Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
PLANT NUTRIENTS
Economics of nutrient application
When plant nutrients limit crop growth, low to medium application rates provide the greater plant response and often the greatest economic return per dollar invested. As the application rate increases, the plant response per unit of nutrient applied decreases, as does the economic return per unit applied. As plant yields approach their maximums, other factors may become limiting, but the economic return from fertilizers still may be several dollars for each dollar invested. The highest profit per acre and the lowest cost per unit of crop produced also result as maximum plant yields are approached. Adding plant nutrients in amounts beyond those required for maximum plant growth actually may produce no plant growth response or positive economic return. Nutrient recommendations for agronomic crops in Pennsylvania are based on research to estimate the amount of a nutrient needed to assure economic optimum crop production. As the economic conditions change, eg. fertilizer or crop prices change, the recommended amount of nutrients may also change.
Too much of a plant nutrient may cause a nutrient imbalance in the soil and in the plant. For instance, too much phosphorus may limit the availability of zinc for corn, or too much potassium may interfere with the uptake of magnesium by corn and other grasses. Building phosphorus and potassium to high soil nutrient levels may be advantageous in order to have sufficient nutrients available for an above-average growing season. However, additional broadcast applications from fertilizers or manures that boost soil levels into the excessive range not only result in unsatisfactory economic returns, but also can adversely affect plant growth, animal health, and environmental quality.