and wheat 0.47 lb. These are 75th percentile values, so three in four measured samples were below them.

Chart

Phosphorus removed in harvested grain

lb P2O5 per bushel, 75th percentile values

Illinois also explains which soil tests apply. Mehlich-3 is now the most widely used test at commercial labs. Bray P1 should only be used on soils with pH below 7.3. The Olsen test suits alkaline soils. In south west Victoria, Australia, the state’s soil health guide on superphosphate uses Olsen P and says soils in the “adequate” range will generally reach 95% of potential growth, while soils in the “high” range are more likely to lose P to runoff.

So the rate question has two parts. If your soil test is low, you may need to build up soil P above what the crop removes. If it is high, many guides suggest applying less than removal, or nothing, for a season or two. Your local extension service or a certified crop adviser can set the rate for your soil and crop.

From pounds of P2O5 to gallons of acid

Labels state phosphorus as P2O5. To find how much product you need, divide your P2O5 target by the product’s P2O5 percentage. For liquids, divide pounds by the product’s weight per gallon, which is on the supplier’s data sheet. To convert P2O5 to elemental P, multiply by 0.436, the ratio of their molecular weights. The calculator below does this for you.

A worked example

Say a field is expected to yield 160 bushels of corn per acre and the soil test is in the optimum range, so you plan to replace what the crop removes. At 0.37 lb P2O5 per bushel, removal is about 59 lb of P2O5 per acre. With a liquid acid labelled 0-54-0, you need about 109 lb of product per acre (59 divided by 0.54). If the supplier’s sheet gives a weight of 13 lb per gallon, that is about 8.4 gallons per acre. These numbers are only an example. Your soil test, yield goal and product sheet will give different ones.

In fertigation, that total is usually split into several smaller doses through the season, timed with crop growth. Splitting lowers the risk of tie-up in the soil and of losses in heavy rain.

Try the numbers for your own farm below.

Prices of phosphate fertilizers

Phosphoric acid itself has no widely published farm price, but the fertilizers made from it do. The World Bank tracks DAP (spot, f.o.b. US Gulf) and TSP (spot, import US Gulf) in its monthly Pink Sheet of commodity prices. In the November 2026 edition, DAP averaged US$563.7 per metric ton in 2024 and US$685.2 in 2025, and reached US$803.1 in October 2026. TSP averaged US$474.6 in 2024 and US$577.6 in 2025.

Chart

DAP and TSP spot prices

US$ per metric ton; annual averages 2023-2025, then quarterly averages

These are export and import spot prices for bulk cargoes, not the price a farm pays at the dealer. Retail prices add freight, storage and margin. Use them to see the trend, then ask local suppliers for a quote.

Where the phosphate comes from

All phosphoric acid for fertilizer starts as phosphate rock. The USGS estimates world mine output at 250 million metric tons in 2025. China, Morocco, the United States and Russia were the leading producers, in that order. Morocco holds the largest reserves listed, about 50 billion tons out of a world total of 73 billion.

Chart

Phosphate rock mine production, top producers

Million metric tons of marketable rock, 2025 estimates, the eight largest producers

US mines in Florida, Idaho, North Carolina and Utah produced about 20 million tons of marketable rock in 2025, worth US$1.9 billion at the mine, says the USGS. The same report notes that US apparent use of phosphate rock fell about 7% from 2024, because less phosphoric acid was made. Phosphate rock was added to the US critical minerals list in November 2025. The USGS also states there are no substitutes for phosphorus in agriculture, and no imminent shortage of the rock.

World use of P2O5 in fertilizers was estimated at 47.8 million tons in 2025, up from 47.1 million tons in 2024, and was projected to reach 51.5 million tons by 2029 (USGS, 2026).

Recycled phosphorus and other sources

Some phosphorus now comes from waste streams instead of mines. These products do not replace phosphoric acid in drip lines, but they can replace part of the granular P on a farm.

• Struvite (magnesium ammonium phosphate) is recovered from wastewater at some treatment plants. It dissolves slowly, so it suits soils and crops that need a steady supply.
• Biosolids and manure carry phosphorus too. Their P content varies, so test each batch and follow local rules on spreading.
• Rock phosphate can be spread directly on acid soils, though NCH-13 lists its water-soluble P at only 0 to 10%, so it acts slowly.

