How Fertilizer Recommendations Are Made

Recommendation, as we use the word here, means advice from a soil or crop specialist to those deciding how much of, what form of, and when to apply a fertilizer to a soil to benefit growth of a plant or crop. Recommendations of fertilizers and soil amendments must consider the interactions of all the factors discussed in this manual, including the soil, the crop, the climate, the beneficial effects of soil organisms such as rhizobia and mycorrhizal fungi, and the harmful effects of plant diseases, insects, and nematodes. The relations among factors affecting plant growth are dynamic and complex. Any of them can become growth-limiting at any time, necessitating a specific remedy to permit the crop to achieve its genetic potential. The major tasks of the diagnosis and recommendation effort are to (1) identify growthlimiting (or yield-limiting) factors and (2) suggest economical, environmentally sound, and practical management alternatives. In the process of modifying the amounts of nutrients in the soil, fertilizer applications have broad effects on the existing soil physical, chemical, and biological properties. Careful consideration of these impacts is needed, because unanticipated effects can limit plant growth and yield. For example, achieving sufficiency of phosphorus (P) is not solely a question of increasing the amount of “available” P (see box at right). If the soil has been fumigated and the crop is one that is highly dependent on symbiotic associations with soil fungi (arbuscular mycorrhizal fungi), the crop—lacking aid from the fungi—will not obtain sufficient P even when soil analyses show high levels of extractable P, or when large amounts of P fertilizer have been added. In such cases, the most practical solution is to ensure that the crop is adequately colonized by the appropriate fungi. Similarly, even adequate fungal colonization of the plant roots will not ensure good yields if nematode infestation is severe. These examples illustrate how all parts of the soil-plant system are potentially growth-limiting. Consequently, a critical part of plant nutrient management is noticing suboptimal plant performance, identifying the cause (or causes), and devising a remedy. In the overall diagnosis process that leads to a recommendation, individual soil and plant tissue samples are just part of the evidence that must be considered. It is a common misconception that the soil analysis or the plant tissue analysis is the only diagnostic informa-