||As farmers make the transition from traditional to modern agriculture their total operation changes. One of the important decisions they have to make during this period is which fertilizer to apply and how much. An important function of adaptive research is to assist farmers in making this kind of decision so that they will realize maximum returns from a given investment. Useful data for determining optimal fertilization practice are derived primarily from field experiments. Data obtained in these experiments are also used for calibrating soil and plant testing procedures, which, subsequently, are useful in making recommendations on fertilizer use for specific farming conditions. The modern approach to fertilizer use experimentation assumes that variability in fertilizer response among sites in an area of study arises mainly from measurable differences in the soil, plant, climate, and management factors. Experiments are located to sample the major differences in soil and climate; plant and management factors are controlled at near-optimal levels; observations are made during the course of the experiments to characterize the productivity factors; and the results are expressed in the form of a general yield equation. Through field experiments conducted in this manner it is possible to determine a quantitative relationship among crop yield, applied fertilizers and the productivity conditions for a given region. This general relationship serves as a basis for making fertilizer recommendations for specific farming conditions. However, certain principles must be carefully observed in locating the experiments, selecting the treatment design, defining the rates of fertilization, conducting the experiments, and analyzing and interpreting the results. This manual treats in detail only one part of the general methodology, the technique of carrying out the field experiments. Fertilizer trials conducted in farmer's field are frequently lost or fail to produce useful information due to an oversight or failure to perform some function properly at the correct time. Careful attention to the principles and details described here should greatly reduce the frequency of such loss and increase the usefulness of data obtained in the harvested trials. This manual has been prepared as a guide for agronomists who are initiating adaptive research on fertilizer needs of crops. A study of the fertilization of unirrigated maize in an important agricultural region of Central Mexico is described. This manner of presentation permits a detailed examination of the several activities involved in fertilizer use studies with maize and, in addition, shows how these activities are organized chronologically. The details described here will vary somewhat for plantings of maize under other conditions and may be greatly different for other crops. However, the principles apply equally well to maize and to other crops.