pH - What It Means in Your Garden

If you are serious about gardening, then you have to get serious about soil. Soil is the carrier of "nutrients" needed to help plants process their only source of "food" - sunlight. Water carries these elemental nutrients from the soil into the roots, and from the roots up through the vascular system of the plants to the leaves, where they are used to process photo energy - that's light - into sugars the plant can use to grow, bloom, and reproduce. OK, that's the very simplistic view of a complex process, but basically, that's about all there is to it.

All soil is made up of 1) a parent material -rocks that are broken down by 2) climate-through freezing and cracking, for example. Three other things determine the type of soil also: the nature of the 3) organisms which live in and break down the material; the 4) and slope and drainage of the site, and the 5) length of time (geologically speaking) that these things have been working on the soil. The rock-free sandy loam of most of the Mississippi drainage (from Beaumont, Texas to Mobile Bay) has been laid down over eons from glacial deposits dumped in Wisconsin and Minnesota during the last ice age. (It's an exaggeration, but you get the point.) The sandy loam in Gulf Shores is a very different soil from the hard clay-base in Montgomery and points north. The "nutrients" in these soils vary widely, as well, and give rise to a host of problems if you insist on treating them the same.

You can think about it this way: Soil's four physical components, minerals, air, water, and organic matter determine the quality of the plants you can grow in your yard. The minerals (is it sandy or more clay?) determine how loose or compacted the soil will be, leaving room (or not) for air, water, and organic material. Sand is coarse, with a large particle, so sandy soil is looser and has room to contain more of all three, but it doesn't hold water well, and coarse organics sift right out or float away. Clay is a smaller particle, so clay soil is usually compacted, but it has a terrific elemental nutrient content. It holds water too well, and drains poorly. The best soil is a mixture of sand and clay, with lots of organics (think compost) thrown in. Ideally, soil should be about 48% mineral (sand and clay mix), 25% water, 25% air, and 2% organics.

The small particle size of clay gives it a larger surface area to react chemically and physically with other particles (and roots) in the soil. Like little magnets, clay (which has a negative charge) can attract and hold positively charged elements. Negatively charged ions are not held, and therefore leach right through the soil. Some important nutrients with a positive charge include calcium (Ca), magnesium (Mg), potassium (K), ammonium (NH4), aluminum (Al) and hydrogen (H).

The problem is that highly acid soils attract and hold highly charged particles such as aluminum and hydrogen, which makes them highly toxic to plants. The only way to tell if your soil is acidic is to take some soil samples and send them off to your local AgCenter for testing. Auburn University has a good publication about how to do this entitled "Home Soil Testing: Taking a Sample" - at their Web site (www.acesag.auburn.edu) and then search for Circular ANR6a. You must send it to YOUR state's AgCenter, and there is a link to the coastal state's at the beginning of this page.

In my next blog, I'll discuss what the Soil Test Report will really tell you about your garden.