Gel in an orchard conversion year: knocking out old rows, planting new

Four point two hectares of 22-year-old Granny Smith came out in the second week of July. The stumps were pushed into a heap, the rows were ripped, and 3,100 new trees went in during the first week of September. By December, 240 of the new trees were dead, all of them in the two rows that ran through the old headland where the bins had been stacked every season for two decades.

The grower’s first conclusion was that the gel had burned the roots. The second was that the nursery had sent a bad batch. Neither was true, and the third possibility, that he had planted into a compacted, herbicide-loaded headland, took a soil pit and one laboratory test to confirm.

A conversion year is the most demanding thing a block of gel will ever be asked to do. Old root systems, residual chemistry, changed soil structure and a new planting all land in the same 12 months.

What the old orchard leaves behind

Removing trees is not the same as removing an orchard. The old root system decays over two to four years, and while it does, it supports fungi, nematodes and bacteria that were adapted to the previous crop. In apple and stone fruit ground this is the replant disease complex, and its effects look exactly like a water problem: poor extension, small leaves, slow establishment.

Add a gel application and you have introduced a second variable right when the first one is hardest to see. That is why the pit matters more than the product in a conversion year. Dig in the new row and look at what the roots are growing into: old root fragments, a plough pan at 250 mm, or a layer of clay that came up with the ripper.

Sequence the removal before you sequence the planting

The order of operations decides how much of your gel survives to do its job.

• Push and burn first, then rip. Ripping through a standing stump drags it and smears the slot.
• Rip to below the old root plate. On mature apples that means 600 to 700 mm, not 400. If the ripper stops in the old root zone, the new roots meet the same compacted layer the old ones did.
• Let the ground settle. A ripped slot that is still open when you plant will leave the new tree sitting in a channel that drains away from the root ball.
• Test before you treat. One composite sample per management zone, for pH, salinity, and at least a nematode count in stone fruit ground.

The old row lines are still there

In a conversion, the old tree lines have different soil chemistry from the inter-row. Twenty years of herbicide strips, fertiliser placement and root turnover leave a band that is often more compacted, higher in some nutrients, and lower in organic matter than the rows between them.

If the new rows sit exactly on the old rows, which is what most conversions do because the trellis and irrigation are already there, then your gel is going into the most difficult soil on the property. Doubling the rate does not fix that. Sub-soiling the old line, adding organic matter, and applying the gel in a wider band around the planting spot does.

The failure worth remembering

The 240 dead trees in the headland had three things in common. The soil was compacted to 320 mm, the pH was 8.4 from years of lime spillage around the bins, and the gel had been applied at the full rate into a planting hole that was only 250 mm deep because the auger could not get further. The roots sat in a small pocket of treated soil, surrounded by a hardpan, in a high-pH zone.

None of that is a polymer failure, but the diagnosis took six weeks because the first assumption was that the new material was at fault, and six weeks is long enough to lose a season. If a conversion-year block fails in a pattern, look for the pattern first: rows, headlands, old tree lines, compaction from machinery, or a drainage path. Polymer failures are usually uniform, because the material was mixed uniformly. Site failures are patchy.

The water budget in year one

A converted block has two water demands in the first season: the new trees and the decaying old root system. The old roots do not transpire, but their decay changes soil structure and can create preferential flow paths straight down the old root channels, bypassing the new root zone entirely.

That is the argument for placing gel in a band around the new planting spot rather than in a narrow hole. A band 300 to 400 mm wide intercepts water moving down the old channels and holds it where the new roots are spreading.

Conversion years are also the one case where I would not cut irrigation in the first season in response to a gel application. The block is rebuilding a root system in disturbed soil.

If you are planning a conversion, the application guide covers banded placement for replant situations. Send the block plan, the age of the old trees and the ripping depth, and we will tell you which rows are likely to need the higher rate before the first tree goes in.