Soil is the key to successful plant growth, but often times existing landscape and garden soil are less than ideal, being too compacted, too heavy, too porous (sandy) or too alkaline. Now is a great time to consider the current quality of your garden and landscape soil, and take steps to improve it if necessary.
Soil amendments can help with many common landscape problems by improving soil's water-holding capacity, increasing soil drainage, increasing nutrient levels and modifying pH level. A soil amendment is any material mixed into the soil to change one of these characteristics — compared to mulch that is applied on top of the soil. Mulches are used to moderate the soil temperature, prevent weed growth and reduce soil moisture evaporation.
Organic vs. inorganic
There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was once living like leaves, wood chips, wood ash, grass clippings and peat moss. Compost is also an organic soil amendment, consisting of decomposed leaves, grass clippings, etc.
Inorganic amendments originate from inanimate sources, that may have been processed further by man. Common inorganic amendments include vermiculite, perlite, sulfur, lime and sand. Vermiculite and perlite are often found in potting soil mixes.
Adjusting soil pH
Sulfur is used on alkaline soils to lower soil pH to an ideal range for plant growth, typically pH 6-7. Soil pH is measured using a logarithmic scale, so pH 7 is ten times more alkaline than pH 6. Below pH 5.5 or above 7.5, soil modification may be necessary to grow pH sensitive plants such as pin oak, river birch, rhododendrons or azaleas, because soil pH directly influences the availability of many nutrients. Many soils in central Oklahoma that have not been farmed are naturally pH 7.0 or above.
Elemental sulfur or aluminum sulfate are the products most commonly used to lower soil pH, however, large amounts of sulfur are required to make even a small change. For example, a loam soil with a pH of 7.5 requires 15 to 20 pounds of elemental sulfur per 1000 square feet to reach soil pH 6.5. Modified soil will revert in time to its original pH level, so it is often better to use pH adaptable plants in your landscape than to use plants with more strict pH requirements.
Lime is used on acidic soil to raise pH. Always base your addition of sulfur or lime on soil test recommendations, rather adding products on a hunch.
However, not all soil amendments are recommended for our soils. For example, wood ashes are high in salt and pH, something our alkaline soils don't need, and sand added to heavy clay soils creates a texture similar to concrete. Gypsum is another soil amendment commonly discussed by homeowners to “sweeten” soil or lower soil pH, but in fact it does neither and is seldom a helpful amendment for our soils. Gypsum should only be used on soil with high levels of salt, where it can bind with the salt molecules and aid in moving them out of the soil profile.
Benefits of organic
Organic matter supplied by organic amendments improves soil growing conditions in the landscape and garden. Adding organic matter to the soil increases pore space, creating a lighter soil that drains more quickly, allows better oxygen penetration for root growth, and is physically easier for roots to grow in than heavy compacted soil. Organic matter also increases water retention in sandy soils, thus reducing the frequency of watering required. Finally, as organic material breaks down, it provides nutrients for plant growth. Many of our soils are low in organic matter, often less that 1 percent. For a vegetable garden 15 percent organic matter is ideal for optimum root development and vegetable production. However, it will take many years of soil amendment with organic matter to reach this level.
Ray Ridlen is an agriculture/horticulture educator for the Oklahoma County Extension Service. For more information, call 713-1125.