Soil Types

Climatic factors have the greatest overall influence on soil structure and composition. Climate largely controls organic matter production and its rate of decay. Climate also influences the rates of erosion and leaching. Nevertheless, the parent material from which the soil derives does have some influence on the types of plants the soil can support.

Soils are generally profiled in terms of their three prominent horizons: A, B and C. These are often visible as the gradation between the dark humus rich 'A' surface layer and the 'C' layer, which is composed mostly of parent material sans humus. Sometimes when 'A' and 'B' are combined, they are called an 'O' layer. Bedrock, or other base material, composes the 'D' horizon. Below, are some examples of common soil orders found in eastern Canada. They are named according to the Canadian System of Soil Classification. (The FAO Unesco system is very similar.)

Specific soil types are often dominated by a distinct set of tree species. Some trees prefer rich loams. Other trees can only survive in dry sandy soils where competitor plants are excluded. Forests can often be typified by soil fertility, moisture and texture.

The first fully scientific soil classification systems were developed in the Union of Soviet Socialist Republics. Soil scientists (pedologists), such as K.D. Glinka, named the great soil groups and orders in Russian. (Dark prairie soil for example, is called chernozem, the Russian name for ‘black earth’.) Unfortunately, most of the major nations use their own classification schemes. Fortunately, many of these systems use similar terminology.

Acidic soils (pedalfers) tend to occur in moist forested areas. Basic soils (pedocals) tend to occur in dry areas. Tropical and sub-tropical soils (lateritic) tend to be leached and low in surface humus. Prairie pedocals tend to accumulate carbonate, and sometimes salt, under their fairly rich humus layer. Generally dry and cool climates, which have slower decay rates, allow for the greatest accumulations of humus.

Soil pH:

Rainwater is a mild solution of carbonic acid (H₂CO₃). Upon contact with a mineral surface this acid dissolves alkali and alkaline elements to form carbonates (K₂CO₃, MgCO₃ & CaCO₃). The alkali halides or 'salts' (NaCl & KCl) can also form by this rainwater action. Therefore, while rain tends to drive soils toward the acidic range (low pH), electropositive ions tend to force the soil to become more basic (high pH).

Pedalfers: Where the rainwater supply is plentiful, water tends to leach carbonates out of the soil. They are driven either downstream or into the groundwater. Without carbonates the soil tends to become saturated with hydrogen ions (H⁺). Such acidic soils are common in humid regions. Lateritic soils are an extreme version of these types of pedalfers. They occur mostly in the tropics. Laterites can be so acidic that iron (Fe²⁺) and aluminium (Al³⁺) ions are available in excess. Podzolic soils can have a degree of this acidic build-up. They occur in sub-humid temperate forests.

Pedocals: Where rainwater is in short supply the carbonates can be mobilised only to a limited extent. Water may evaporate away before it soaks very far down. Consequently, carbonates, and sometimes even salts, build-up into concretions. Solonetzic soils have extreme carbonate build-ups. They occur in semi-deserts, to dry prairies. Chernozemic soils have medium thick build-ups of carbonates. The more fertile zones of prairies and steppes typically have chernozemic soils.

Organics:

Trees generally grow best when the organic content of the ‘A’ and 'B' horizons are at least three percent (3%). Usually, the more humus the better the soil is for plants. (Within limits.) Where humus accumulation is extreme, the milieu can become very acidic. In such soils, the cation exchange capacity can become too low for proper plant growth. This excessive acidification tends to happen mostly in bogs, muskeg or other unusual situations. Thick soils in the Organic order (Histosols) are more likely to lack sufficient amounts of the heavier nutrient elements. After many generations of plants growing on dead plants, the lighter elements can be lost faster than they are added. The other elements can become diluted as the mass of dead plant tissue expands. Bog soils may even lose so much phosphorus over time that they become very poor at supporting plant growth.

Sand:

Sand is composed of small (2.0-0.05 mm) particles of assorted silicates. Often these grains are dominated by quartz. Quartz has a low solubility in water. It is the most common mineral ‘left behind’ when granite erodes. Course quartz sand has a very low capacity to hold ions. Sand can easily loose plant-useable ions to leaching. On the plus side, sand does allow for the diffusion of both air and water through the soil.

Silt:

Silt is composed of medium sized particles (0.05-0.002 mm). These grains often consist of assorted silicates, such as feldspar, mica, and quartz. Silt has a medium ability to absorb ions. It has a fairly good porosity, and allows for fairly good diffusion of air and water through the soil.

