Soil Balancing, part 6 (trace minerals)

December 4, 2023

All of the major trace minerals are important for plant health and frequently improve the uptake of a macronutrient. Although these trace minerals are needed only in very small amounts, they are just as important as the macronutrients.

Boron:

Boron improves the uptake and translocation of calcium, magnesium, and phosphorus. Boron is an anion, meaning it is negatively charged, which allows it to leach easily, and thus needs to be applied annually in most soils. An ideal level of boron is considered to be 3 ppm – although in reality, boron should be balanced in ratio to calcium ppm or pounds per acre – otherwise, it can be toxic to crops, especially corn, strawberries, raspberries, etc. The rule of thumb I use if boron is lower than 1 ppm is to use 2.5 pounds of actual boron at 1000 ppm, and 5 pounds actual boron at 2000 ppm.

Iron:

Iron levels should be at about 100 ppm. Iron is rarely low because of its abundance on the earth’s crust. The question is, “Is it available?”. Iron availability decreases as soil pH climbs to 7 or higher. The same goes for anything else that increases oxidation values in the soil, such as excessive tillage. We seldom add iron to the soil for this reason. I would much rather lower the pH, reduce tillage, get reducing cover crops into the rotation, and keep the soil covered so the soil stays cool (and thus the iron stays in reduced form), than apply more iron – because excessive iron will tie up phosphorus.

Manganese:

Manganese is similar to iron, in that its availability drops in oxidizing environments and excessive tillage, etc. Levels should be half of iron, with a minimum of 50 ppm. Only if it is very low do we add manganese sulfate.

Copper:

Copper is critical for disease resistance and protein production. Ideal copper numbers are about 4-5 ppm in soil. Also the numbers should be half of zinc. Copper can also oxidize similar to iron and manganese, but to a much lesser degree. Thus, we frequently apply 5-20 pounds copper sulfate to help bring copper levels into balance in the soil.

Zinc:

Zinc is critical for leaf formation and root quality. Levels should be 8-10 ppm in soil and should be double that of copper. Zinc generally follows phosphorus levels – the higher the phosphorus numbers, the higher zinc will climb as well. So although we certainly apply zinc if needed, it is almost more critical to simply get the phosphorus numbers up to the desired levels. It is also important to not let phosphorus numbers get out of hand because it will increase the zinc levels and create an imbalanced copper to zinc ratio, which means poor crop production.

Aluminum:

This is a tricky one and one that there is so much more to learn about. Aluminum can be toxic. It is not uncommon to see levels as high as 800 to 1000 ppm in local soils. At this point, I am not too concerned if I see 800 ppm, as long as calcium and silica levels are in line.