In Europe, the EU fertilising products regulation sets rules for recovered materials such as precipitated phosphate salts. Check what your country allows before you buy or spread any recycled product.

Environment: keeping phosphorus out of water

Phosphorus that leaves the field can feed algae in lakes and rivers. The US EPA on nutrient pollution from agriculture explains that nitrogen and phosphorus not used by crops can wash into waterways in rain and snowmelt, or leach to groundwater. High levels cause eutrophication, which can lead to low-oxygen “dead zones”.

The usual advice is known as the 4Rs: the right source, at the right rate, at the right time, in the right place. In practice that means:

• Apply P based on a recent soil test, not habit.
• Band or inject P near the roots instead of leaving it on the surface.
• Do not spread on frozen or saturated ground, or just before heavy rain.
• Keep buffer strips along streams and ditches.
• Split fertigation doses to match crop growth stages.

Rules and safety for phosphoric acid on the farm

Concentrated phosphoric acid is corrosive. It can burn skin and eyes and damage metal. Wear chemical-resistant gloves, goggles or a face shield, and protective clothing when you handle it. Store it in labelled, acid-resistant containers away from bases, chlorine products and metals that react with it. Keep an eyewash and clean water close by. Read the safety data sheet from your supplier before use.

Fertilizer rules differ by country. In the EU, Regulation (EU) 2019/1009 limits cadmium in phosphate fertilizers (those with 5% P2O5 or more) to 60 mg per kg of P2O5 for products sold under its CE marking. In the US, states register and label fertilizers. Organic growers should check with their certifier before using any phosphoric acid product. Check the label, your local rules, and get professional advice for your farm.

Questions to ask your supplier

Before you buy a phosphoric acid product for the farm, ask for the details that change how you use it:

• What is the guaranteed analysis? Merchant acid is sold at different strengths. The P2O5 percentage sets your dose.
• What does a gallon weigh? You need the density to convert pounds to gallons for the injector.
• Is it green or black acid? Clarified green acid is less likely to leave deposits in fine emitters.
• What are the cadmium and heavy metal levels? This matters most in the EU and for buyers with sustainability rules.
• What containers and pumps does it need? Ask which plastics and seals resist it.
• Is there a safety data sheet? Read it before delivery and keep a copy near the store.
• Is it approved for organic use? If you farm organically, get the answer in writing and check with your certifier.

Also ask about delivery size. A bulk tank may lower the unit price, but it needs a bunded, secure store and someone trained to handle it.

Where satellite crop monitoring fits

A satellite cannot measure soil phosphorus. What it can show is where crops grow more weakly than the rest of the field, so you know where to take soil and tissue samples first. Farmonaut Satellite + AI Farm Monitoring System gives vegetation index maps of each field over the season, which can help you compare zones before and after a fertigation program.

Related pages on our site: uses of phosphate rock · carbon footprint tracking · supply chain traceability · crop loans and insurance · large farm management · farm fleet management · weather and satellite data API docs · mobile apps

▶ A walk-through of monitoring crop health with satellite imagery and vegetation indices in the Farmonaut web app. · 8:43

FAQ

What is phosphoric acid used for in agriculture?

Mostly to make phosphate fertilizers such as MAP, DAP, TSP and liquid 10-34-0. Growers also inject diluted acid into drip lines to supply phosphorus and lower water pH. Some is used to make feed phosphates for livestock.

Can I apply phosphoric acid directly to soil?

It is possible, but NCH-13 notes that handling problems mean it is normally processed into other fertilizers first. Direct use is mostly through fertigation, where the acid is diluted in irrigation water. Follow the label and local rules.

How much phosphoric acid per acre?

There is no single dose. Start with a soil test and the crop’s removal, for example 0.37 lb P2O5 per bushel of corn (Illinois Extension). Divide the P2O5 target by the product’s P2O5 percentage to get pounds of product, then by its weight per gallon. The calculator on this page does the maths.

Is phosphoric acid better than sulfuric acid for drip lines?

Phosphoric acid supplies phosphorus while it lowers pH, which suits fertigation. In hard water it can form calcium phosphate deposits, so some growers use sulfuric acid there. A water test and a jar test will show which works for your water.

What pH should fertigation water have?

The University of Florida guide says most crops grow best between pH 5.5 and 6.5, while blueberries and some other crops prefer more acidic conditions. Set your target with your crop adviser.