Skip Richter
National Gardening Association Regional Editor

Here's the dirt on Austin soils

Some gardeners seem able to make anything grow. Their secret is not a green thumb. It's the right soil.

The Austin area is blessed with many wonderful things, but deep and rich native soil is not one of them. The black clays of Pflugerville and many hillsides west of Interstate 35 are dense soils that absorb water slowly and pack tightly, making it tough for plant roots to get a foothold.

To the west, limestone outcroppings are covered with thin soil. Thin soil makes plants susceptible to drought because roots quickly deplete the limited moisture. Along the river bottoms and the broad plains to the southeast of town, the soils are often deep loams. They absorb and drain water well, providing a good foundation for growing plants. But these soils, like all soils in our area, are usually low in organic matter.

So, if you want to grow things in Central Texas, you have to improve the soil. The two things most often needed are compost to improve the quality of soil and deeper soil to support a strong, extensive and resilient root system. Lawn grasses require at least 6 inches of soil, but more is better. Turf on shallower soils will be weak and require constant watering. Flowers and vegetables also need 6 to 12 inches of soil. Building up raised planting beds can turn shallow, rocky soil into a beautiful garden spot.

Central Texas soils are usually high in pH levels, making them well-suited to our Western native plants and ill-adapted to acid-loving plants of the Southeast, such as azaleas and blueberries. Our soils typically have high levels of phosphorus and potassium but may need some additions of nutrients for optimum plant growth. The best way to tell what your soil needs is to have it tested. The Travis County Extension Office has free soil-testing packets for area residents. These provide instructions on how to take a soil test and how to have it analyzed. These are also available on the Grow Green information racks at more than 40 area garden centers.

Before fertilizing your lawn or garden, start with a soil test. You will likely discover that the fertilizer blend you have been using is not the best for your soil. Misapplication of fertilizer wastes money, contributes to pollution in streams and aquifers and gives less-than-desired results.

The right soil
There are two basic ways to improve your soil: Buy or make compost to add to the soil you have, or buy a soil mix to build raised beds for flowers, shrubs and vegetables.

If your soil is deep enough and simply needs some improvement, spread several inches of compost over an area and mix it in as deeply as you can. This will enhance the soil in the root zone of the plants and improve your gardening results dramatically.

If you have a spot that is poorly drained or has only a thin layer of soil over underlying rock, it may be best to bring in a special soil mix to build up your depth. These mixes come in many forms and typically have a blend of loam or sand along with composted bark, manure or other decomposed organic matter. They have names like bed mix, planting mix or landscaper's mix.

Rather than lay this material directly on the surface, it is best to spread out a few inches and then till or spade it into the existing soil. Then spread more over the area to build up the soil to the desired depth. This blending technique will prevent the dramatic interface between two types of soil that can impede root penetration and could create a "perched water table" because water will not readily move from the loose, compost-enhanced mix into an underlying clay layer.

Whichever method you choose will require considerable digging and mixing, so it's best to improve the soil before you plant.

If you've already planted, you should work the soil or compost as deeply into the root zone as you can. For lawns, aerate turf by removing plugs with an aerator machine, or for small turf areas, punch vertical holes with a spading fork. Then work compost or soil into the entire area and water well. In nonturf areas, mix or till the soil or compost as deeply as you can without disturbing roots.

You may have heard the old adage that you should not put a $10 plant in a 10-cent hole. To that wisdom, I would add that you should not spend a dollar on a plant without first spending a dollar on your soil. Soil building saves you money in the long run because it reduces the need to replace plants, lowers water use and reduces fertilizer applications.

Compost breeds life
We tend to think of our soil based only on its texture and mineral content. Yet a fertile garden soil is teeming with life. Trillions of tiny organisms can be found in a spoonful of rich garden soil. Bacteria, actinomycetes, fungi, protozoa and a host of other organisms live in healthy soil.

