Duflo, Kremer and Robinson sold fertilizer vouchers at harvest, not planting

Years of agricultural-extension campaigns in Western Kenya had told smallholder maize farmers, correctly, that fertilizer raised yields well above its cost. Surveyed farmers largely agreed. Adoption stayed low anyway — only about a quarter of farmers used any fertilizer in a given season — and the standard remedy was more of the same: more demonstration plots, more persuasion, subsidies offered at planting time, on the assumption that holdouts either didn't believe the numbers or couldn't afford the input.

Economists Esther Duflo, Michael Kremer and Jonathan Robinson suspected the real barrier was neither belief nor price but timing: by planting season, when fertilizer purchases and their price pressure actually arrived, harvest cash had usually already gone to school fees and debts, leaving farmers who genuinely intended to buy fertilizer without the money to do it. A subsidy experiment alone would only show whether price mattered, so they designed a field trial across two planting seasons in Western Province built specifically to separate the timing question from the price question.

Sold full-price vouchers at harvest, delivered at planting, catching cash when plentiful and locking in the purchase before it leaks away.

After harvest, farmers had cash, so they could buy the voucher without competing expenses.

The voucher locked in the purchase, acting as a commitment device against later impatience.

Fertilizer delivered later meant farmers never had to hold cash and resolve simultaneously.

The effect appeared even at full price, showing timing, not price, was the barrier.

When people defer a worthwhile action until resources are gone, sell the commitment at the moment of liquidity, not at the moment of need.

The early full-price voucher raised fertilizer adoption 14 percentage points in the first season (from a base of 24%) and 18 points in the second (from 26%). A 50% subsidy offered later in the season raised adoption by only 13 points, and a free-delivery offer made later had no significant effect at all — moving when the offer arrived beat cutting what it cost.