Do organic fertilizers contain sufficient micronutrients?

Micronutrients for plants obtained from organic fertilizers are often insufficient. Although compost and manure are reliable sources of organic matter, they generally contain low amounts of zinc and boron, less than 2 ppm, much lower than crops require. I have worked on organic apple orchards where the compost had only 0.3 ppm of copper, so it would have been necessary to apply organic sources of copper.

Soil testing continues to be essential within organic farming systems. A vineyard in California that used only compost produced soil zinc levels of 0.8 ppm, which is less than half the allowable threshold for grapes of 1.5 ppm. We applied 20 lb/acre of zinc sulfate (OMRI-approved) in that vineyard just before bud break, and we improved grape yields by 22% while staying organically certified.

In organic soils with a pH below 5.5, molybdenum can be rendered unavailable. To mitigate this in my farming system, I routinely mix compost piles before application with 5 lb/acre of sodium molybdate. The composting process integrates molybdenum with organic matter, forming packets of slow-release molybdenum availability. Legumes take up molybdenum in a deliberate manner, allowing it to stay in compliance with organic regulations.

Foliar applications serve to fill gaps during mid-season. For instance, a 0.2% solution of chelated iron (from molasses) was applied to organic spinach at the four-leaf stage and the chlorosis was reversed in five days. As a reminder to be an informed consumer, the OMRI certification for a chelate should be checked because some chelates, like EDTA, are derived from natural sources, but violate the organic standard.

Read the full article: 7 Essential Micronutrients for Plants: Complete Guide