Home > Site analysis > Evaluate below-ground characteristics > Areas of compacted soil
Areas with compacted soil, poor drainage and low oxygen
Compacted and poorly-drained soils contain little oxygen--a gas that tree roots need to survive and grow. Many trees die or grow poorly and succumb from an insect or disease problem because they are planted in soil that is too wet for them during certain times of the year. Only species and cultivars tolerant of wet sites can survive in these difficult soils.
To check for compaction and drainage, dig several holes at least 18" deep (24 to 30" preferably) in each section of the site. If soil is very difficult to dig with a shovel, it may be compacted. Another sign of compaction is standing water for a day or more after a period of rain. If a pick ax appears to be the best tool for digging a planting hole, then soil is probably too compacted. If soil is fairly easy to dig into with a shovel, it is probably not compacted.
See: figure for checking soil drainage
Soil bulk density is one reliable measure of compaction. It is used by professional landscape architects, urban foresters and others. A quick method (Lichter and Costello, 1994) of measuring it is to dig a 12-inch-deep hole with a shovel, placing all excavated soil in a bucket or other container. Line the hole with a plastic bag and fill the bag with water. Measure the volume of water in the bag. Place soil in an oven for several days at 180 to 200 F until it is completely dry. Weigh the dried soil. Bulk density is calculated by dividing the weight of dried soil by the volume of water. This technique can underestimate bulk density by 3 to 9% compared to the more traditional (and more accurate) undisturbed soil core technique. If the soil is too compacted root and tree growth will be poor (see table).
| CRITICAL BULK DENSITY VALUES FOR DIFFERENT SOIL TEXTURES | |
| SOIL
TEXTURE |
CRITICAL
BULK DENSITY¹ RANGE |
| Clay, silt loam | 1.4 - 1.55 |
| Silty clay, silty clay loam, silt | 1.4 - 1.45 |
| Clay loam | 1.45 - 1.55 |
| Loam | 1.45 - 1.6 |
| Sandy clay | 1.55 - 1.65 |
| Sandy clay loam | 1.55 - 1.75 |
| Sandy loam | 1.55 - 1.75 |
| Loamy sand, sand | > 1.75 |
¹
Bulk densities greater than these values could restrict root growth |
|
If soil is very compacted and hard all the way down to the bottom of the planting hole, then wet site tolerant trees are most appropriate. Occasionally soil is loose underneath and compacted only on the surface. If you can break up the surface compacted layer for 15 feet or more around the tree before planting, drainage and tree growth may improve. In this case, trees can be chosen regardless of their wet site tolerance.
One method for determining if soil is poorly drained is to smell it. Sour smelling soil may be a gray color indicating that it contains little oxygen. Occasionally, the smell may be strong enough to detect while standing close to the hole. More often, a soil aggregate or clump must be broken open close to your nose to detect the smell.
If the soil does not smell sour when you dig, it may still be anaerobic (without oxygen) during certain times of the year. This could limit tree growth unless the species to be planted are tolerant of wet sites. To complete a thorough soil evaluation, check the soil several times during the year (especially during cool, wet weather) prior to planting. Many poorly-drained soils are wettest in the dormant season when there is little if any foliage to draw water from the soil. You may also be able to gather soil drainage information from the Soil Survey or people more familiar with that particular planting site. These techniques will allow you to develop a year-round picture of the drainage characteristics of the planting site.
Other clues to help determine soil drainage characteristics and oxygen content include presence of subsurface compacted layers (hardpans) and artificial soil layering. Oxygen content and diffusion rate in the soil can also be measured with special devices. These machines may become useful and gain wider acceptance as professionals gain more experience with them and they become easier to use. On undisturbed sites, the presence of only wet site tolerant trees such as bald cypress can also indicate that soil is poorly drained.
Trees
with aggressive root systems should not be planted in compacted or poorly-drained
soil because surface roots often form. These can disrupt lawn mowing operations
and can damage curbs, sidewalks, pavement and other nearby structures.
