Counting Survivors: How to Measure Whether Gel Paid for Itself

Survival rate is the number that decides whether a planting programme gets repeat funding, and it is also the number most often estimated rather than counted. A site walk in September, a glance at the rows, and a figure of “about 90 percent” goes into the report. If you are spending money on water retaining gel, that is not good enough to tell you whether the spend worked — you need a count you can defend in a meeting, and you need the replacement cost arithmetic beside it.

Pick a sample you can defend

Decide before planting which rows are monitored, and mark them. A workable design is five fixed transects of 40 plants each per treatment block, giving 200 plants per treatment. That is enough to detect a difference of about 8 percentage points with reasonable confidence; if you need to resolve 4 points, you need roughly 800 plants per block, which is why most commercial trials settle for detecting only large effects.

Randomise the transects, not the treatment. Alternate treated and untreated strips across the site so that soil type and aspect do not line up with the treatment. If the gel block happens to be the flat bottom of the paddock and the control is the dry ridge, you will measure topography and call it polymer.

Define death before you start, in writing. Our working definition: no green cambium when the stem is scraped 2 cm above the root collar, and no bud break by week eight in the first growing season. A plant that is alive but not growing is not a survivor in any commercial sense, so record it separately as “alive, unthrifty” — it will usually be dead by month 18, and it will distort your numbers if you count it as a win.

The arithmetic that decides payback

Take a 5,000-plant scheme. A bare-root seedling costs about $0.60, planting labour about $1.50 per stem, and the gel about $0.05 per stem at 3 g and a bulk price near $17/kg. Total per stem without gel is $2.10; with gel it is $2.15.

Now suppose gel lifts first-year survival from 82 to 88 percent — 6 points, a realistic outcome on a dry site, not a spectacular one. That is 300 fewer dead plants. Replacing them costs $2.10 each in materials and labour, plus the fact that a replacement planted in year two is a year behind and often dies too. Avoided cost is $630 against a gel spend of $250. The payback is real, but it is a 2.5:1 return, not a 10:1 one.

That ratio is the honest shape of the case. It also tells you where the deal breaks: if gel costs $40/kg rather than $17, or if your baseline survival is already 92 percent, the same 6-point gain cannot happen and the arithmetic collapses. Run the sum with your own numbers before you order, and be suspicious of any supplier case study that quotes survival gains without stating the control’s baseline.

What survival numbers hide

A survival rate is an average across a site that is never uniform. Break your count down by block, by soil type and by slope position, or you will miss the fact that gel saved the ridge and did nothing in the hollow. Three of five blocks improving by 10 points and two unchanged is a much more useful finding than “+6 overall”.

Timing matters as much. Count at week six, at the end of the first dry season, and at 24 months. Most mortality happens in the first dry season, and a 12-month count flatters every treatment because the losses that define a planting’s reputation happen in year two. Also record the weather: a 6-point gain in a wet year means nothing, and a 6-point gain in a 1-in-10 dry year is worth more than the table suggests.

Finally, be straight about what a survival count cannot tell you. It does not measure growth, and a treated plant that survives at half the size of the control has not obviously paid back. Add a height measurement on 50 stems per block and you will catch that case before it reaches a client report. If you want the dosage basis for the cost side of the calculation, the product data by application gives grams per stem for each planting method.