Choosing a Soil Amendment

Choosing a Soil Amendment

Soil quality is one of the most important factors in growing and maintaining a healthy lawn. The Austin area has several different soil types, but all of them are clay-textured, low in organic matter, and somewhat alkaline. These characteristics cause the following problems in lawns:

  • Clay absorbs and traps water for prolonged periods, causing pooling and drainage problems.
  • Clay hardens and cracks when dry.
  • Clay easily compacts, preventing oxygen and water from reaching roots.
  • Roots find clay hard to penetrate, making it difficult for them to become established.
  • Low organic matter makes grass more susceptible to disease, insects, and erosion.
  • Alkaline soil prevents plants from absorbing essential nutrients such as iron and manganese.

Soil Amendments

Fortunately, poor soil can be improved using soil amendments. A soil amendment, or soil conditioner, is any organic or inorganic substance added to soil that improves its physical properties for supporting plant growth. Usually, an amendment is used to repair such factors as: 

  1. Soil structure. Soil particles tend to stick together in definite lumps or shapes known as aggregates. Their size and the space separating them determines the ability of the soil to move water and air through its layers.
  2. Water retention. Soil should be able to temporarily store water for root uptake, plant growth, and soil organisms, while at the same time having the ability to drain at a moderate rate without pooling. 
  3. Nutrient retention. Soil needs to attract, retain, and supply nutrients to plants.
  4. Organic matter. Five to six percent is the ideal amount of organic matter for lawns.

Types of Soil Amendments

Organic

Organic amendments include organic matter from animals, plants, or plant products that occur naturally, such as: straw, sawdust, wood ash, grass clippings, wood chips, hay, leaf mold, bone meal, compost, sphagnum peat moss, manure, and biosolids (treated sewage sludge). In order to be classified as an amendment, organic matter has to be fully composted. Sphagnum peat moss is an exception and doesn’t require further composting.

A popular compost in the Austin area is Dillo Dirt.  The City of Austin has been making it since 1989, using brush, some yard trimmings collected curbside, and some biosolids. These items are then combined and composted at Hornsby Bend to produce the amendment.

Benefits of Organic Amendments

Adding organic matter to clay soil loosens it and creates new holes or spaces, which allows air and water to move more freely. Roots can then reach deeper into the soil, and water retention and drainage improve. Organic matter also provides plant nutrients, along with boosting the soil’s ability to retain them as nourishment for the roots.

Fungi, bacteria, and earthworms living in the soil benefit as well from organic matter. It increases their bioactivity, further releasing nutrients into the soil. Over time organic amendments get used up by plants and microorganisms and will have to be replenished. 

Inorganic

Inorganic amendments include vermiculite, perlite, tire chunks, pea gravel, sand, lime, and gypsum. These are either mined or man-made, and the machinery and industrial processing required in their production makes them more expensive and less available.

Benefits of Inorganic Amendments

Inorganic amendments are used to increase soil aeration and drainage, decrease the capacity for excessive water retention, and for decreasing or increasing soil weight. Because soil microbes can’t break inorganic amendments down, they don’t need to be applied as often as organic matter.

Are Soil Amendments Fertilizers?

Organic and inorganic amendments affect plant growth indirectly by improving the condition of the soil, which in turn boosts the performance of fertilizers. Fertilizers are organic or inorganic nutrients that directly affect plant growth. Most soil amendments, however, supply some nutrients to plant roots (e.g. sphagnum peat moss, hay, leaf mold, sawdust, and straw), and can act as fertilizers. As well, organic fertilizers contain considerable amounts of organic matter and therefore many double as amendments (e.g. manures, bone meal, biosolids, and some compost materials).

Soil Testing

Always test your soil first before applying amendments. Unnecessary amendments can cause toxic conditions or nutrient imbalance. The Texas A&M AgriLife Extension Service Soil, Water and Forage Testing Laboratory offers this testing service. Download the urban soil submittal form for homeowners, which also gives instructions on how to take a soil sample. Test results will provide your soil’s pH (measure of the acidity or alkalinity) along with nutrient levels and recommendations on the types and amounts of soil amendments to use.

Choosing an Amendment

The best overall amendment is organic matter. All soil types benefit from it, especially clay. The best method for adding organic matter to a lawn is top dressing, where a thin layer of sand or compost rich soil is spread over the lawn. In the Austin area, a top dressing of compost is recommended, such as in a blend of topsoil and compost. Spring and fall are the best times to top dress.

Basic Top-Dressing Steps

  1. Dethatch first if your lawn has more than 1/2 inch of thatch. For lawns with heavy clay, core aerate. Remove the cores and thatch debris.
  2. Mow your lawn to its recommended height.
  3. Shovel out a small mound of the top-dressing mixture from the wheelbarrow onto the grass, working a few square feet at a time.
  4. Spread the top dressing evenly using something flat, like the back side of a rake. Work it into the aeration holes and be sure to cover the low spots. Keep the layer 1/4 to 1/2 inch. If the grass is 3 or more inches high, layer up to 1 inch thick.
  5. Continue working the top dressing until it’s settled evenly on the soil surface and the grass shows through. If using a rake, flip it back and forth from the flat to the tine side.
  6. Give the area a good watering, or top dress before a rainfall. Let the top dressing sit for one or two days, then rake the lawn to smooth out any bumps or hollows.
  7. Replant any grass in bare spots if needed.

Sound irrigation practices are indispensable to a lush and healthy lawn.  Call the licensed irrigators at Smart Earth Sprinklers at (512) 694-1147 or contact us online to keep your sprinkler system operating at its best.

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