15N‐Labelled Single‐treatment Fertility Experiments 1

There is only one way of measuring directly the uptake of nutrient from applied fertilizer as affected by timing, placement, fertilizer source, and environmental interactions. This is by isotope labelling of the fertilizer. The purpose of the present study is not only to demonstrate the effectiveness of this technique in field experiments, but to extend the technique to the direct measurement of these fertilizer efficiencies without interactions. A further extension permits the direct quantitative measurement of interactions. The method has been referred to as “single‐treatment fertility experiments” because all treatments, as far as the plant soil or environment are concerned, are identical. The only difference is which of the fertilizer applications within a given treatment was isotopically labelled. Thus growth, nutrient uptake, amount, and pattern in all plots is identical within normal experimental variation. The presence of the label permits the quantitative measure of treatment effects without interaction. The method has been successfully used in field experiments in various areas of the world with rice ( Oryza sativa L.), maize ( Zea mays L.), and wheat ( Triticum aestivum L.), Problems quantitatively evaluated were: the relative effectiveness of NH 4 + and NO 3 − as a source of N to rice at different placements; the effect of NH 4 + ‐N in the fertilizer band on the uptake of P and of P on the uptake of NH 4 ‐N; and the uptake of fertilizer N by corn, rice, and wheat as related to growth stage at which it was applied. Interactions can be directly and quantitatively measured by adding additional treatments.