As we add composted leaves, grass clippings, manure and other organic matter to the soil, these organisms go to work, breaking down the materials into humus and releasing their nutrients to the plants. Heavy clays become looser as their structure improves. That clod of dry clay soil that is as hard as a chunk of concrete will become a clod that easily breaks apart in your hand. Organic matter interacts with a clay soil to turn the solid mass into loose clumps of particles. Aeration is improved, internal drainage is enhanced and roots are able to more thoroughly fill the soil and mine its nutrients.

Sandy soils cannot hold water and nutrients well. Everything just runs right through a sandy soil as if it were crushed glass. Compost can improve this problem, too. Think of organic matter in a sandy soil as tiny sponges mixed in with the finely crushed glass. The soil can now hold water and nutrients for plants to use days and weeks later.

Plants were designed to live with their roots surrounded by the decaying materials they produced in previous seasons. Like the forest floor or the soil surface in a meadow, these materials provide the fuel for soil life that in turn feeds plants.

Soil built with regular additions of compost gets more fertile each year. It feeds microbes, earthworms and other organisms that produce glues that hold soil particles together in loose clumps, acids that wash over the mineral components releasing more nutrient elements, and growth-promoting substances that supercharge plant roots. Some microbes even take nitrogen from the air and turn it into fertilizer.

After a few seasons of adding compost, I usually find the need to fertilize is limited to occasional slight corrections and perhaps the use of a starter solution for new transplants. In a soil high in organic content, nutrient imbalances are seldom noticed. All the macro- and micronutrients from the leaves, grass clippings and other materials are now available to the growing plants.

I've exhorted countless gardeners to add organic matter to cure a variety of soil ills. Yet despite seeing it work miracles over and over, I still marvel when I see it again. If you feed your soil with compost, your soil will feed your plants. And feeding your plants is truly the secret to a good garden.

Don Legacy contributed to this article. Richter is the Travis County extension agent for horticulture. Legacy, who owns a dirt yard, is chairman of the Central Texas Horticulture Council.

What is mulch?
Unlike soil amendments, which are worked down into the soil to improve the root zone of our plants, mulch is a covering on the surface of the soil. The best mulches are made from natural ingredients like bark chips, shredded tree trimmings, leaves, grass clippings, compost, hay or pine needles. Mulch protects the surface from getting a hard crust following a rain or sprinkler irrigation. It moderates soil temperatures and helps deter weeds. In time, the mulch will decompose, providing compost for the growing plant roots.

How much?

To determine how much mulch or soil is needed for a specific area, measure the area to be covered and use this formula:

Area to cover (in square feet) x depth of mulch or soil desired (in inches) x 0.0031 = cubic yards of mulch or soil required.

For example, to cover an area of 100 square feet with 3 inches of mulch: 100 square feet x 3 inches deep x 0.0031 = 0.93 cubic yards needed.

Note: If buying mulch in 1-cubic-foot bags, multiply the cubic yards needed by 27 (there are 27 cubic feet in 1 cubic yard) to find the number of bags needed. Or, use this alternate formula: Area to cover (in square feet) x depth of mulch or soil (in inches) / 324 = cubic yards of mulch or soil required.

If you are not mathematically inclined, visit this web site for a calculator. Just enter the size of the area you want to cover and the depth of mulch or soil you want to apply. It will calculate how many cubic feet or cubic yards you need.

Approximate amounts
Approximate amount of soil (or mulch) needed to cover an area

Depth required in inches
Area in square feet 1 inch 3 inches 6 inches 8 inches 12 inches
50 4 cubic feet 1/2 yard 1 yard 1 1/4 yards 2 yards
100 8 cubic feet 1 yard 2 yards 2 1/2 yards 4 yards
500 1 1/2 yards 4 1/2 yards 9 1/4 yards 12 1/3 yards 18 1/2 yards
1000 3 yards 9 1/4 yards 18 1/2 yards 24 3/4 yards 37 yards