Wind and dry topsoil: why surface-applied gel migrates

Twenty-eight kilometres an hour, gusting to thirty-six, a relative humidity of 19 percent, and a spreader working across 40 ha of sand. By the time the operator stopped for lunch, the hopper had emptied 26 percent faster than the area covered justified. Some of that material was in the soil. A good deal of it was in the fence line 300 m downwind, visible as a band of swollen beads after the first rain.

Dry polyacrylate granules are light. Bulk density sits around 0.6 to 0.8 grams per cubic centimetre, which puts a 0.5 mm particle in the same weight class as a grain of dry sand of twice its diameter. On a windy day, on bare ground, with no water to stick it down, it behaves like dust with a bad attitude.

How the material leaves the site

Wind. The obvious one. Granules under about 0.3 mm are gone at almost any wind speed above 15 km/h once the surface is dry. The 0.5 to 1 mm fraction moves by saltation, bouncing along the surface until it hits a fence or a windrow.

Water. Less obvious, and more expensive. Granules broadcast on a slope or a compacted surface are the first thing to move when rain arrives, because they are lighter than the soil and they sit on top of it. A 20 mm event three days after a surface application can move a large share of the product into the drain, and the block looks treated because a thin scatter remains.

The migration cycle nobody accounts for

Surface-applied gel goes through a cycle that moves it upward and outward. Dry granules sit on the surface. Rain swells them. They swell to several times their dry diameter, lift the soil crust slightly, and then dry out and shrink. Shrunken granules are lighter and looser than the crust around them, and the next wind moves them further. Repeat this five times and the material has sorted itself: the fines have blown away, the coarse fraction has collected in hollows, and the effective dose across the block is uneven.

That sorting is the reason a block can be treated at the correct average rate and still perform badly. The average is right; the distribution is not. Half the block has twice the dose and half has almost none.

The 300 kg day that ended up in a drain

A restoration job on 12 ha of light sandy soil specified 25 kg/ha broadcast and watered in. The contractor broadcast in one pass behind a tip truck, intending to water in with a bowser the following day. The bowser broke down. Forty-eight hours later, 18 mm of rain fell in 40 minutes on a block with a 3 percent slope.

Of the 300 kg applied, a post-event inspection found usable concentrations in roughly half the area, a heavy concentration in a sediment trap at the low corner, and a visible bead line along 400 m of table drain. The effective dose on the upper slopes was close to zero. The block was re-treated at a third of the original rate in the areas that had lost material, and the total cost of that job was about 40 percent above the original budget.

The lesson is not that broadcasting is wrong. It is that a surface application without incorporation has a shelf life measured in hours, not days, and the weather does not respect the plan.

What actually keeps it in place

Four approaches, in order of reliability:

• Incorporate. Mixing through the top 100 to 150 mm removes the problem almost entirely. It costs a machine pass and it is the only method that survives a wind event and a storm in the same week.
• Band it. A narrow band under the plant row, covered by the planting operation, is protected by soil from the start and uses less material.
• Hydrate before applying. Pre-swollen gel is heavy, does not blow, and stays where it lands. It is harder to handle and needs water on site, but on a windy site it is often the only practical option.
• Water in immediately. If you must broadcast dry, have the water on site and apply it the same day, in the same pass pattern, at a rate that wets the top 50 mm. A bowser booked for tomorrow is not a plan.

Measuring your own loss

Put four shallow trays of known area on the ground in the application path, weigh them before and after the pass, and weigh them again after 24 hours. The difference between the first two weights is what landed. The difference after a day of wind is what left. It takes ten minutes and it will tell you more about your spreading technique than any specification sheet.

Do it once in calm conditions and once in wind, and you will have a working rule for when to stop spreading. On most sites, that rule lands somewhere around 20 km/h for dry granules on bare ground.

If you are planning a surface application on an exposed site, the application guide covers pre-hydration and banding methods in detail. Send the wind rose for the site and the soil texture, and we will tell you which method is likely to leave the material where you put it.