Soil Conditions

As mentioned previously, important soil conditions to consider for tree health are texture, drainage, compaction, pH, salinity, and structure. All of these conditions can be modified to some extent. However, when soil conditions cannot be adequately modified, or modification is too expensive, tree species and cultivars often can be selected that are tolerant of extremes.

  • Soil Texture - Soil texture is the proportion of sand, silt, and clay particles in a soil. Sand particles are large and coarse, silt is smaller, and clay particles are very small. Ideal soils for root growth tend to have a mixture of all three particle sizes and are called loams. Such soils have good water and mineral nutrient (element) holding capacity because of the silt and clay, and good drainage, porosity, and air movement because of the sand. Very clayey soils tend to waterlog and are hard for roots to grow through. Sandy soils may be droughty and low in mineral content. Soil texture is not easy to change but it is good to be aware of.
  • Soil Drainage - Drainage refers to how quickly a soil drains after water is applied and is affected by texture, structure, porosity, precipitation or irrigation levels, and the slope and landscape position of a site. Soils can be excessively drained, well drained, or poorly drained. Excessively drained soils tend to be coarse textured (sandy) and drain too rapidly to be able to supply much water for plants. Poorly drained soils usually are fine textured (clayey) and hold water too long which limits soil oxygen. Excessively drained soils can be improved by addition of organic matter or by bringing

in new soil. Poor drainage can be improved with drain tiles or other artificial drainage systems. Special planting methods can also be used to deal with poor drainage.

  • Soil Compaction - Good soils for root growth are made up of about one-half soil particles and one-half pore space. Soil compaction occurs when a soil is compressed enough to reduce pore space, decreasing root growth and soil oxygen levels. Vehicle and pedestrian traffic, construction activities, and working of wet soils all increase compaction, especially in fine soils. Severely compacted sites, like old parking lots, construction sites, or walkways, may need to be deeply tilled over the entire site before trees are planted. Local compaction can be reduced or prevented with mulch beds and traffic control. Augering 2-3 inch diameter holes 8-16 inches deep in a grid around a tree reduces compaction. Holes should be left empty and allowed to collapse, or can be filled with compost or other coarse organic material (called vertical mulching). Avoid augering into large roots by staying away from the soil near the trunk. Research has shown that systems that inject air below the soil surface have little or no effect on compaction. Core aerators reduce compaction but only very near the soil surface.
  • Soil pH - Soil pH or reaction is the acidity or alkalinity of a soil and is a good indicator of the availability of many mineral nutrients (elements). Seven is a neutral pH with lower numbers being acidic and higher numbers alkaline or basic. Trees generally do best in a slightly acidic soil with a pH of 5 to 6.5. Below and above this range certain minerals become less available. High soil pH's (alkaline soils) are common in Utah, causing problems with availability of iron and manganese for trees. Researchers have lowered soil pH with regular additions of granular sulfur, but this may be impractical if the pH is very high and if the soil is very fine. Sulfur applications may damage turf, but such damage can be prevented by applying the material to a mulch bed under the tree. A mixture of sulfur and ferrous sulfate can be placed in augered holes to locally reduce soil pH. Iron chelates also can be used to provide iron in high soil pH situations, but they are very expensive and do not last long (see Control of Iron Chlorosis, AG SO01, a fact sheet available through your Extension office or at publica/agpubs/agso01.pdf).
  • Soil Salinity - Salinity refers to the relative amount of salts in the soil. Saline soils have high accumulations of sodium, chloride, and certain other ions. These ions interfere with nutrient and water uptake, reduce growth and alter plant metabolism, and can cause increased soil compaction. Utah has large areas of naturally saline soils and areas that are becoming saline because of salts present in irrigation water, combined with over-irrigation. Localized areas can also become saline due to runoff of deicing salts along roads and sidewalks. Such salts may also be deposited directly on above-ground plant parts. Salts can be leached out of well drained soils if the salinity is mild and the irrigation water is fairly salt-free. Planting salt-tolerant trees may be the only choice if salinity is very high.
  • Soil Structure - Structure refers to the arrangement of soil particles into granules, clods, or other forms with spaces in between. Loose, granular soils are good for trees because air, water, and roots easily penetrate the many spaces in the soil. Hard, dense, cloddy soils have little room for roots to grow or oxygen and water to penetrate. Good soil structure may be ruined by working or disturbing the soil while it is wet. Though poor structure may improve over time through the action of soil microbes and freezing and thawing, deep tillage is the only way to quickly improve soil structure (do not till around established trees). Addition of organic matter at planting time may improve soil structure, but should be avoided in all but the worst cases since roots must eventually grow into the surrounding native soil. If soil conditions are that bad, changes should be made or trees should not be planted.
  • Soil Testing - Soils tests for mineral content, pH, and other characteristics can be done through commercial labs or through Utah State University's Soils Testing Laboratory, Utah State University, Logan, UT 84322-4830. Contact your Extension office for directions for gathering and transporting samples or obtain the fact sheet Soil Testing Guide for Home Gardens, HG H05 ( gardpubs/hfs05.pdf). Be sure that you have several samples analyzed separately if you think that your landscape differs from one area to another.

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