Deficiencies of Secondary Nutrients in Crop Plants—A Real Challenge to Improve Nitrogen Management

Secondary nutrient (e.g., calcium, magnesium, sulfur) deficiencies in crop plants disturb the nitrogen balance in the plants, thus reducing the overall yield. This hypothesis was analyzed based on the physiological functions of these nutrients, in relation to the uptake and utilization of N, in crop plants. Nitrogen uptake by plants requires a well-developed root system, the size of which depends on the supply of calcium. This process is largely controlled by the content of toxic aluminum in the soil, which can be mitigated through the application of lime and/or gypsum. In humid climates, the excessive uptake of calcium by plants occurs during water shortages; this process significantly interferes with N uptake. Magnesium, which affects plant growth throughout the growing season, can effectively control excessive calcium uptake. Magnesium deficiency can be ameliorated with soil- or foliar-applied fertilizers. These stages define the timing of plant sampling and determination of the N:S ratio, as an indicator of plant nutritional status. The application of Mg, S, or MgS facilitates higher productivity of fertilizer N by narrowing the N:Mg and N:S ratios in plants. The use of secondary nutrients can allow farmers to obtain high yields while reducing both production costs and environmental risks.