By Christoph Kessel, Nutrition (Horticulture) – Program Lead, OMAFRA
Crops require boron, copper, iron, manganese, molybdenum or zinc in relatively small amounts. If your soil test report recommends the application of one of these micronutrients, there are several organic and inorganic sources from which to choose.
Soil organic matter and organic sources such as composts and manures are an often overlooked micronutrient storehouse. For example a one ton manure application can provide (in lbs) 0.02-0.1 boron, 0.04-06 copper, 0.4-2 iron, 0.2-1.0 manganese, 0.0002-0.01 molybdenum, and 0.2-1 zinc. Micronutrient content depends on the source but an added benefit is that they are generally already in the plant available forms. With planning and careful management, organic sources can provide a long term sustainable micronutrient source to plants.
Inorganic micronutrient fertilizer sources are oxy-sulphates, sulphates, chelates and soluble powders. The most appropriate form for application depends on the specific nutrient as well as the crop species and soil conditions. Granular micronutrient products are blended with other fertilizer ingredients for broadcast application or used in a starter fertilizer.
Oxy-sulphates
- combination of oxide & sulphate form
- oxide – much more stable in a blended product, slowly available to the crop
- sulphates – much more soluble & available than oxides
- inconsistency in plant availability & crop response
Sulphates
- quite soluble
- tend to be hygroscopic (adsorbs moisture from air) & can cause problems with caking or clumping when mixed with other fertilizer ingredients
- consistent plant availability
- generally as effective as chelates in foliar sprays but lower cost
Chelates
- complex organic molecule
- keeps nutrients in soluble forms prevent them from reacting with other minerals to form insoluble compounds
- allows many of these nutrients to be mixed with liquid fertilizers without forming insoluble precipitates
- may increase the availability in soil
- most commonly used chelating agents are EDTA & DTPA
- other organic materials (humic acids, lignosulphates, glucoheptonates) will form complexes with metallic ions but do not hold them as tightly as a true chelate
- considerably more expensive than other soluble forms of micronutrients
- use with care since they can complex minerals already in the soil & possibly make the deficiency worse
Soluble powders
- least expensive form of micronutrient for foliar application and the most consistently reliable
- most require sprayer with good agitation to keep materials in solution
- sticker-spreader needed to get the nutrient through the cuticle & into the leaf
Choosing an inorganic micronutrient source depends on the product’s effectiveness (soli or foliar), soil pH, crop, application method (soil or foliar) and of course price. To meet the crop’s micronutrient needs only a small amount of the micronutrient is required. This means that applying inorganic micronutrient sources uniformly to a field is very important.