A block office plans to hand out free soil-testing cards so that each farmer can learn which nutrient his field lacks and then buy only the fertiliser that is needed, saving money and avoiding over-application. Which of the following, if true, raises the most serious doubt about the plan?

Knowing what the field lacks helps only if the farmer can then buy that and nothing else. Where the sole dealer sells fixed mixtures, the test result cannot change the purchase, so neither the saving nor the reduction in over-application follows. Storage of the cards, cropping patterns and earlier seed distribution leave the farmer's buying choice exactly where it was.