Water-soluble phosphate for foliar and soluble programmes
Water-soluble MAP 12-61-0, MKP 0-52-34 and urea phosphate for foliar sprays, soluble NPK manufacture and hydroponic systems — including the residue limits that decide suitability.
A water-soluble phosphate is bought for one reason: it has to dissolve completely and stay dissolved. Everything else — the grade, the packaging, the price — is secondary to the water-insoluble residue figure, because a phosphate that leaves residue blocks an emitter, scorches a leaf or settles out of a tank.
This is a different product family from the granular material sold as 11-52-0. The chemical name is sometimes the same and nothing else is: the granular product contains filler and a partly soluble phosphate, and substituting one for the other in a soluble programme is a straightforward failure.
Foliar application
Foliar phosphate is used where root uptake is compromised — cold soil, poor root system, a crop at a stage when demand outruns supply — or where a small corrective dose is cheaper than a soil application. The dose is small and the concentration tolerance is narrow, so the product has to be clean and the solution has to be accurately made up.
Water-soluble MAP 12-61-0 is the usual phosphate source in a foliar programme. MKP is used where potash is also wanted and nitrogen is not, which is typical at flowering and fruit fill.
Soluble NPK manufacture
A formulator building a fully soluble NPK needs inputs that will not precipitate when they are combined. Calcium and phosphate are the pair that causes trouble: mix a calcium salt with a phosphate in the same tank and calcium phosphate comes out of solution, and both nutrients are lost.
That is a formulation constraint, not a product defect. It is handled by keeping calcium and phosphate in separate tanks, by using a chelated calcium, or by sequencing the application. A supplier who does not raise this at enquiry stage has not understood the application.
Hydroponics and substrate growing
In recirculating systems the phosphate source has to be both fully soluble and consistent, because the nutrient solution is made to a recipe and any variation in the input changes the recipe. Lot-to-lot consistency matters more here than in any other application.
Water chemistry matters too: hard water brings bicarbonate, which reacts with phosphate and with the acid used to control pH. Urea phosphate is sometimes used precisely because its acidity helps bring bicarbonate down.
The mistake that costs the most
Accepting a description instead of a number. "Fully water-soluble" on a product page is marketing; the water-insoluble percentage on the lot COA, with the test method stated, is the specification. Put the limit in the contract.
Questions this application raises
Can water-soluble MAP be used in a drip system?
Yes — that is what it is made for. The requirement is that the water-insoluble content is low enough for the emitter type, and that the figure is agreed in the contract rather than assumed from a product description.
Why does my tank go cloudy when I mix calcium nitrate with MAP?
Calcium phosphate is precipitating. The two nutrients are incompatible in the same tank at working concentration. Apply them separately, or use a formulation designed to keep both in solution.
Is MKP a foliar product or a fertigation product?
Both. It is fully soluble and near-neutral in solution, so it suits either route. The choice between MKP and water-soluble MAP is about whether potash is wanted alongside the phosphate.
Products used here
Tell us what you are applying it to
The crop, the soil or the system decides the grade. Give us the application and the quantity, and the recommendation comes back with the specification.
Request a phosphate quotation
Send the product, grade, quantity and destination port. We reply with a specification, packing options, the shipping basis and a price for your market.