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Soil Amendments

You want a healthy, green lawn that is the envy of your neighbors. So how do you accomplish this?

Soil is the key factor, whether you’re dealing with seeds, sprigs, plugs, sod, or an already established lawn. You want to provide the best environment for roots and plants to grow. This is where soil amendments, also called soil conditioners, play a role. A soil amendment is any organic or inorganic material added to the soil to improve its physical properties, such as structure, drainage, water retention, aeration (extent of air gaps in soil), permeability (ability of soil to allow water, air, or plant roots to penetrate or pass through), and water infiltration. It must be thoroughly mixed into the soil to do its work.

Organic Amendments

Organic amendments consist of materials derived from plants or animals.  Examples of organic amendments include: grass clippings, compost, biosolids (treated sewage sludge), manure, sawdust, straw, sphagnum peat moss, wood ash, wood chips, hay, leaf mold, and bone meal. These organic materials should be fully composted or finished to be classified as amendments. Sphagnum peat moss doesn’t require any further decomposition or composting.

The main purpose of organic amendments is to loosen the soil and increase its porosity (create more pores or holes). They also increase the organic matter content in the soil, which over time increases:

  • soil aeration
  • soil drainage
  • water and nutrient-holding capacity
  • water infiltration

It’s important to remember that organic amendments are a temporary measure and will have to be replaced once soil organisms digest them.

Inorganic Amendments

Inorganic amendments are mined or man-made. Examples include: sand, pea gravel, lime, perlite, and vermiculite.

The costs of heavy machinery and technical know-how to produce these amendments make them more expensive and less available than the organic ones. They also don’t add nutrients where they may be needed, although some types store them. However, inorganic amendments have some advantages. They improve soil structure and have long lives because soil microbes do not break them down.

Inorganic amendments are used to:

  • increase soil drainage
  • increase soil aeration
  • decrease excessive water holding capacity
  • decrease or increase soil weight

Amendments vs Mulch vs Fertilizers

These terms can be confusing, especially because some of the same materials are used for all three categories. Basically, the differences are in how and why you use these materials, and how decomposed they are. Just keep in mind your purpose when choosing and applying them.

Mulch is the layer of organic or inorganic materials placed on top of the soil to act as a protective cover. Grass clippings, straw, sawdust, shredded bark, pine needles, wood chips, compost, gravel, black plastic, and rubber chips are examples of mulches. The purpose of mulch on soil is to help prevent weed growth, reduce erosion, hold in moisture, and moderate temperature. Organic mulches add nutrients as they break down slowly. After they have decomposed they can be mixed into the soil as amendments.

Fertilizers are concentrated nutrients added to the soil, directly affecting plant growth, while amendments indirectly affect plant growth by improving the physical condition of the soil. However, most organic fertilizers provide significant amounts of organic matter and can be classified as soil amendments (e.g. sewage sludge, bone meal, manures). Organic amendments, to varying degrees, also provide nutrients to plant roots, thus acting as fertilizers (e.g. sphagnum peat moss, straw, leaf mold, sawdust, hay).

Texas Soils

Urban area soils in Texas are clay-textured. Problems with this type of soil include:

  • hard for roots to penetrate
  • tend to be alkaline, preventing iron from reaching plants
  • compacts easily, preventing oxygen from reaching the roots
  • absorbs and traps water for long periods of time, causing pooling and drainage problems
  • hardens when dry

The permeability of clay soils is low, so choose an amendment with high permeability like composted wood chips, composted hardwood bark, or perlite. These amendments are also low on water retention to offset the high water retention of clay. Make sure the wood products you’re using are finished or mature compost when using them as amendments. They should be dark brown or black, crumbly, and have a rich, earthy smell. Uncomposted wood products are slow to break down, tie up nitrogen, and interfere with soil and water movement.

Choosing an Amendment

The first step before choosing an amendment is to test your soil. A routine test will give you the pH of the soil, which is its acidity or alkalinity, as well as the nitrogen, phosphorous, and potassium percentages.  It will also tell you what nutrients are missing and what kind of fertilizers you might need.

One place to get your soil tested is at the soil testing labs at Texas A&M. Submission forms and collection bags are available at county A&M AgriLife Extension offices. Submission forms are also available at their website  Choose the “Urban Soil Submittal Form”.  Instructions for taking the sample are on the form.

You can also order specialty tests which measure organic matter, micronutrients, boron, detailed salinity, and percentages of sand, silt and clay. The best time for taking a soil sample is winter or very early spring. You should get your results in 2–3 weeks. If you wait until spring, it may take longer to get them.

The results of your soil test will also help you determine the type and quality of amendments for your lawn. You will also need to consider the length of time you want your amendments to last. If you want to improve your soil quickly you will have to choose amendments that break down rapidly (e.g. grass clippings, manures, composts). On the other hand, if you want long-lasting improvements you need to choose an amendment that breaks down slowly (e.g. composted wood chips, sphagnum peat moss, hardwood bark). Then there’s the third option: do you want a quick improvement that’s long lasting? If so, choose a combination of amendments that break down quickly and slowly.

Amending Soil By Top-Dressing

Tilling or turning over the soil to add an amendment can’t be done when your lawn is already established. Instead, a procedure called top-dressing is used, which is the process of spreading compost or another amendment over the surface of your lawn to improve the quality of the soil and fix lawn problems.

The best time to top-dress is early spring or fall. Be sure to choose a top-dressing that matches the soil in your lawn (use those test results). Here are the basic steps to top-dress your lawn:

  • de-thatch your lawn if more than 1/2 inch of thatch (built-up plant material between green top growth and roots)
  • mow your lawn to the lowest recommended height for its type
  • aerate your lawn (removing plugs of soil from the ground)
  • work a small area at a time, shovel a small pile of top-dressing onto the grass and spread it evenly to a depth of 1/4 to 1/2 inch, filling in the aeration holes
  • use a rake to gently rake the lawn so that the grass shows through and the depth is even, with the top-dressing touching the soil
  • water the area to settle and keep the top-dressing in place
  • go back and rake the area again after a day or two to ensure evenness
  • now is a good time to overseed if you wish.

With the correct use of soil amendments your neighbors will be asking you how you created such a beautiful lawn.

Your sprinkler system is vital to keeping your lawn and garden looking it’s best.  Call Smart Earth Sprinklers at (512) 694-1147 for all your maintenance and repair needs.

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