Gel Below the Rip Line: Placement Depth in Compacted Profiles

The ripper went down 320 mm, the planting holes were augered to 400 mm, and the gel went into the bottom of every hole. It looked thorough. Two summers later the survival on that block was 61 percent, and the test pits showed why: the gel was sitting in a 100 mm band beneath a compacted layer that the roots had never crossed, fully swollen and completely unreachable. The trees had died above a reservoir they could not reach.

Placement depth is the most under-specified variable in gel application. Dose gets argued over for hours; depth usually gets whatever the auger happens to do. But a granule 100 mm outside the root zone is worth nothing, and in a compacted profile it can be worse than nothing, because it holds water that would otherwise have moved upward by capillarity.

Where the roots actually are

For a container-grown tree at planting, 80 to 90 percent of the root mass is in the original root ball, in the top 300 mm. New roots extend outward and slightly downward in the first season, mostly in the top 200 to 250 mm, and they follow pores rather than pushing through dense soil. In a compacted subsoil with a bulk density above 1.6 g/cm3, root penetration drops sharply and the tree effectively grows in a shallow disc.

So the gel should be where the disc is, plus a margin at the edge where new roots will explore. In practice that means the backfill annulus between the root ball and the pit wall, from 100 to 300 mm below the surface, mixed through the backfill rather than dumped in a layer.

Three placement faults worth recognising

The first is the bottom-of-hole dump, described above. It is popular because it is fast and because a supervisor watching from a vehicle can see material going in. The second is the surface dressing, where granules are spread on top and watered in; they swell, dry, and mostly stay in the top 30 mm where the sun and the wind work on them. The third is the sidewall strip, where gel is placed against a glazed pit wall. That one is subtly worse than it looks, because a smooth auger wall is a barrier and the gel sits in a film of water that runs down the wall rather than into the soil.

If you see any of the three on a site visit, the survival numbers six months later will tell the story before the pits do.

Matching depth to the tillage

The rule is simple: the gel goes in the same tilth as the roots, not below it. If you rip deeper than you plant, either plant deeper or accept that the deeper looseness will not be used in year one, and dose accordingly.

Scarifying the pit wall

One cheap fix solves both the glazed wall and the layer problem. Before backfilling, run a hand fork or a crowbar down the pit wall to break the smear, four or five strokes per side. Roots find those cracks, and water moves laterally instead of sitting in the hole. On the 61 percent block, the retrofit was a deep watering lance pushed 400 mm at the pit edge on the surviving trees, which broke the barrier and lifted survival on the remainder to 84 percent by the following spring.

None of this requires more product. It requires the supervisor to specify depth in the method statement and to check it in the first ten holes, with a tape, on the first morning. After that, the crew holds the pattern.

Before the next planting, add two lines to the method statement: gel mixed through the backfill from 100 to 300 mm, and pit walls scarified. Then dig three holes after the first 50 trees and photograph them with a tape. The grades and mixing guidance for our soil-applied SAP powder assume incorporation rather than layering, which is worth knowing when you write the clause.