Soil Amendment

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Soil Amendment

When living plants are allowed to decompose over time, the result is an incredibly rich and fertile soil amendment that we call compost. Fully composted organic material is the most beneficial element you can add to your soil.

Imagine a forest floor with an undisturbed layer of leaves and bark slowly rotting over time. Fungi are working to decompose the organic material, changing it into a nutrient form that is available for absorption by the plant roots. You can create your own compost using grass clippings, leaves, vegetative kitchen scraps, and plant debris from your garden. Create an enclosure that will fit into your landscape for your compost bin. Add water frequently, turn it regularly, give it time, and you’ll have fresh compost for your landscape.

There are two composting methods you can follow: the hot method, which is accelerated, and the cold method. The hot method will result in fully composted organic matter in as little as four weeks. The cold method may take as long as two years. Utilizing a compost pile doesn’t just give you a source of free soil amendment; it also keeps the waste clippings from your landscape from entering the urban landfill. Composting is a great way to conserve our natural resources.

You may choose to purchase compost from your local nursery or garden center. It is critical that the compost process be complete. Fully composted organic material will be dark and fresh smelling, consisting of very small and soft particles. Amending your soil with partially decomposted organic material will actually draw nitrogen from the soil, making it unavailable for your plants. If your compost has large chunks with hard or sharp edges, the process may be incomplete.

You can use manure to improve your soil, but caution is recommended. Manure is commercially available and packaged for convenient handling. These products often have plant nutrients added to bring it to a standard fertility level. Commercial manures are an excellent soil amendment. Manure that is processed by individuals has varying degrees of decomposition and nutrient value. There is the potential for disease organisms as well as weed seeds in untreated and unprocessed manure. Some types of manure need to be aged appropriately to avoid burning the plants. Other manures such as rabbit pellets and earthworm casting are ready to use with no aging needed.

Various types of sand are commonly recommended as a soil amendment; however, sand has little water-holding capacity and no nutrient value. It will not improve the composition of clay, and will not improve the nutrient value of sandy loam.

Mulching is a long-established horticultural practice that involves spreading a layer of material on the soil around the plants. Mulch can be classified as inorganic or organic.

  • Inorganic mulch includes plastic, rocks, rock chips, and other non-plant materials.
  • Organic mulches include straw, compost, shredded bark, newspaper, or similar materials.

While both have their place in the landscape, organic mulches have the benefit of decomposing slowly, which improves the condition of your soil by adding decomposed organic matter consistently over time. This improves root growth, adds nutrients, increases water absorption, and provides an ideal environment for earthworms and other beneficial soil organisms.

The type of mulch you use depends to some degree on personal preference. Pine bark mulch, shredded hardwood, cedar, pecan shells, and pine straw are among our favorites. Apply mulch to your trees, landscape beds, shrubs, and vegetable garden. In other words, everything except your lawn should be mulched. The one exception to this general rule is a meadow or pocket prairie filled with wildflowers and/or native grasses. Apply 3 to 4 inches of mulch over your entire landscape area, leaving a little space next to the trunk or stem of the plants so moisture will not stay in contact. If you want to add an extra barrier of weed control, the addition of newspaper (4 to 8 sheets thick) under the 3 inches of mulch will stop most weeds and grasses.

Next to proper soil preparation, mulching is the second most important thing you can do to improve your landscape. However, it is important to avoid working mulch into the soil. Non-decomposed material will leach nitrogen from the soil. Be sure to rake mulch aside before working the soil. Mulching will achieve the following results:

  • Conserve water by reducing evaporation and runoff,
  • Increase the soil’s ability to absorb and retain moisture,
  • Help control weed growth and prevent seed germination,
  • Regulate soil temperature – cooler in summer, warmer in winter, and
  • Add organic matter over time.

If you use commercial fertilizers, you need to know what you are buying. All fertilizers are required by law to list the amount of each of the three main elements as a percentage of the total: (N) for Nitrogen, (P) for Phosphorus, and (K) for Potassium. For example, a 40 pound bag of 15-5-10 fertilizer will contain six pounds of nitrogen (40 x .15 = 6), two pounds of phosphorus (40 x .05 = 2) and four pounds of potassium (40 x .10 = 4). The other 28 pounds of material (40 – 6 – 2 – 4 = 28) is filler to help distribute the fertilizer evenly. Commercial fertilizers provide instructions on how much fertilizer to apply. These instructions need to be used in conjunction with the results from your soil test.

Because plants need nutrients continuously, it is best to use “slow-release” fertilizer. Most organic fertilizers and many commercial fertilizers are slow release. Check the ingredients if you are uncertain.

As a rule, most Parker County soils are deficient in nitrogen. Most soils in the urban landscape have too much phosphorus from too many years of over application of fertilizers. Too much phosphorous can cause plants to appear to have an iron deficiency (e.g., yellowing of leaves). Because of the general needs of most plants, and the prevalence of too much phosphorus in most soils in Parker County, fertilizers with ratios of 4-1-3 (e.g., 20-5-7) or fertilizers containing only nitrogen (e.g., cottonseed meal or a 21-0-0 commercial fertilizer) should be used if a soil test has not been performed.

Many nurseries and other stores sell lawn fertilizers combined with herbicides. Even though it may be convenient, this practice is not recommended. In our geographic area, the timing for application of fertilizer and herbicide rarely coincide. An herbicide is intended to kill unwanted plants, mainly weeds. It is important to understand the life cycle of the weed in order to establish an appropriate control method. One control method does not work for all plants, and certain times of the year are not ideal for intervention. The same is true for fertilizer. All plants do not require the same supplemental nutrients. Both fertilizers and pesticides should be applied accurately and sparingly, only when necessary.

Note: Cottonseed meal has little soil conditioning ability, but it is a good organic fertilizer. It contains from 6-10% nitrogen, and is useful as a side-dressing for many ornamental plants and vegetables.
by Carol Welch