Dicalcium phosphate is a calcium salt of phosphoric acid used as a mineral supplement in compound animal feed. It is a feed material, not a soil fertilizer, and the two must not be conflated — this product is formulated, specified and regulated for animal nutrition, and its critical quality parameters are biological availability and contaminant limits rather than agronomic nutrient grade. It supplies calcium and phosphorus in a fixed ratio close to that required for skeletal development, which is the reason it is one of the most widely used inorganic feed phosphates.

Substance identity

⚠ CAS number caution. The identifier 7557-93-9 appears among the synonyms attached to this substance in some database records. It is a known transcription error and must not be used in specifications, customs documentation or safety data sheets. The correct CAS for anhydrous dicalcium phosphate is 7757-93-9. The dihydrate carries its own separate CAS number; where the traded form is specifically the dihydrate, that identifier should be confirmed and used rather than the anhydrous one.

Specification

Theoretical composition, calculated:

Typical commercial feed-grade specification — ranges only, verify on certificate of analysis:

Note that the composition table above shows why the hydrate question is not academic. Anhydrous and dihydrate differ by roughly 4.8 percentage points of phosphorus — a real and substantial difference in a feed formulation. A specification that says only “DCP 18 %” without stating the form is incomplete.

Properties

• Appearance — white to off-white, odorless, free-flowing powder or granule.

• Solubility in water — very low; on the order of 0.02 g per 100 mL. DCP is not a soluble phosphate and is not suitable for solution applications.

• Solubility in dilute acid — soluble. This is the property that makes it bioavailable: dissolution occurs in the acidic environment of the stomach or proventriculus, releasing calcium and phosphate ions for absorption further along the tract.
• Hydrate interconversion. The dihydrate loses its water of crystallization on heating — reported from around 100–110 °C — converting to the anhydrous form. This matters during drying and pelleting: a product specified as dihydrate can partially dehydrate in processing, shifting its assay on a mass basis.
• Thermal decomposition — on stronger heating, anhydrous CaHPO₄ condenses to calcium pyrophosphate with loss of water, reported in the region of 400–425 °C.
• Density — approximately 2.93 g/cm³ (anhydrous), 2.31 g/cm³ (dihydrate).

Applications

• Compound feed for poultry, swine, ruminants, aquaculture and pets as the inorganic calcium and phosphorus supplement.

• The nutritional rationale for supplementing at all. Much of the phosphorus in cereal grains and oilseed meals is bound as phytate, which monogastric animals digest poorly without added phytase. Plant-derived rations therefore under-deliver available phosphorus relative to requirement, and an inorganic phosphate source closes that gap. Phosphorus is required for skeletal mineralization, energy metabolism and cell membrane structure; deficiency presents as poor bone development, reduced growth rate and, in laying birds, degraded shell quality.

• Calcium-to-phosphorus ratio. DCP supplies Ca and P at close to 1.3 : 1, which sits near the ratio required by many species and reduces the amount of separate calcium correction the formulator has to make. The ratio itself matters as much as the absolute amounts — excess calcium impairs phosphorus absorption.
• Premix and mineral block manufacture.
• NOT a soil fertilizer in this listing. Do not substitute this product where a water-soluble agricultural phosphate is specified. Feed-grade material is manufactured and tested to a different set of parameters entirely.

Handling and storage

Store in sealed bags in a dry, covered, ventilated warehouse on pallets, protected from moisture and from contamination by other materials — cross-contamination control is a formal requirement in feed manufacturing, not merely good practice. Keep segregated from fertilizers, pesticides, veterinary products and any non-feed chemicals; use dedicated handling equipment where the feed-safety system requires it. Observe stock rotation and retain lot traceability records. Control dust at transfer and mixing points; use eye protection and dust masks when handling open powder. Keep away from strong acids.

Notes

• Use CAS 7757-93-9. Do not use 7557-93-9 — that string is a known typographical error in synonym listings.

• State the hydrate form on every specification. Anhydrous CaHPO₄ and the dihydrate CaHPO₄·2H₂O differ by approximately 4.8 percentage points of phosphorus. Commercial “DCP” is often a mixture of both.

• Fluorine is the critical impurity in feed phosphates, because it accumulates in bone and dental tissue with chronic intake. Fluorine limits and heavy-metal ceilings are set by the feed legislation of the destination market and differ between jurisdictions — the applicable limits must be confirmed against that legislation, not against a supplier’s standard specification.
• Solubility in 2 % citric acid is the conventional proxy for biological availability of feed phosphates and is a more useful figure than total P alone.

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PART 2 — COMPOUND NP / NPS FERTILIZERS

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HOW TO READ A COMPOUND FERTILIZER GRADE

(Suggested as a shared block reproduced on each compound page, or as a single explainer linked from all five. It is written once here to avoid duplicated copy.)

A fertilizer grade is a sequence of percentages by weight, always in the same order:

N — P₂O₅ — K₂O, optionally followed by declared secondary nutrients such as S.

So NPS [CAS?]-4S means: 23 % nitrogen, 21 % phosphorus as P₂O₅, 0 % potassium as K₂O, and 4 % sulfur. One tonne carries 230 kg N, 210 kg P₂O₅, no K, and 40 kg S. The remaining ~500 kg is counter-ions, filler, moisture and coating — this is normal and is true of every compound fertilizer.

