Healthy soil is the key to a beautiful lawn. Some chronic lawn issues — browning, thinning, and wilting — can stem from poor soil conditions. Cracked, dry, and compacted soil are other signs the soil supporting the lawn might be unhealthy.
The good news is that poor soil can be improved by soil conditioning. Soil conditioning is the process of improving different features of soil quality, such as:
- Tilth — the physical condition and structure of the soil in relation to how suitable it is for plant growth. This includes the presence or absence of clumps (aggregates) and their size, the aeration level, and whether the soil has channels where water can enter and drain. If a soil has good tilth, it has the structure for supporting healthy root growth.
- Nutrient-holding capacity — the ability of the soil to retain minerals that plants use as nutrients. Clay soils generally have higher nutrient-holding capacity, giving them the potential to be very fertile. But they also compact and clump easily.
- Water-holding capacity — varies by soil type (sandy soil drains quickly, and clay holds on to water longer), but other things affect it as well. Soil should be well drained, while still holding enough water for healthy plant growth.
- Organic matter — ideally 5 percent. Organic matter promotes soil biological activity, and also affects tilth, and nutrient- and water-holding capacity.
What are Soil Amendments and Conditioners?
A soil amendment is any material, organic or inorganic, added to soil to improve its physical qualities so it can support plant growth. The term is often used interchangeably with soil conditioner.
Organic amendments contain materials derived from animals or plants, and include manure, biosolids (treated sewage sludge), compost, grass clippings, straw, hay, sawdust, wood chips, wood ash, bone meal, leaf mold, and sphagnum peat moss. To be classified as amendments, organic materials should be fully composted. The exception is sphagnum peat moss, which doesn’t require further decomposition.
Adding organic matter to the soil increases pore space, allowing for better oxygenation for root growth, and making it physically easier for roots to penetrate the soil. Organic matter also improves soil structure, aeration, water- and nutrient-holding capacity, drainage, water infiltration, and workability. Many organic amendments also provide plant nutrients and act as organic fertilizers.
Organic matter as well feeds bacteria, fungi, and earthworms that live in the soil. This increases bioactivity, which releases even more nutrients for plant roots. Once the earthworms and microorganisms have digested the organic matter, more of it will have to be added.
Inorganic amendments are either man-made or mined, and are often more expensive and less available than organic ones. Some examples include vermiculite, sand, perlite, lime, pea gravel, and gypsum.
Inorganic amendments improve water filtration, increase soil aeration and drainage, decrease or increase soil weight, and decrease excessive water holding capacity.
Fertilizers and Organic Amendments
Fertilizers are concentrated organic or inorganic nutrients added to the soil that directly affect plant growth. In contrast, amendments are organic or inorganic materials that indirectly affect plant growth by improving the condition of the soil.
However, because of the substantial amounts of organic matter in organic fertilizers, many are considered soil amendments (e.g. manures, bone meal, biosolids) and organic amendments, because they provide nutrients in varying degrees to plant roots, can act as fertilizers (e.g. sawdust, straw, hay, leaf mold, sphagnum peat moss).
Nearly all soils in Central Texas lack organic matter, are somewhat alkaline, and are clay-textured. These characteristics create the following problems:
- It’s hard for roots to penetrate
- Pooling and drainage problems occur when clay absorbs water and traps it for extended periods
- Clay becomes hard when dry
- The alkalinity prevents plants from absorbing iron
- The clay compacts easily and prevents water and oxygen from reaching plant roots
Before using any soil amendments always test your soil first. One place to get it tested is at the Texas A&M AgriLife Extension Service soil testing labs. Download and print the applicable submittal form from their site. For homes and gardens select the Urban and Homeowner form. If unable to obtain soil sample collection bags from the laboratory you can use plastic zip lock bags. Information is provided on the forms on how to collect and submit a soil sample, as well as the fee amounts for each type of test.
A number of different soil tests are available, including ones for routine nutrients, micronutrients, organic matter, boron, lime requirement, detailed salinity, and texture. The routine test detects the soil acidity or alkalinity (pH), salinity, nitrates (NO3-N), plus the levels of primary nutrients (P — phosphorus, K — potassium, Ca — calcium, Mg — magnesium, Na — sodium, and S — sulphur) that are available to plants.
A routine test report will tell you what nutrients are already in your soil and recommend which ones to add and how much to apply. This takes the guesswork out of what type of fertilizer or amendment to use. You’ll also get a lime recommendation for any low pH.
Amending Your Lawn by Top-Dressing
Top-dressing a lawn is the process of adding a thin layer of compost or other soil amendment over the surface to improve soil quality. Using compost is best for achieving better soil structure and a healthy environment for microorganisms (e.g. bacteria, fungi, protozoa, nematodes) and macroorganisms (e.g. earth worms, spiders, beetles).
The best time of the year for top-dressing is in the spring for warm season grasses, and the fall for cool season grasses. Cool season grasses can also be top-dressed in the spring if you don’t plan to overseed in the fall. Be sure to match the nutrients in your compost or other soil amendment with your soil test results.
Steps for Top-Dressing
- For lawns with more than 1/2 inch of thatch, dethatch or core aerate first. Core aerate if you have poor soil, such as heavy clay. Remove the thatch debris and cores.
- Mow the lawn to the lowest recommended height for its grass type.
- Shovel a small pile of top-dressing onto the grass, and evenly spread a layer over a small area 1/4 to 1/2-inch-thick, depending on the grass height. You can layer up to 1 inch thick for grasses 3 or more inches high. Fill in any aeration holes, especially if the soil is poor.
- The top-dressing must touch the soil, so brush the grass using the backside of a rake until the grass shows through. Make sure the top-dressing is at an even depth.
- Water the grass to settle the top-dressing and keep it in place.
- In a day or two rake the area again to make sure the top-dressing is even. With enough moisture, much of the top-dressing should work into the soil within a few weeks.
- If you’re overseeding, do it after applying the top-dressing. Keep the soil moist until the seeds germinate.
Proper irrigation of your lawn is indispensable to its health and vitality. You can rely on the pros at South Austin Irrigation for irrigation and sprinkler system alterations and repairs.
Call us today at (512) 534-7449 or complete our online form.
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