Turf on Fill and Compacted Subsoil: Getting Gel Into the Top 100 mm

Turf is the most unforgiving client a water retaining gel will ever have, because the crop is a 20 mm mat of roots sitting on whatever the site handed you. On a good loam that mat knits in twelve days. On compacted fill, brick rubble, or a skim of poor subsoil over hardcore, the same mat takes three weeks and dies twice on the way. Gel helps in the second case and does very little in the first, and working out which case you have is most of the decision.

What poor ground does to germinating seed

Three failures arrive together on fill. Infiltration is slow, so irrigation runs off sideways before it wets the seedbed. Water-holding capacity in the top 50 mm is low, so what does soak in dries out within a day of wind. And the surface crusts, so seedlings that do emerge snap off at the collar.

Gel addresses the second problem and only partly the first. It cannot fix crusting, and it cannot fix a compacted layer 150 mm down that stops roots dead. If a spade will not go in, rip or aerate first. Polymer spread over a hardpan is money sitting in a puddle.

Rates per square metre, and why 100 mm is the number

For turf establishment, work the granules into the top 100 mm of seedbed. Not the top 30 mm, because that layer dries and takes the gel with it. Not 300 mm, because the roots are not there yet and the money is buried out of reach.

Bands by ground condition:
Good loam with 150 mm of topsoil: 15 to 20 g per m2
Thin topsoil over clay: 25 to 30 g per m2
Compacted fill or sandy subsoil: 30 to 40 g per m2
Hydroseeding with mulch slurry: 15 to 25 g per m2, applied in the same pass

A worked figure: 500 m2 of compacted fill at 35 g per m2 is 17.5 kg, which is most of a 25 kg bag and about the point where buying by the pallet starts to make sense. On a 2,000 m2 verge the same rate needs 70 kg, so specify by weight and not by area alone.

In a hydroseeder, add the gel last, after mulch and seed are dispersed, and keep the tank moving. Granules that settle in a still slurry swell unevenly and block the nozzle. Use a fine grade, roughly 0.3 to 1 mm, for slurry work. Coarse 2 to 4 mm granules belong in incorporation with a rotavator or a tine rake, where they can be buried rather than sprayed.

Watering a new turf surface: light and often, then less often

Germinating turf needs surface moisture while gel holds water at depth, so the two have to be reconciled rather than averaged. The rule: keep the top 25 mm damp from the hose and let the gel look after the 25 to 100 mm zone.

Days 1 to 10: three passes a day of 3 to 4 minutes, at first light, midday and early afternoon. That is roughly 4 to 6 mm a day.
Days 11 to 21: two passes a day, morning and mid-afternoon, 5 to 6 minutes each.
Weeks 4 to 6: one deep pass every second or third day, 10 to 12 mm, timed to the weather.
Week 7 onward: weekly deep irrigation of 15 to 20 mm, judged by feel at 50 mm rather than by surface colour.

On a slope, halve each pass and double the frequency, because runoff beats evaporation as a loss. On a windy site, water early and accept that the surface will look dry by two in the afternoon. The gel is what carries the roots through that gap.

Pick the grade for the method, not the cheapest bag: fine granules for slurry, medium for incorporation, and an absorption figure quoted in tap water rather than distilled. Our SAP powder product range sets out particle size and absorption bands so a grade can be matched to a seedbed before a pallet is ordered.