Monopotassium phosphate is the potassium salt of phosphoric acid with two acidic protons remaining. It occupies a distinct position in the phosphate range: it carries phosphorus and potassium together with no nitrogen at all, it is free of chloride and sodium, and it dissolves completely and rapidly in water. That combination makes it a specialty product for irrigation systems and foliar programs rather than a bulk soil-applied commodity, and it makes it useful in the specific situations where a grower wants to supply P and K while deliberately withholding N.
Substance identity
| Field | Value |
|---|---|
| Chemical name | Potassium dihydrogen phosphate |
| Common names | MKP, KDP, potassium phosphate monobasic |
| Formula | KH₂PO₄ |
| CAS | 7778-77-0 |
| PubChem CID | 516951 |
| Molar mass | 136.08 g/mol |
| EC/EINECS | listed — confirm the current EC number against the ECHA substance record |
Specification
Theoretical composition of the pure compound:
| Parameter | Value |
|---|---|
| P | 22.8 % |
| P₂O₅ equivalent | 52.2 % |
| K | 28.7 % |
| K₂O equivalent | 34.6 % |
| Grade | 0-52-34 |
Typical commercial specification — confirm on certificate of analysis:
| Parameter | Typical |
|---|---|
| Purity (KH₂PO₄) | ≥ 98 % |
| P₂O₅ | ≥ 51 – 52 % |
| K₂O | ≥ 33 – 34 % |
| pH, 1 % solution | 4.3 – 4.7 |
| Water-insoluble matter | ≤ 0.1 % |
| Moisture | ≤ 0.2 % |
| Chloride | low; declare limit on CoA |
| Appearance | white crystalline powder or granular crystal |
Properties
-
Solubility — approximately 22.6 g per 100 mL of water at 20 °C, rising substantially with temperature. Lower than MAP or DAP on a gram-per-litre basis, which matters when preparing concentrated stock solutions in cold water: dissolve with agitation and allow adequate time, or warm the make-up water.
-
Solution reaction — mildly acidic, around pH 4.5 for a 1 % solution. This helps keep phosphate in solution in hard, alkaline irrigation water and reduces the tendency to form calcium and magnesium phosphate scale in drip lines, although it does not eliminate it.
- Salt index — among the lowest of the common phosphorus and potassium carriers per unit of nutrient delivered. Potassium chloride sits at the opposite end of that scale. Low salt index is the property that makes MKP suitable for foliar spraying and for concentrated root-zone delivery in drip systems, where osmotic injury and leaf scorch are the limiting risks.
- Chloride-free and sodium-free. This is the decisive property for chloride-sensitive crops — tobacco, many fruit trees, avocado, grapevine, strawberry, potato for processing — and for saline or sodic soils where chloride and sodium loading must be minimized.
- No nitrogen. MKP allows P and K to be supplied without adding N.
- Thermal behavior — melts near 253 °C; on stronger heating it condenses to potassium metaphosphate with loss of water.
- Density — approximately 2.34 g/cm³.
- Non-hygroscopic and physically stable in normal storage.
Applications
-
Fertigation and drip irrigation — dissolves completely, leaves no residue to block emitters, and the mildly acidic reaction assists compatibility with moderately alkaline irrigation water.
-
Hydroponics and soilless culture — a standard P and K source in nutrient solution recipes, valued because it introduces no counter-ion that has to be managed.
- Foliar application — low salt index and low burn potential permit foliar delivery of P and K, which is useful when root uptake is constrained by cold, waterlogged or compacted soil, or by low soil temperature early in the season.
- Reproductive-stage nutrition — the specific agronomic case. Late in the season many crops need continued P and K for fruit fill, tuber bulking and grain filling, while additional nitrogen at that point promotes vegetative growth, delays maturity, and can degrade fruit firmness, color and storage life. MKP supplies P and K into that window with zero N, which is precisely why it is used there and why a straight NPK product cannot substitute.
- Chloride-sensitive crops as the P and K source in place of potassium chloride.
- Industrial and technical uses — buffering agent, and, as high-purity single crystals grown from solution, the nonlinear-optical and piezoelectric material known as KDP. Those applications require purity tiers well above fertilizer grade.
Handling and storage
Store in sealed bags in a dry, covered, ventilated area on pallets. Keep away from moisture; although MKP is not strongly hygroscopic, absorbed water causes caking and complicates dissolution. Segregate from strong alkalis and from strong oxidizing agents. When preparing stock solutions, add product to water with agitation rather than the reverse, respect the solubility limit at the actual water temperature, and do not prepare concentrated stock solutions of MKP together with concentrated calcium or magnesium sources — insoluble phosphates will precipitate in the tank. Use separate stock tanks and combine only at the dilute injection stage. Wear eye protection when handling powder.
Notes
-
0-52-34 is the composition of the pure compound; it is not a blend and cannot be varied.
-
Conversions: P₂O₅ × 0.436 = P; K₂O × 0.830 = K. One tonne of MKP therefore delivers about 228 kg elemental P and 287 kg elemental K.
- Where an N-containing soluble phosphate is acceptable, technical-grade MAP ([CAS?]) is the usual higher-P alternative; where the requirement is specifically no nitrogen, MKP has no close substitute among soluble solids.
- MKP is not a soil-acidifying amendment. Its acidic solution reaction is local and transient.
