Using Compost as a Soil Amendment to Improve Plant Health and Recycle Green Materials - Part II

By Janet Hartin, Environmental Horticulturist, University of California Cooperative Extension, San Bernardino and Los Angeles Counties

Here, a landscaper prepares an area for large shrubs. He begins by breaking up the soil with a roto-tiller. Following this, he adds amendments, making sure to thoroughly and uniformly mixed them into the soil to a depth of at least 18 inches to two feet deep. For plants small woody plants, the area should be tilled to one foot, and for ground cover, it should be tilled no deeper than six inches. Amending only a few inches of soil encourages shallow rooting and should be avoided.

The correct use of compost as a soil amendment to improve poor and moderate quality soils can greatly enhance the growth and development of landscape plants and turf.

However, it is important to remember that proper site design, plant selection, spacing and planting technique, and maintenance are essential for assuring the health of landscape plants, and that no amount of soil amending will eliminate these requirements.

While compost makes an excellent soil amendment for bedding plant, turf, and small woody plant establishment, it should not be added to holes excavated for the purpose of planting landscape trees. This is because developing tree roots tend to prefer the higher quality amended soil, often leading to a pot-bound plant with a shallow root system. It is better to plant trees in the parent soil that has been removed, loosened, and added back into the hole after the sides and bottom of the hole have been etched and cut into to encourage downward and outward root growth. This will help ensure safe and high quality tree development throughout its lifespan.

Even badly dehydrated clay-based soil such as the soil shown here can be saved using compost. When applied in adequate concentrations, compost can significantly improve the texture of such soils. Improvements in soil structure occur in two ways. First, the compost itself contains particles that improve soil tilth and porosity. To result in immediate improvements, approximately 30 percent of the final soil volume should be amended with high quality compost.

Amending Soil with Compost to Improve Physical Properties

Clay-based soil amended in this way will lead to more productive and healthy plant growth for less cost than amending the same soil with the necessary 45-percent sand. Second, composts may also be effective at lower application rates, although changes will be gradual, rather than immediate, and repeated applications may be necessary before observable differences are noted. As compost decomposes in soil, it encourages the formation of soil aggregates. These resulting aggregates are composed of parent soil particles and are not merely decomposed compost. Because composts encourage the formation of soil aggregates, they can be particularly useful in restoring a crumb-like structure where construction activities have damaged and altered the natural structure of the soil.

If the whole planting area cannot be amended, minimally each planting hole should be dug at least three times the size of the root ball of the plant. Compost can then be incorporated back into the hole after being thoroughly mixed with some of the original soil. This will encourage high-quality root growth beyond the constraints of the original container, adding both stability and a greater absorptive rooting area. Do not plant woody plants deeper than the original soil level in the pot they are being transplanted out of; this can lead to crown rot.

Amending Soil with Compost to Remediate Chemical Problems

Twelve to 20 lbs. of sulfur per 1,000 square foot of soil can reduce pH from 8.0 to 6.5. Multiple applications of no more than five pounds of sulfur per 1,000 square foot of soil are recommended to avoid plant injury. The sulfur should be incorporated to a depth of at least eight inches for maximum benefit. Although rarely necessary in California, the pH of an acid soil may be raised by the addition of lime (calcium carbonate).

Most ornamentals, such as this one, grow optimally in soils with a pH range from 5.5 to 8.0, with soils near 7 (neutral) being optimal. Many California soils are alkaline. A neutral pH compost applied in sufficient volume to a slightly to moderately alkaline soil can reduce the pH and promote healthy plant growth. When soil pH is extremely high and/or the pH of the compost is also alkaline, however, the addition of elemental sulfur, iron sulfate, or ammonium sulfate may be necessary to neutralize the pH. It is important to note that results of remediation might not be seen for several months to a year after application.

Improving Soil

Soils with poor structure and infiltration rates due to high concentrations of sodium can be improved by applying gypsum (calcium sulfate). During this process, the calcium in the gypsum replaces the sodium. The displaced sodium readily dissolves in the soil water and can be removed by saturating the soil so that the sodium is rinsed, or leached, below the root zone. Applications of about 20 pounds per 1,000 square foot of soil followed by repeated leaching (saturation of the soil to below the root zone followed by drainage) can appreciably reduce sodium concentrations and improve infiltration rates. Repeated leaching over several months may be required.

Gypsum use only improves soils damaged by sodium. Unlike compost, gypsum does not alter pH appreciably. Nor will it improve water infiltration and structural components of soils suffering from problems other than high sodium. During the incorporation of gypsum, soils are loosened and aerated; the resulting improvement in drainage is often mistaken for a gypsum-induced benefit even though the gypsum itself had no effect.

The absorption of high levels of certain soluble salts found in the root-zone of a plant can significantly reduce growth and development. Leaf scorch and leaf drop are common symptoms. Repeated leaching, as recommended above for sodic soils, is sometimes effective in reducing damage. It is important to irrigate plants thoroughly after fertilizing and to avoid applying more fertilizer than necessary. Frequently, sodic soils (where 15 percent or more of the cation exchange capacity (CEC) is occupied by exchangeable sodium affecting plant growth) are also high in soluble salts. These soils require a combination of the above treatments. Compost can also improve sodic soils, and composts and gypsum can be used together. Gypsum is a component of drywall and some composters cull this material from construction and demolition wastes for use in their products.

Remember that rates of compost and fertilizer, and any necessary pH adjustment, are influenced by the plant selected, soil and site characteristics, compost quality and feedstock, and other factors. Before planting, it is advisable to have compost, soil, and soil/compost blends tested by an STA certified laboratory. Consider discussing the results and recommendations with a trained professional before proceeding.

In a future issue, Janet Hartin explores specification guidelines for compost use as a soil amendment, and preferred compost characteristics for soil amendments.