Two conventions cause recurring confusion in trade:

1. P and K are declared as oxides, not as elements. Neither P₂O₅ nor K₂O is actually present in the product; both are historical accounting conventions. To convert: P₂O₅ × 0.436 = elemental P, K₂O × 0.830 = elemental K.
2. Sulfur may be declared as S or as SO₃, and the two numbers differ by a factor of 2.5. 13 % S = 32.5 % SO₃. A specification that quotes “32.5 % sulfur” without stating the basis is ambiguous and must be clarified before it is compared against anything.

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Frequently asked questions

What is the difference between anhydrous and dihydrate dicalcium phosphate, and does it matter when buying?

Two solids share the name. The dihydrate, CaHPO4·2H2O, and the anhydrous salt, CaHPO4, differ in molar mass (172.09 vs 136.06 g/mol), so their theoretical elemental contents differ: about 18.0% P and 23.3% Ca for the dihydrate, against 22.8% P and 29.5% Ca for the anhydrous form. Commercial material is frequently a mixture, and the declared assay indicates which form dominates. The dihydrate is generally more reactive in dilute acid and tends to show higher solubility in 2% citric acid; the anhydrous form is more stable during drying and storage. Always confirm which form a quotation refers to, because a tonne of anhydrous material carries roughly a quarter more phosphorus than a tonne of dihydrate.

How do I compare two offers fairly when the specifications are not identical?

Convert everything to cost per unit of phosphorus before comparing. Divide the delivered price by the declared P content to obtain cost per kilogram of P; if one offer is quoted as P2O5, convert using P2O5 = P × 2.29. Then adjust for what else you are paying for: moisture and loss on ignition are inert weight, calcium may or may not have value in your formulation, and low citric-acid solubility means part of the declared phosphorus will not be used by the animal. Freight, packaging and physical form also count — granular material handles with less dust and less segregation, while powder disperses faster in a premix. The cheapest tonne is often not the cheapest kilogram of usable phosphorus.

What does “solubility in 2% citric acid” on the specification actually tell me?

It is a laboratory proxy for how much of the phosphorus is released under acid conditions broadly comparable to the stomach, and it is the figure most feed specifications use to rank phosphate sources. A high value indicates the phosphate dissolves readily. A low value suggests that part of the material has been converted to less reactive species — for example by over-drying, dehydration of the dihydrate, or pyrophosphate formation during processing — or that the product has been extended with poorly reactive filler. Ask which analytical method the laboratory used, since results from different protocols are not interchangeable. The figure ranks sources; it does not measure digestibility in a given species and does not replace published availability data or a feeding trial.

Which contaminants and declarations should I insist on seeing before accepting a batch?

Phosphate rock supplies both the phosphorus and the contaminants, so a certificate showing only Ca and P is incomplete. Ask for fluorine: feed phosphates must be defluorinated, legal ceilings apply in most markets, and you should verify both the limit in force at the destination and the value for the batch. Ask for cadmium, lead, arsenic and mercury; sedimentary rock generally carries more cadmium than igneous rock, and that difference follows the raw material rather than the producer’s intentions. Confirm the grade designation in writing, because feed grade and technical grade are chemically similar but controlled to different purity requirements, and technical material must never be substituted into a feed formulation. Request a batch certificate, not a typical analysis.

How should it be stored, and what actually degrades it in the warehouse?

Keep it dry, cool, off the floor and in closed packaging. The usual failure is caking: the powder takes up atmospheric moisture, forms lumps, and then needs re-milling, which generates dust and increases segregation in mash feed. Heat is the second problem. The dihydrate loses its water of crystallisation at moderate temperatures, above roughly 100 °C, which raises the assay per unit weight but reduces reactivity, so a hot store or an aggressive drying step can change what you are actually dosing. Keep it away from strong acids, which decompose it and release heat, and away from any source of free water. Granular grades tolerate handling and storage better than powders, which carry the higher caking and dust risk.

When is dicalcium phosphate the wrong choice in a feed formulation?

Several cases. If the formulation is already at its calcium ceiling and only phosphorus is short, this is the wrong tool: it delivers roughly 1.3 parts calcium per part phosphorus, whereas monocalcium or monosodium phosphate lets you add P without moving the Ca:P ratio. In early nursery pig diets, where a low acid-binding capacity is wanted so the young stomach can acidify, calcium-rich phosphates work against that objective and monocalcium phosphate is usually preferred. Wherever the phosphate must dissolve — liquid feed lines, drinking-water supplementation, spray application — it is unsuitable, being practically insoluble in water; it settles out and blocks equipment. And if the specification calls for food or technical grade, do not substitute feed grade, because the purity controls differ.

Can feed-grade dicalcium phosphate be used as a phosphorus fertilizer?

Chemically it supplies phosphorus and calcium, but it behaves as a low-solubility source. Being practically insoluble in water, it cannot be used in fertigation, drip systems or foliar sprays — it settles in the tank and plugs emitters. In soil, release depends on acidity: on acid soils some phosphorus becomes available over time, while on neutral to calcareous soils calcium phosphates remain largely unavailable and water-soluble sources such as MAP, DAP or superphosphate perform considerably better per unit of P applied. Feed grade is also an expensive way to buy fertilizer phosphorus, since it is purified for a different purpose. In most markets fertilizer use requires its own product registration, which a feed-grade declaration does not cover. Application rates depend on soil test and crop.

Related substances

• MAP dissolves completely in water while dicalcium phosphate does not, and that one property decides which phosphate can go through an irrigation line and which cannot.
• DAP and DCP are routinely confused in enquiries: the ammonium salt is a fertilizer, the calcium salt is a feed phosphate, and the DAP page makes the distinction explicit.
• Dicalcium phosphate is what soluble phosphate turns into once it meets calcium, so the MKP page is the place to see the conditions under which phosphate stays in solution.