Per Cubic Metre of Peat: 0.6 kg or 2 kg, and What Changes In Between

The question arrives in almost the same words every time: how much per cubic metre? The honest answer is a range, and the range is wide. A nursery growing young plugs in a peat substrate might use 0.4 kg per cubic metre. A landscape contractor establishing shrubs in a sandy loam on a slope might use 2.5 kg. Same material, six times the dose, and both are correct for their job.

What separates them is not the plant. It is the substrate, the irrigation regime, and — the part that gets skipped — the salt load in the fertility programme.

What the polymer does in a substrate

In a container substrate, the polymer acts as a small reservoir. It takes up water during irrigation, holds it against gravity, and releases a portion of it as the substrate dries and the matric potential falls. In a peat mix, which already holds a lot of water, the benefit is modest and mostly shows up as a longer interval before wilting. In a coarse, free-draining mix of bark, perlite or sand, the effect is much larger because the substrate itself holds very little.

That is why dose per cubic metre means nothing without the substrate description. Peat at 85 percent moisture by volume already holds 400 litres per cubic metre. Adding a kilogram of polymer that holds 300 litres sounds impressive until you realise it is competing with a substrate that is already a reservoir.

The useful way to think about it: dose by the water the substrate cannot hold, not by the volume of the bed.

The dose bands and what each one is for

At 0.3 to 0.6 kg per cubic metre, the polymer is a buffer. It smooths out irrigation lapses, reduces the peak moisture loss between waterings, and costs little enough to add to a general-purpose mix. Most nursery substrates sit here.

At 0.8 to 1.5 kg per cubic metre, it becomes a functional component. This is the range for hanging baskets, green roofs, and any container that dries faster than the irrigation system can respond. Watering frequency can typically drop by 30 to 50 percent, which is a labour saving, not just a water saving.

At 2 kg and above, it starts to change the physical structure of the mix. The swollen gel occupies volume, reduces air-filled porosity, and can keep the substrate wetter than the roots want. In a peat-heavy mix this is where root disease problems begin, and the grower usually blames the irrigation.

The counter-intuitive finding is that the highest dose is often the worst performer in a wet season. A substrate at 2.5 kg per cubic metre that stays saturated for three days after heavy rain has less oxygen at the root zone than an untreated control, and the plants show it as yellowing that looks exactly like a nutrient deficiency.

Salt cuts the number, and fertiliser is salt

Any liquid feed programme runs at an electrical conductivity of 1.0 to 2.5 mS/cm, and that is a salt solution. In a substrate fed weekly at 1.8 mS/cm, a polymer that holds 350 g/g in distilled water might hold 90 to 120 g/g in the pore solution. The difference is not a rounding error; it is the difference between a reservoir and a decoration.

Two practical consequences follow. First, do not size the dose from a deionised water figure. Second, expect performance to fall as the crop moves into a heavier feed regime, which is exactly when the plants need the water most.

If the feed water is also hard, the calcium will stiffen the gel further. A nursery on borehole water at 300 mg/L hardness should assume the polymer works at roughly half the capacity quoted, and dose accordingly.

Mixing is where the dose quietly disappears

A substrate mixer runs for 45 to 90 seconds. Polymer added dry at the start disperses reasonably; polymer added on top of an already wet mix clumps into beads that never fully hydrate and sit in the mix as hard spots for the life of the crop. A grower who inspects a handful of finished substrate and finds visible white specks has a mixing problem, not a dosing problem.

Feed the powder into the mixer with the dry components, before the water, and give it 30 seconds of dry blending. If the line adds water first, add the polymer as a pre-slurried gel instead, at a rate slow enough that it does not form a plug in the nozzle.

Uniformity is worth measuring. Take ten handfuls from different points in a batch, wash each through a 2 mm sieve, and count the swollen particles. A coefficient of variation above 30 percent across the ten samples means the mixer is not doing the job, and the average dose on the batch sheet is not what any individual plant receives.

Testing a mix before you commit to a rate

Fill six pots with the candidate mix and six with the control, water both to container capacity, and weigh them daily for two weeks without irrigating. The day the control reaches 50 percent of its starting mass is the day the polymer either buys you time or does not. That single number, in days, is the commercial case for the dose.

Then check air-filled porosity on the same mixes at container capacity. If the treated mix has dropped below about 10 percent by volume, the dose is too high for that substrate regardless of how long it held water.

Run the trial in the season you will actually grow in. A rate that works in a cool spring can waterlog in July, and the difference is not the polymer’s fault. The SAP for growing media grades are usually selected on the salt figure first and the capacity second, which is the right order for a fertilised crop.