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A bag or other container of fertilizer indicates the percentages by weight of the primary plant nutrients -- nitrogen, phosphate and potash. That analysis of the primary nutrients is on the container by law, in a format like this: 10-15-10.
Plants though, don't thrive on primary nutrients alone. They need the secondary nutrients -- calcium, magnesium and sulfur -- and the micronutrients -- boron, chlorine, copper, iron, manganese, molybdenum and zinc.
Typically a bag of mixed fertilizer for gardens doesn't contain secondary nutrients or micronutrients. So where do you find these other nutrients and how often and in what amounts do you apply them?
Micronutrient deficiency is not a common problem in home gardens. Plants don't require large amounts of these elements; what's available naturally in the soil usually suffices. Adding micronutrients is recommended only when certain sensitive crops develop deficiency symptoms. Some gardeners purchase expensive soil amendments as a precaution because they do not know if any micronutrients are needed, but compost is an excellent source and improves soil in many other ways, too. If you want to ensure that your soil has sufficient amounts of micronutrients, kelp meal and unprocessed seaweed itself are also good sources. Greensand and many organic fertilizers contain micronutrients, but, again, such products may be rather expensive and unnecessary.
Exceptions are when growing plants in soilless media such as vermiculite, perlite and sphagnum peat, or hydroponically, where all nutrients must be available in the water. Good quality commercial soilless potting mixes should contain micro and macro nutrients, but if you make up your own, some sterilized soil, compost or other materials can be added to supply them.
Local soils may be low in one or more micronutrients and your county Extension service is generally aware of such problems typical for your area. In many cases, there is an adequate level of the nutrient in the soil but high or low pH makes it unavailable to plant roots. Many Michigan soils have pH higher than 7. This can cause certain shrubs and trees, for example, to show iron or manganese deficiency symptoms. Applying those elements to the soil is not as effective as applying them to the foliage, or by injection, and/or changing the soil pH. See pH.
(If, shortly after having a basic soil test, you decide to test for additional micronutrients, the lab may still have a portion of your soil sample. You can call with the reference number on your report, ask for the additional tests, and send a check for the additional cost.)
This site was last updated 04/18/07