Soil with acceptable levels of pH, plant essential nutrients, and soluble salts can provide adequate nutrition and chemical properties for plant growth and development. No amount or type of soil amendment will overcome poor initial site design or improper horticultural management.

Sometimes the soil to be landscaped has been graded, layered, compacted or otherwise modified to construct a suitable lot for a building foundation, a landscape berm, or other feature. Unfortunately, such soil alterations are performed to engineering or architectural standards, not horticultural standards. The discrepancy in the two standards often presents considerable problems in establishing and maintaining landscape and garden plant materials.

In mature landscapes, detrimental changes in soil chemical or physical properties can occur with use and management of the area. Soil compaction is one of the most common and potentially harmful changes that occur as a result of foot and vehicle traffic. Over fertilization can create high soluble-salt levels, and the long-term use of acid-forming fertilizer can lower pH, possibly to an undesirable level. As woody plants grow and develop more extensive root systems, they also may encounter subsoil conditions that limit plant growth because of original construction or parent material.

WHEN TO AMEND

Adding one or more soil amendments, regardless of existing soil conditions, is commonly considered essential to successful establishment of new plantings or rejuvenation of poorly-growing existing ones. However, not all sites require soil amendment, and most do not.

It is usually impractical to effectively amend the physical properties of whole landscape areas because of the expense and volume of soil required to effectively amend soil needed by trees, shrubs, and other perennials. To have a long-term effect, the entire landscape area needs to be amended to the breadth and depth that the mature roots of trees and shrubs will grow.

While individual planting holes can be amended, the effect is temporary because roots of woody and other perennial plants eventually grow out of the planting hole into native soil. Sometimes trees grown where soil was amended only in the planting hole develop poor structural integrity because their roots grew less extensively beyond the original planting hole.

Research findings show that amending tree and shrub planting holes or entire landscape and garden areas is warranted only when soil physical or chemical properties are extremely limiting to plant growth. If a situation arises where amending individual planting holes is deemed necessary, the holes should be constructed at least four times the diameter but the same depth of transplants’ root systems.

Evaluating Soil Infiltration Rate & Evaluation/Test Take a large empty coffee or juice can with the bottom removed and push it into the soil until just 2 or3 inches remain above the surface. This may require hammering on a board placed across the top of the can to prevent damage to the can. Try to push the can straight into the soil. Be certain the surface of the soil inside the can is undisturbed and free of plant residue. Measure and record the height of the inside of the can from the rim to the soil surface. Fill the can with water and measure the water depth after one hour. This is the soil’s infiltration rate. After all the water has drained from inside the can, repeat the filling and measuring process. The second measurement provides a rough estimate of the soil’s percolation rate. An infiltration or percolation rate significantly less than 0.5 inches (1.25 cm) per hour is an indication of dense, compacted, or layered soil.

Additions of physical soil amendments can enhance the tilth and manageability of small beds and landscape areas where annuals are regularly removed and replanted or where other small perennial ornamental plants are to be grown. Occasional additions of organic matter plantings of annual flowers will improve and maintain soil structure and enhance transplanting and establishing these color beds.

An appropriate physical or chemical amendment may also be feasible in beds or containers where small woody or perennial plants with unique demanding soil requirements will be located. In existing woody or perennial plantings in a landscape, serious physical soil problems are difficult and normally impractical to remedy.

Before proceeding to amend a new or established landscape area, the site should be evaluated and analyzed to determine if an amendable problem exists. Soil across the site should be examined with probes, augers, and shovels to check for layered soils and compaction zones at or below the surface. Original grading specifications and techniques should also be reviewed and considered to identify subsurface changes that could cause soil structure problems.

The soil should be evaluated for extremes in soil texture, such as high clay or high sand content. The rate of water infiltration and drainage (percolation) should be estimated (see side bar). A soil test of pH and soluble salts can be helpful before planting an area, but it is sometimes more appropriate as a tool for diagnosing poor performance of established plants.

The first rule for amending soil is that there must be an amendable problem present. Amendment of the soil is usually warranted and should be considered if any of the following are discovered:

• soil texture analysis reveals extremely high percentages of clay (>30%), which greatly reduces aeration and tilth of a soil, or extremely high percentage of sand (>70%), which greatly reduces water and nutrient holding;
• infiltration or percolation less than rate 0.25 in per hour (0.6 cm per hour);
• layers of soil with different textures are present;
• compaction is present in the soil surface or in a layer below the surface;
• pH is beyond the acceptable range of the plants being grown (?5.0 or ?8.0 for most plants);
• soluble salts level is too high (an EC test value of ?2.0 dS/m or greater for sensitive plants, ?4.0 dS/m for other plants) and/or
• high concentrations of sodium (Na+) as determined by sodium adsorption ratio (SAR) evaluation.

The second rule for amending soil is that the amendment must be uniformly mixed and completed to the depth that will solve the problem. For example, surface applications of organic material will have no effect on a subsurface plow plan layer. Similarly, incorporating soil sulfur in the top 6 inches (15 cm) of soil will have little effect on soil pH below that depth.