NPK Fertilizer

NPK is shorthand for a compound fertilizer carrying nitrogen, phosphorus and potassium together, and it is defined by the specification the buyer sets rather than by one fixed composition. The three numbers on the bag are percentages by mass, but only the first means what it appears to: nitrogen is stated as the element, while phosphorus is stated as P₂O₅ and potassium as K₂O — an oxide convention inherited from nineteenth-century ash analysis that survives because the entire trade quotes it. A 15-15-15 therefore carries 15% nitrogen, about 6.5% actual phosphorus and about 12.5% actual potassium, and anyone converting between conventions has to carry the factors or the numbers stop agreeing. The second point is complex versus blend: a chemically granulated complex puts every nutrient in every granule and cannot separate, while a bulk blend is cheaper and more flexible but will segregate on a long conveyor or a rough road if the particle sizes are not matched. Ehlinar supplies granular complexes, bulk blends and water-soluble grades, built to the ratio and the nutrient sources the crop and the soil call for.

Appearance: Granules of roughly 2–4 mm whose look gives away the route: mottled and multi-coloured from a bulk blend, uniform off-white, grey, brown or pink from a chemically granulated complex, and frequently dyed blue or green. Water-soluble grades for fertigation are a fine white to pale crystalline powder instead. Almost every grade is hygroscopic and coated.

Applications

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Also known as

Compound fertilizer · Complex fertilizer · Multi-nutrient fertilizer · Bulk blend (BB) · NPK 15-15-15 · NPK 20-10-10 · Water-soluble NPK 20-20-20

Frequently asked questions

What do the three numbers on the bag actually mean?

Percentages by mass of nitrogen, phosphate and potash — but in different currencies. Nitrogen is given as elemental N; phosphorus is given as P₂O₅ and potassium as K₂O, neither of which is actually present as an oxide in the bag. To convert to elements, multiply P₂O₅ by 0.436 and K₂O by 0.830; to go the other way, multiply P by 2.29 and K by 1.20. So 15-15-15 supplies 15 kg N, 6.5 kg P and 12.5 kg K per 100 kg. The convention is an accident of history that the trade has never shaken off, and it is the single most common source of arithmetic errors in a fertilizer specification.

Complex NPK or a bulk blend?

A complex is chemically granulated, so every granule holds the full ratio and the product cannot separate however far it travels or how it is spread — which matters most for wide bout widths and for crops sensitive to uneven nutrition. A bulk blend physically mixes finished granules of urea, MAP or DAP and MOP or SOP: it is cheaper, it can be made to any ratio at short notice close to the farm, and it segregates if the components differ in size or density. If you are buying a blend, ask for the size-guide number of each component and keep them close; if you are buying to spread at 24 metres or more, or shipping a long way in bulk, a complex usually repays the premium.

Which potash — MOP or SOP?

Muriate of potash (potassium chloride) is the default: it carries more potassium, around 60% K₂O, and costs substantially less. Sulphate of potash (potassium sulphate) carries about 50% K₂O, adds sulphur, and — the actual reason to buy it — brings almost no chloride. Chloride-sensitive crops such as tobacco, potatoes, grapes, citrus, avocado and most greenhouse crops lose quality or yield when chloride accumulates, and so does any crop on a saline soil or irrigated with saline water. If chloride is not a problem for your crop and your water, MOP is the rational choice.

Can NPK be shipped in bulk by sea?

It depends on the ammonium nitrate content and on one test. An NPK containing ammonium nitrate can undergo self-sustaining decomposition: once a local heat source starts it, the reaction keeps travelling through the pile after the heat is removed, giving off nitrogen oxides. The trough test measures how fast it propagates, and a grade above 25 cm/h is not permitted in bulk at sea. Grades held to no more than 70% ammonium nitrate with no more than 0.4% combustible organic carbon — or no more than 45% with combustible material unrestricted — fall under UN 2071, Class 9, and sit outside that schedule altogether when a trough test shows they are not liable to self-sustaining decomposition. If the cargo is going into a hold, ask for the trough-test result alongside the offer.

Why is my blend arriving separated?

Because the components are not the same size. A bulk blend stays mixed only while its particles behave identically when they are poured, vibrated or conveyed, and small differences compound over a long journey: the larger granules migrate to the outside of a cone and the fines to the middle, so the top of the pile and the bottom of the bag analyse differently. The fix is specification, not handling — match the size guide number and the uniformity index across every component, keep the density spread narrow, and avoid free-flowing fines. Where a blend cannot be kept uniform, a chemically granulated complex removes the problem entirely.

Why must urea and ammonium nitrate never be blended dry?

Because the mixture pulls water out of air that neither component would react to. Every soluble salt has a critical relative humidity above which it absorbs atmospheric moisture, and a mixture's is always lower than that of its components — sometimes startlingly so. Urea holds to about 72% relative humidity and ammonium nitrate to about 59%, but a dry blend of the two goes damp and then slumps into a slurry at under 20%, which is ordinary indoor air. That is why urea-based and ammonium-nitrate-based grades are kept apart in a blending plant and in a store, and why a compatibility check belongs in the blend specification rather than in the complaint that follows.