Quote:
Originally Posted by Leafmite
Iron is a good thing for plants so if that is the combo you bought, your plants should be quite happy. I use Ironite with all my plants to give them a boost.
Which plant likes the magnesium? I know my Camellia seems to really like it. My Cinnamon tree really loves iron.... 
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A single magnesium atom is the business part of each chlorophyll molecule. Without magnesium plants cannot make chlorophyll, and turn yellow (chlorotic.) Many plants accustomed to neutral to acid soils can't extract enough magnesium and/or iron from alkaline soils, and turn chlorotic. Then you have the choice of 1) Epsom salts to the soil and foliar spraying of chelated iron; or, 2) acidifying the soil plus Epsom salts plus iron fertilizer.
Most plants from high-rainfall regions lie in this neutral-to-acid category. Minerals are washed down into the soil, and decaying organic matter near the surface provides organic acids that lower the pH at the root zone. Wetlands tend to have very low pH (very acidic) due to all the soil microflora generating lots of acidic organic matter breakdown products. This is why most bog plants, and many firs, are considered acid-loving plants.
A great example is Cestrum nocturnum in my garden. It's native to volcanic Caribbean soils with lots of rain. Here in our mineral-laden soils, with pH 8.0-8.5 tap water, it is very chlorotic unless I acidify the soil around it, or use a foliar iron spray.
Alkaline soils tend to be found only in areas with little rainfall. Here the little rain that falls tends to evaporate from the surface rather than move lower in the ground. As it evaporates it brings minerals (mostly sodium, calcium and magnesium salts) up to the surface. Desert plants tend to favor alkaline soil for best health. This is why it's hard to keep desert plants alive in typical peat-based potting mixes unless repotted every 1-2 years: the breakdown of peat changes the soil chemistry to acid, and these plants are highly susceptible to acid-loving fungi like Phytophthora.