Micronutrients Chapter 15

B and Mo exist as oxyanions.

High yields = fast depletion. High analysis = little input. By high analysis what is meant is few impurities, including micronutrients. Older forms (low analysis) contained who knows what but some micros. Two soil conditions affecting micronutrient availability are pH and Eh (redox potential).

You ought to be concerned about what you add to your soil –you might be adding something you don’t want there. There are regulations governing the composition of land-farmed waste, including contents of the waste and build-up of metals in the soil. But nothing is perfect. pH and Eh can cause high concentrations as well as low. Let’s look at these factors.

Availability of Cations pH Fe OH -  Fe(OH) 3 ↓ Will over-liming induce deficiencies? Again, good to appreciate chemical equilibrium. Here, increasing [OH - ], i.e., decreasing [H + ] and increasing pH leads to precipitation of the metal micronutrients, as shown for Fe, decreasing their solution concentration and availability for plant uptake. The answer, obviously, is yes.

Curious thing but true that the chemically reduced form (e.g., Fe 2+ ) is more soluble at any pH than the chemically oxidized form. Thus, couple wet (anoxic, anaerobic) conditions conducive to microbially-mediated reduction with low pH and you might get toxically high concentrations. On the other hand, there is a tendency for wet soils to approach neutral pH. Whether conditions are right for toxicity depends on initial pH and concentration of the metal.

Availability of Anions Cl - generally plentiful B deficiency common H 3 BO 3 + H 2 O  H + + B(OH) 4 - Borate anion bound to soil colloids Availability greater at (low / high) pH? As per the equilibrium, as pH increases, there is more B(OH) 4 -, so availability decreases.

Exception to generality that availability greater low pH Mo Liming increases availability? Yes. Liming increases pH and decreases the adsorption of the Mo oxyanion. So, we’ve a contradictory situation with availability and pH. This, too, is part of the reason why a compromise in soil pH is best for most plants, i.e., 5.5 or 6.0 to 7.0.

Cu 2+ +EDTA 4-  CuEDTA 2- The below equilibrium is shifted strongly to right. When the total concentration of Cu in solution produced by adding X amount of Cu as a salt to the soil is compare to the total concentration of Cu in solution when X is added as the EDTA complex, there is more in solution with the complex. Free Cu 2+ is subject to adsorption and precipita- tion whereas the complex isn’t. The downside is that EDTA (example chelate) can form complexes with other metals, like Ca 2+ which is vastly more abundant in the soil than Cu 2+. Fortunately, the Cu complex is strongly favored, off-setting this problem.

Very stable complexes exist with these metals so that competition with Ca, etc. is not a serious limitation to the use of chelated complexes to supply the micronutrients.

Yes, so long as you don’t burn the foliage this works because the dose is small. Been around for a while. Same principle as with P.