Post-Mortem: The Six-Hectare Trial That Got No Rain for 31 Days

The trial was supposed to settle an argument between two agronomists, and it did, just not the way either expected. Six hectares of almond replant near Lleida were split into treated and untreated strips in February 2023, with 7 kg per hectare of gel worked into the planting rows. The treated strips were planted on 3 March. The first meaningful rain after planting fell on 3 April. In between, the site recorded 2.4 mm, spread across four events that never wet the top 20 mm.

By late April, survival in the treated strips was 71 percent and in the control strips 69 percent. The trial cost 9,000 euro and proved nothing, which is exactly the outcome that most gel failures produce when there is no water to work with. This is the post-mortem, and the lessons apply to every dryland planting program that treats polymer as a substitute for the first watering.

What the Site Looked Like on Day 31

The treated rows were easy to identify, and not in a good way. Granules that had been incorporated at 150 mm were still visible as hard, translucent chips in the loose soil around each tree, some the size of a pea, some fused into lumps. The trees themselves had not died of thirst in the way the crew expected. They had died of root desiccation in a soil that was dry to 300 mm, with the granules competing for whatever moisture remained.

Two soil pits told the story better than the survival counts. In the treated pit, moisture at 100 mm was 4.1 percent, lower than the 5.3 percent in the control pit three metres away. The polymer had drawn water out of the surrounding soil during the dry month and held it inside the bead, where a newly planted root could not access it. That is not a product defect. That is what a dry granule does when it is placed in dry soil and never wetted through.

Gel Needs One Wetting to Work: The Physics of a Dry Granule

Dry polyacrylate is a glassy solid with a strong affinity for water. Placed in soil at 5 percent moisture, it will absorb until it reaches equilibrium with the surrounding matric potential, which in a dry soil means it takes very little and holds it tightly. The absorption that the datasheet reports, 300 to 400 times its weight, is measured in free water, not in soil at wilting point. In a soil at 4 percent moisture, the same granule may hold only 20 to 40 times its weight, and that water is bound at a tension the plant cannot overcome.

This is why every credible protocol says the polymer must be pre-swelled or the soil must be wetted at planting. A granule that starts dry and stays dry is a competitor, not a reservoir. In the Lleida trial, the planting crew had no water cart and worked on the assumption that February rain had primed the profile. February had delivered 38 mm, but it fell three weeks before planting and the top 200 mm had dried to 6 percent by the time the trees went in.

What the Trial Should Have Done Differently

Three changes would have made the data usable. First, pre-swell the granules in a tank and plant into a slurry, which costs about 0.4 euro per tree in water and labour but guarantees the polymer starts hydrated. Second, plant into a moisture profile above 12 percent at 200 mm, which means either waiting for a 25 mm rain event or running a water cart down the rows before the planter. Third, record soil moisture at planting and at days 14, 21 and 31 in every strip, so a dry month becomes a documented variable rather than an excuse.

The trial also made a design error that is worth naming. Incorporation depth of 150 mm was chosen for drought protection, but on a site with no irrigation the roots were still in the top 80 mm at day 31. The polymer was below the roots, holding water they could not reach, and above them the soil was bare and drying. For dryland planting with no follow-up water, a split placement, half at 60 mm and half at 150 mm, would have given the young roots a chance at the shallower reserve.

How to Plan for a 30-Day Gap

Assume a 30-day gap with no effective rain and design for it. That means a water cart at planting, pre-swollen gel, a surface mulch of 20 to 30 mm, and a dose that does not exceed 6 kg per hectare in the row. It also means choosing a planting window rather than a planting date: on Mediterranean sites, an autumn planting that catches the first 20 mm front is worth more than any polymer dose, because it gives the root system eight weeks of mild weather before the dry season starts.

Finally, budget a partial replant. Every dryland program should carry 10 percent of stock in reserve, and the honest ones carry 15. If a trial has no reserve trees, the first dry month turns a research question into a commercial loss. When you next price a program, ask the supplier for absorption data at 10 percent soil moisture as well as free swell, and check whether the grade re-wets after drying; the agricultural grades in the SAP powder product listing are supplied with both figures for exactly this reason.