Price per Tonne Is the Wrong Number: Cost per Cubic Metre of Water Held

Two quotes landed on the same desk. Supplier A wanted 2,600 per tonne, supplier B wanted 3,400. The buyer took A and felt good about it for about eleven weeks. Then the irrigation logs came in and the block dosed with B was running 30 percent fewer hours. The price per tonne had nothing to do with the cost per cubic metre of water actually stored in the root zone, and that second number is the only one that shows up in a budget.

This is not an argument for buying the expensive bag. It is an argument for doing four minutes of arithmetic before the purchase order goes out.

The calculation, with real numbers

Start with absorption measured in the water you will actually use, not deionised water. Then discount for what the plant can extract and for what survives a season. A reasonable working set: 55 percent of held water is plant-available, and 60 percent of the original capacity is still there at the end of a six-month season. Multiply the three.

The cheap tonne costs 35 percent more per cubic metre of water the tree can reach. That gap is what the irrigation log picked up in week eleven.

Where the arithmetic gets misused

The same table can be made to say anything if you leave out delivered cost. Freight on a 24-tonne load of polymer is not trivial, especially to a site 300 km from a port, and a supplier quoting ex-works will look 8 to 12 percent cheaper than the number that actually arrives. Add freight, duty and the cost of returning pallets, then divide.

Also add the dose correction. If a product is dosed by weight and its bulk density varies between lots, the same 25 kg bag covers a different volume each time. Bulk density around 0.75 to 0.85 kg per litre is normal; a lot at 0.65 means the crew’s scoop is delivering 20 percent less than they think. That is a cost line too, and it hides inside the bag price.

What the water is worth

Irrigation water in most agricultural districts costs somewhere between 0.20 and 1.20 per cubic metre, and in municipal landscape work it is often free at the meter but expensive in labour and truck time. If water is 0.60 per cubic metre, then 99 cubic metres of stored water is worth about 59 in avoided purchases, which does not come close to the 3,400 tonne price. The value is not the water. The value is the timing: water available in week six of a dry spell, at the root, without a truck.

That reframing matters when you are writing a justification. Do not claim a water bill saving unless you have metered it. Claim the reduction in irrigation passes, the survival delta, and the labour hours not spent dragging a hose. Those are the numbers a finance manager can check against a timesheet.

A counterexample worth remembering

A 14-hectare revegetation job chose a 400 g/g product at a premium and applied it at the standard rate. Six months later the ground looked worse than the control strip. The product was fine. The dose had been set for a 250 g/g gel, and the higher-capacity material swelled so much in the backfill that it lifted seedlings out of the ground after the first heavy rain, a failure mode nobody had budgeted for. Higher capacity is not free; it changes the volume maths in the hole.

Before the next quote comparison, build the four-line table above in a spreadsheet and force the supplier to give you absorption in your water, delivered price and bulk density. If they cannot supply bulk density, ask for a 5 kg sample and measure it yourself with a 10-litre bucket. Our potassium polyacrylate gel range is quoted with all three figures for exactly this reason.