by Dr. Alan Blaylock
Dr. Alan Blaylock, who holds a MSc in plant nutrition, concludes that “Micronutrients are not magic, you need to understand them to maximize them".
Maximum benefit from micronutrients is obtained through good management. We are seeing more benefit and response to micros over time, as soils become more depleted and more responsive hybrid crops are introduced.
Nutrients are taken up by the plant through either mass flow, diffusion or root interception. The micronutrients are primarily taken up by diffusion into the root. The "Root Rhizophere" is a busy place, colonized by micro-organisms that live in an environment largely enriched by exudates which is leakage from the roots themselves. Activity involved in nutrient availability can be affected by many factors, including background levels, pH, organic matter and temperature.
For example, for each point of pH change (say from pH 7 to pH 8), there is a 100 x’s decrease in the availability of Zinc, Manganese and Copper. With each point increase in pH, it decreases Iron availability by 1000 X. Oxygen also affects nutrient availability such as Manganese and Iron.
Adverse soil conditions also affect micronutrient uptake because "the micros don’t come running to the roots" and since roots only explore 1-2% of the soil volume, the way you apply your micronutrient fertilizer is important. You need to pay attention to interactions. For example, high Nitrogen levels delay the transport of Copper from the lower leaves to the upper leaves in a plant. One must realize that high organic matter, complexes or ties up Copper, while lower organic matter means slower decomposition and lower micronutrient release. While we have slow diffusion in high clay soils, we can experience leaching of cations in low CEC or coarse textured soils.
For example broadcasting, while being an acceptable strategy for Boron, is not a great idea for Manganese, Zinc or Iron especially in high pH soils where banding can be 2 to 4X more effective.