Garden Chemistry

Considering all the time I’ve spent gardening, it’s amazing how little I’ve thought about all the chemistry that surrounds me in the garden. Of course I realize there is a lot of organic and bioinorganic chemistry going on in the plants, but there is actually a fair amount of inorganic chemistry occurring in the soil itself. Almost 99% of a typical fertilizer consists of nitrogen, phosphorus, and potassium, but in order to survive, however, plants also need small amounts of magnesium, iron, calcium, boron, sulfur, copper, zinc, and manganese. Although these elements are usually present in the soil, their bioavailability isn’t always guaranteed. Most plants perform best in a pH range of 6-7.5 and much of this has to do with the availability of these trace elements as shown on the following chart obtained from this website.

Much of this availability has to do with the form of the element. To be useful to the plant, the elements generally have to be in an ionic form, which is why some fertilizers chelate their metals. Nice healthy soils with lots of organic matter or clay essentially behave like ion exchange resins, holding on to and stabilizing the ionic species for eventual use by the plant. If the pH is too high, insoluble metal oxides (Cu, Fe, Mn) can form. If it’s too low, too much Al³⁺ is released from rocks, which displaces all the other elements on the ion exchange sites.

Redox properties can also have an effect. Well aerated and well drained soils tend to be oxidizing while wet, decomposing, clay soils tend to be reducing. In the presence of sulfur, anaerobic conditions lead to formation of insoluble metal sulfides. Anaerobic conditions also increase Fe availability by changing its oxidation state from Fe³⁺ to Fe²⁺ (which I assume makes it easier for its release from the ion absorption sites. There is a lot of complicated chemistry going on here.

I admit that I was totally unaware of any of this when I started gardening. In fact, the only reason I even know this much is due to having to understanding how to change soil pH. Lime is used to raise soil pH while iron salts and elemental sulfur are used to lower soil pH. The lime and iron reactions I understood from freshman chemistry, but sulfur? Gardeners would simply tell me that sulfur is acidic, but that obviously wasn't true. Eventually I guessed (correctly) that the sulfur oxidizes to SO₂ which dissolves in water to make H₂SO₃.

What do I do with all this knowledge? Well, nothing really. I just plant the flowers, add Miracle-Gro, and hope everything grows.