Which micronutrient becomes more available at pH 7 compared to pH 5.1?

Molybdenum is the micronutrient that becomes more available at a pH of 7 compared to a pH of 5.1. This is primarily due to the chemical behavior of molybdenum in the soil. At higher pH levels, molybdenum exists largely as the molybdate ion (MoO4^2-), which is more soluble and thus more readily absorbed by plants. Conversely, at lower pH levels, molybdenum can convert to forms that are less available for plant uptake.

For the other micronutrients mentioned, such as iron, manganese, and copper, their availability tends to decrease at neutral to alkaline pH levels (around pH 7 and above). These micronutrients can precipitate or form insoluble compounds as the soil pH increases, making them less accessible to plants. This contrast underscores why molybdenum is particularly noteworthy for its increased availability as pH rises in the soil, highlighting its role in various physiological processes in plants, such as nitrogen fixation and enzyme activation.