Summer gardens have just about run out. The ambitious among us are well into plantings for their autumn-vegetable gardens.
The time between plantings, especially if there is going to be a rest period before the next planting, is a great time to stir in some organic matter. What passes for soil along coastal Georgia is little more than beach sand. Folk who move to coastal Georgia from places that have real soil have trouble gardening here until they realize that they are essentially growing hydroponic. If they don’t supply all the minerals and nutrients ? and I mean all - the plants do not perform well.
Also, what you add to the ground does not hang around long. Sand does not have many “sticky” places on it for fertilizer metals to cling. Silt has more sticky surfaces; clays have even more.
The measure for the ability of a soil to hold and retain fertilizer elements is called its cation exchange capacity, or CEC. All our fertilizer elements are positively charged ions, called cations. Ions with a negative charge are called anions. Nitrogen exists in soils either as a positive ion, ammonium (NH4), or as a negative ion, nitrate (NO3). Ammonium is positively charged and tends to stick to soil particles. Nitrate, the form of nitrogen most easily taken up by plants, is negatively charged and tends to stay in the soil solution, where it easily is leached out by rain. Negative charges repel other negative charges.
Yes, the soil has a negative electrical charge. You knew that, you just don’t think about it very often. The negative terminal in an electrical circuit is called the ground. Your house has a grounding rod driven into the earth to complete the circuit for your home. So the negatively charged soil particles tend to attract and hold positively charges ions. The more “sticky” sites the soil has, the more fertilizer metals the soil can hold, the better the plants grow.
CEC is one important measure of soil productivity. Sandy soils typically have CECs of 0-5. Silt has a CEC in the range of 10-20. Clays have CECs in the range of 20-45. If you wanted to improve the CEC of your sandy garden soil, you could add clay to it. Unfortunately for the gardener, one part sand plus one part clay is the recipe for brick. No, clay is not the way to go.
What about organic matter, which has lots of sticky sites? The CEC range for organic matter (soil humus) is 150-250. If you want to improve soil structure, add organic matter. If you want to improve available water-holding capacity, add organic matter. If you want to improve soil porosity, add organic matter. If you want to improve soil aerofication, add organic matter. If you want to improve soil, add organic matter.
But the organic matter you add will not give the immediate benefits of soil humus. Finished compost has a CEC of 37-70, which is better than clay but not as great as soil humus. Soil humus takes hundreds to thousands of years to form and is the reason our sandy soils have any CEC at all. The compost you till in will have all the immediate benefits of adding organic matter, but practically all of it will be consumed by soil microflora and microfauna and evolve off as carbon dioxide in a very few years.
So you need to continue to till in organic matter in your Coastal Georgia garden. You will get immediate benefits and leave a small but positive legacy to the land that will last thousands of years. Stirring in organic matter is most definitely a good thing.
“OK, Don, you sold me on the organic matter. Where do I get this stuff?” First, don’t buy it. You can recycle fallen leaves, limb clippings and other green waste from your yard. If your neighbors have leaves they bag up for the trash truck to haul off, save the landfill space and compost their leaves. Somebody in your neighborhood is throwing away perfectly good leaves that could be growing you better vegetables and healthier flowers next year.
If you really want to get into composting and building up soil as a neighborhood, commercial landscape-care companies have leaves they bag up from common areas and properties they service. Instead of having to pay tipping fees at the landfill, this “waste” organic matter can be composted into highly productive soil amendment for your garden.
Some landfills direct tree-care trucks to drop wood chips in a particular spot so homeowners can pick up the chips for composting or to use as mulch. In either case, putting the organic matter back into the soil is a good move. So the raw materials are free. Composting is a fairly easy process once a few rules are followed.
Next time: Tilling the organic matter in versus leaving it on top as a mulch.
Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.