How to Read a Water-Soluble Fertiliser Formulation — NPK Ratio and Chelated Trace Elements

Who this is for and what it covers

This is for importers and distributors buying water-soluble fertiliser for drip irrigation, foliar application or soilless growing. It covers how to read a formulation, how to choose and which documents to ask for. It does not cover prices.

What the N-P2O5-K2O numbers say

The 20-20-20 or 19-19-19 on a water-soluble fertiliser label is the mass percentage of three nutrients: N, P2O5 and K2O. P and K are expressed in oxide form, not as the element — do not read P2O5 as a phosphorus figure.

Broadly, the ratio points to a growth stage: a balanced grade such as 20-20-20 is general purpose, from establishment through fruit fill; a high-nitrogen grade suits vegetative growth and leafy crops; a high-potassium grade is used for fruit sizing, sweetness and colouring.

The difference between 20-20-20 and 19-19-19 is not only the three main numbers: the secondary and trace element content (MgO, TE) in the recipe is often different too. Read the complete formulation sheet, not just the three headline numbers.

What else to read on a formulation sheet

A usable formulation sheet states at least: the main nutrient ratio, the secondary elements (MgO, Ca, S), the types and forms of the trace elements, and whether the product contains chloride. The point buyers most often overlook is the form of the trace elements: the same iron behaves very differently in the field as a chelate and as a sulphate, and that decides whether it is actually available under your soil and irrigation conditions.

Choose by application method

The same 20-20-20 is not right for every situation; selection comes back to application method, water quality and soil pH:

• Foliar spray: fast dissolution and low water-insoluble matter are essential, at a concentration the crop tolerates
• Broadcast and drip irrigation: focus on solubility and compatibility, to avoid precipitation blocking emitters
• Soilless culture and hydroponics: the strictest requirements on formulation completeness and trace-element form, usually chelated
• Seedling and fruit-fill stages: switch between high-nitrogen, high-potassium and balanced grades by growth stage

Secondary and trace elements, and chelated forms

Trace elements are supplied either as chelates or as sulphates. The chelating agent sets the pH window: EDTA is conventional and lower cost and suits neutral to slightly acid conditions; DTPA tolerates a wider pH range; EDDHA and EDDHMA stay stable in high-pH, calcareous soils, where iron is less easily fixed. Start from your soil pH and irrigation-water pH and choose the chelating type accordingly; in high-pH conditions a plain sulphate form can fail. Stability constants and the exact pH windows are specific to each chelating agent — there is no single public figure; use the supplier's technical data.

Solubility and water-insoluble matter

Drip, foliar and soilless systems are highly sensitive to water-insoluble matter: blocked lines and emitters usually trace back to it. When buying, require the COA to state water-insoluble matter and dissolution rate or solubility explicitly, not just the main nutrient content. Two formulations at the same ratio can behave very differently in the field if their water-insoluble matter differs.

Chloride and mixing incompatibilities

For chloride-sensitive crops, choose a potassium-sulphate, low-chloride formulation. When mixing, note that calcium and sulphate or phosphate ions can form a precipitate: run a small jar test before making a stock solution, or apply the products separately. Otherwise the precipitate blocks filters and drippers, and the saving disappears.

Procurement and document checklist

Ask for:

• The applicable standard (for example NY/T 1107-2020, Water-Soluble Fertilisers)
• The formulation sheet (main nutrients, chelating form, trace elements)
• COA including moisture and water-insoluble matter
• Packaging specification and MOQ
• A stock-solution compatibility test report

How to compare (no prices, just the basis)

Do not compare only the price per tonne. Convert to cost per unit of nutrient (N + P2O5 + K2O) and factor in the chelating form and the water-insoluble matter specification. Two offers are only comparable when the ratio, the forms and the content basis are the same.

Sources

Standard NY/T 1107-2020, Water-Soluble Fertilisers (standard number verifiable at the national standards platform): https://openstd.samr.gov.cn/

Product TDS and formulation ratios are manufacturer parameters; see the corresponding product page on this site and the manufacturer's formulation sheet.

Chelating-agent pH windows and stability constants: individual manufacturers' technical data (no single public figure).