Estimating litres of water a gel dose actually buffers between shifts

At the two o’clock handover, the irrigation supervisor asks a fair question: how many litres is this stuff actually holding for us between one shift and the next? Not absorbency in grams per gram, not a lab figure. Litres that a plant can use, in the hours between one watering and the next, on this block, with this water.

You can answer that with three multiplications and one correction factor. Most people skip the correction factor, which is why the answer they give is usually two to three times too high.

The raw arithmetic

Start with the dose. Say 80 g per tree of a granule rated at 300 g/g in distilled water. The theoretical maximum is 80 times 300, which is 24,000 g, or 24 litres per tree. That is the number that appears in a sales conversation, and it is not wrong, it is just not available.

Now apply the corrections. In real soil, a granule that is fully swollen at zero suction will give up water only as the soil around it dries, and it stops releasing well before it is empty. Roots cannot extract the last portion. Salt in the soil or the irrigation water reduces the swelling from the start.

A working set of factors for a moderate soil and clean water:

So the 80 g dose, in a good loam, is worth roughly 10 to 12 litres of plant-available water per tree, not 24.

Turning litres into days

Ten litres per tree is only meaningful against a demand figure. A first-year tree in summer might use 3 to 6 litres a day in a hot climate, less in a cool one. So the gel is covering something like two to three days of demand, not two to three weeks.

That sounds disappointing until you compare it with the alternative. Two to three days of buffer is exactly the gap between a broken pump and a repair, between a missed shift and the next one, between a heat spike and the weather change. It is insurance priced per tree, and it should be sold that way rather than as a water replacement.

Why doubling the dose does not double the buffer

The second 80 g goes into the same soil volume, competes for the same water, and swells against the same confining pressure from the surrounding soil. In a planting hole, the total volume of swollen gel is limited by the pore space available. Once the backfill is saturated with gel, additional granules displace soil rather than adding usable storage.

Field measurements on a sandy loam showed 80 g per tree delivering about 11 litres of usable buffer, 160 g delivering about 17, and 240 g delivering about 19. The curve flattens fast. That is the result that changes purchasing decisions: the second bag buys much less than the first, and the money is usually better spent on a wider treated band, better placement, or more water at planting.

The shift calculation in practice

For a block-level estimate, work in millimetres rather than litres per tree. A dose of 40 kg/ha incorporated through 200 mm, at 40 percent usable and 300 g/g, holds roughly 4.8 mm of plant-available water across the hectare. On a block using 6 mm a day at peak, that is less than one day of full demand.

That number usually surprises people who have been told the material holds 300 times its weight. It is still a real benefit, because the buffer matters most at the margins, in the eight hours before a scheduled irrigation, in the two days a pump is down, in the week after planting when the roots cannot yet forage.

Do the arithmetic before the handover, not during it. The application guide works the same calculation at three soil types; send the dose, the incorporation depth and a water analysis, and the litres and the days come back as numbers you can check.