By Wayne McLaurin
Georgia Extension Service
Productive soil is the single most important factor in successful gardening. Since few of us are blessed with perfect soil, we need to know how to improve what we have.
Improving the tilth
Adding organic matter to the soil improves the soil texture (tilth). This improves the environment for good root growth and the development of soil microorganisms that make nutrients more readily available.
Organic matter supplies some nutrients, too, but most forms are lower in nutrients than commercial fertilizers. To the plant, it makes no difference whether the nutrients come from organic or inorganic sources, since they can use the nutrients only in the basic inorganic form.
The difference is primarily in the availability. For instance, nitrogen from organic sources is released more slowly than nitrogen from most commercial fertilizers.
Slow release of nutrients would be good in a soil already adequate in nutritional levels. Where soils are short of one or more nutrients, though, it's usually best to add commercial fertilizers for the more quickly available nutrients to correct the deficiency.
It's best to add both organic matter and inorganic fertilizers. However, all fertilizers added should be based on a soil test.
First, check your soil
Before adding fertilizers, first find out whether a problem in growing healthy plants is due to nutrition or a physical property of the soil, such as poor texture.
A plant in a poorly aerated soil may do poorly because the root system can't use the nutrients, even though an ample supply may be there.
Soil with a steep slope may have good surface runoff but poor subsurface drainage if the texture is high in clay or if underlying soils block water movement.
Water is always held more tightly in fine soils than in coarse, sandy soils. A fine soil underlaid with compressed organic matter, sand and even gravel won't drain well.
Uniformity, balance, drainage important
The best soils for growing plants are uniform in texture throughout the root zone, with a good balance of minerals, air and organic matter.
One of the easiest ways around drainage problems is to make a raised bed. Raising the bed as much as 8- to 12 inches will allow proper drainage from around the root system.
Improving soil a continual process. It often takes 10 or more years to make a productive garden soil.
If your soil is too sandy or too high in clay, the solution to both extremes is essentially the same -- add organic matter.
In a sandy soil, organic matter acts much like a sponge to hold moisture and nutrients. In clay, organic matter helps aggregate the finer particles, allowing for larger pore spaces, improving aeration and drainage.
It's possible, especially in clay soils, to create a soluble salt problem by adding too much organic matter all at once.
Adding organic matter
The rule of thumb is to incorporate no more than 3 cubic yards of organic matter per 1,000 square feet per year. This is about 1.25 inches on the soil surface before it's tilled in. All amendments should be thoroughly tilled into the soil, making it a uniform mixture.
The best organic amendments include relatively coarse, partially decomposed compost and aged barnyard manure. The type of manure isn't important. But it should be at least one year old if you plan to plant soon after amendment.
Never use fresh manure. Always compost it for at least 120 days before using it. Because of high salts, avoid repeated use of manures unless the salts can be leached first. Dairy cattle manure generally is lower in salt content.
Coarse sphagnum peat is a good amendment but is expensive when compared with manure or compost.
Many gardeners want to add sand to the clay to break it apart. Don't. Adding sand to clay makes good bricks. If you are going to add sand, you must also add organic matter along with it -- about 25 percent of each by volume.
Remember, to build a good soil:
- Check for nutrients (soil test) and texture (ball test).
- Add organic matter (composted manure, compost, etc.).
- Dig and aerate only when the soil is workable (not too wet and not too dry).
- Add nutrients according to the soil test with either organic sources (slow release) or commercial fertilizers (quicker release).
Roll some slightly moistened soil between your thumb and forefinger. If it forms a firm ball, feels smooth and becomes sticky when moistened, it's too high in clay. It's a better texture if you can't form a ball, the soil won't stay together and it feels somewhat grainy. If, on the other hand, the soil feels very coarse, it may be too sandy and won't hold enough water.
Subsoil Drainage Test
Dig a hole in the garden about 12 inches deep and the diameter of a spade. Pour water in the hole to the rim. Refill the hole a day later and see how long it takes all the water to soak in.
If it soaks in within a few minutes, the subsoil drainage may be too good. It may not hold enough water to sustain plant life and can lose valuable nutrients through leaching.
If the water takes more than one hour to soak in, the subsoil drainage may be poor. Plants may suffer from oxygen starvation (drowning) in this soil.
Liquid products break the surface tension of water around the soil particle and allow deeper water penetration. They in no way increase the pore space of a soil.
The liquid "conditioners," then, can't be considered soil amendments. They're properly called "adjuvants." At best, they may temporarily enable water to penetrate better. But they don't break up clay soils as some claim. They're not substitutes for amendments.
What about gypsum?
Gypsum is a salt (calcium sulfate) and does no more than increase the calcium and sulfur content. It doesn't change the soil pH and isn't a substitute for dolomitic lime.
To adjust the pH, always use dolomitic lime, since it also adds magnesium, an essential element. Many claims are made that gypsum breaks up clay soils.
However, adding organic matter will break up clay soils far better than any other product while increasing the ever-important microorganisms. In other words, adding gypsum to a soil that doesn't need it is a waste of money.