Frequently asked questions
The grade is written 0-52-34. What does that actually guarantee, and can a supplier honestly offer more?
Those figures are oxide-basis percentages by weight: 52% P2O5 and 34% K2O. Pure KH2PO4 (molar mass 136.09 g/mol) contains 52.2% P2O5 and 34.6% K2O, so the molecule itself sets a hard ceiling. An offer claiming 53% P2O5 is either quoting on an ignited or dry basis, or is simply wrong. In elemental terms, multiply P2O5 by 0.436 and K2O by 0.830, which gives roughly 22.8% P and 28.7% K. A 52/34 guarantee therefore implies purity close to 99%; the balance is moisture, water-insoluble matter and minor salts. That balance, not the headline number, is where two lots genuinely differ.
What is the practical difference between technical grade, fertiliser (greenhouse) grade and food grade MKP?
All three are the same compound, controlled against different specifications. Fertiliser or greenhouse grade is written around what matters in a drip line: water-insoluble matter, moisture, chloride and heavy metals, cadmium especially, since regulatory limits on phosphate fertilisers are normally expressed per unit of P2O5 rather than per tonne of product. Technical grade targets industrial and buffer use. Food grade adds pharmacopoeia-style limits on arsenic, lead and fluoride. Food grade will perform perfectly well in fertigation, but you pay for attributes the crop cannot use. Because the grade names are not standardised between suppliers, ask for the batch certificate of analysis and compare the actual line items.
Crystal, powder or compacted granular — which physical form should I specify, and how does that affect storage?
Crystalline MKP in a defined sieve fraction flows well, dusts less and stores most easily. Fine powder dissolves faster but bridges in hoppers, generates dust and segregates in dry blends. Compacted or granular material exists for direct soil application and bulk blending, where fines would separate out during handling. On storage: MKP has a relatively high critical humidity compared with calcium nitrate or urea, so it is not aggressively hygroscopic, but it still cakes when bags breathe through day-night humidity cycles or stand on a cold concrete floor. Keep it sealed, palletised, out of direct sun and away from lime or other alkaline materials. Kept dry, shelf life is effectively indefinite.
Why does my stock tank stop dissolving part way through a batch?
MKP dissolves endothermically, so a concentrated batch cools itself as it goes in. Solubility falls with temperature — roughly 220 to 230 g/L in water at 20 °C, several times that near boiling — so a tank that was mixing happily can start dropping crystals as its own dissolution chills it. Add the salt gradually into agitated, tempered water instead of dumping it, and let the tank equilibrate at the coldest temperature it will genuinely see overnight before deciding the concentration holds. A 1% solution sits near pH 4.5, so MKP acidifies the line mildly; judge that against your water’s bicarbonate content rather than assuming it will handle acidification for you.
What must MKP never be mixed with?
The firm rule is never to combine MKP with a calcium or magnesium source in the same concentrated stock tank. Phosphate and calcium precipitate as sparingly soluble calcium phosphates: sludge in the tank, loaded filters and blocked emitters downstream. Calcium nitrate, calcium chloride and Ca/Mg-rich foliar products belong in a separate tank and should meet MKP only after dilution in the main line. Keep it away from strongly alkaline materials as well — lime, alkaline tank mixes — because raising pH shifts dihydrogen phosphate towards less soluble species. Hard, high-bicarbonate irrigation water is the slow version of the same problem. Jar-test any unfamiliar tank-mix partner at working concentration before committing a batch.
Two suppliers quote MKP differently. How do I compare the offers properly?
Convert to cost per unit of nutrient before anything else. MKP carries two nutrients, so a workable method is to value the K2O at what your usual potassium source costs and attribute the remainder to P2O5, which reveals what the phosphate is actually costing you. Then adjust for what the headline price hides: moisture, water-insoluble matter (dead weight plus filter load), the assay basis quoted, particle size distribution, packaging, and whether the certificate of analysis is batch-specific or a generic template. Two lots both labelled 0-52-34 can differ materially in insolubles. Chloride-free and sodium-free status is worth a premium only if your crop, substrate or water actually makes it relevant.
When is MKP the wrong product to buy?
More often than its reputation suggests. It supplies no nitrogen, no calcium, no magnesium and no sulphur, so it cannot carry a nutrition programme alone. Per unit of P2O5 it is far more expensive than MAP, DAP or triple superphosphate, and per unit of K2O far more expensive than MOP or SOP — for broadcast field application on soil with adequate phosphorus, that premium buys nothing agronomically. If a soil test already shows high available P, adding more is pointless, and in catchments with phosphorus restrictions it may be prohibited. MKP justifies its price in fertigation, foliar programmes, soilless culture and difficult water, where solubility, low chloride and low salt index genuinely matter. Elsewhere, commodity sources do the same job for less.
Related substances
- MAP is the cheaper source of water-soluble phosphate when the accompanying nitrogen is welcome and the potassium is not needed – the comparison is mostly about which second nutrient you actually want.
- Magnesium sulphate and MKP appear in the same fertigation programmes but belong in separate stock tanks, because concentrated phosphate and divalent cations precipitate on contact.
- The milky sludge that forms when a phosphate solution meets hard water is calcium phosphate, and the dicalcium phosphate page shows just how stubbornly insoluble that precipitate is.