Clay:

Clay is composed of very fine particles ( <0.002 mm). Clay is composed predominantly of mica crystals and other aluminium silicates. These particles are the fine debris from erosion upstream. These flecks are generally carried long distances by water prior to being deposited. These tiny flake-like particles have a very high cation exchange capacity. The mica crystals hold plant-useable ions very well. On the downside, clay is relatively impermeable. Air and water do not move well through compact clay.

Loam:

Loam is a mixture of silt, sand and clay. Loamy soils are generally better than other soil textures for supporting plant growth. Such soils are preferred for both agricultural and silvicultural purposes.

Solod / Solonetz:

Sometimes in very arid environments, with intermittent rains, carbonates are carried into the deeper soil layers by water. Without a sufficiently constant water flow these carbonates can build-up into concretions in the C horizon, or lower. Significant accumulations of salt can also build-up in semi-arid regions. Often the alkaline layer is deep enough not to greatly harm plants. In fact, the upper layers can have near neutral pH levels (~ pH 7). Mildly basic upper soils layers are actually beneficial. Higher pH levels tend to make many ions more mobile, and more plant-useable. Chernozemic soils have such a limey layer, although it is not as extreme as it is in Solonetzic soils. Solonetzic soils are extreme versions of carbonate enriched soils. These soils may even have carbonate concretions in their 'B' horizon. Such soils are called 'solods'.

Mineral Nutrients:

Mineral elements play a crucial role in plant health. Nitrogen, phosphorus and potassium are the mineral nutrients most commonly lacking in soils. In certain soils essential nutrients may be in forms that are not plant-useable. Sometimes minerals are rendered unavailable to plants because of low pH, or some other factor. For more information:

References

Agriculture Canada. 1987. The Canadian System of Soil Classification. Second Edition.Canadian Government Publishing Centre. Ottawa.

Eyre, S.R. 1968. Vegetation and Soils - a world picture. Second Edition. Aldine Publishing Company. Chicago.

Sims, R.A., Kershaw, H.M. and Wickware, G.M. 1990. The Autecology of Major Tree Species in the North Central Region of Ontario. COFRDA Report 3302, NWOFTDU Technical Report 48. Ontario Ministry of Natural Resources. Thunder Bay.

Pender, Terry. 2003. Our Stressed-out Trees. Ontario Arborist. International society of Arboriculture. 31(6): 10-12.

Soil Nutrient Deficiencies

The elements nitrogen (N), phosphorus (P) and potassium (K) are the mineral nutrients most commonly lacking in soils. Generally this is not because they are totally absent, but rather because they are in a form unusable by plants.

Plants require some of the transition metals as components of their enzymes, co-enzymes, proteins and other important metal-organic compounds. Cadmium, palladium, silver, mercury and even tungsten are used by some organisms. Higher plants do not seem to require most of these metals. Nevertheless, some transition metals are essential for plant melallo-enzymes. Some of the transition metals required by plants include:

Elements in Soil

Most of the heavy metals are toxic in high doses. Even nutrients such as iron and copper can be toxic, if their concentrations in soil are too high. Arsenic, lead and mercury are toxic in fairly small doses. Silver, platinum and gold are relatively inert and of relatively low toxicity. Lithium, aluminium, gallium, germanium, arsenic, selenium, cadmium and other light elements are ubiquitous in soils. Plants almost always have significant traces of these elements in their tissues.

Lithium, gallium, germanium, arsenic and cadmium probably have no actual function in vascular plants. Even though plants may have limited uses for certain of these these elements, they still accumulate them. The halogens are fairly common in soils, and most plants absorb them. For example, iodine is present in most plants. This is why animals, which require iodine, are able to survive on a diet of plants. Several studies suggest that iodine in not an essential nutrient for higher plants.

Root Anoxia

Plants require oxygen for respiration, just as animals do. Usually photosynthetic plants liberate more oxygen than they consume, and they absorb more carbon dioxide than they 'exhale'. But plants do 'breathe'. The roots of a plant absorb oxygen from the air for respiration. Respiration in plants is often quite localised, tissues obtaining it from the nearest source. For most trees this oxygen diffuses through the spaces between soil particles. Which is why soil should not be compacted or waterlogged. Plants have a difficult time growing in soil with a low porosity. Filling these spaces with stationary water can also cause problems. If the water is too immobile the dissolved oxygen can be used up by respiration and the water around the roots becomes anoxic (no oxygen) or hypoxic (low oxygen).

Many people do not seem to know that trees can be smothered. Trees can die after the burying their roots by landfill, or with tarmac, or with flagstone, or even after the soil is compressed by heavy machinery. Blocking, compressing or waterlogging the spaces between soil particles can lead to root anoxia. Usually within one growing season of root smothering a tree will die. This is the reason for the recommendation that grade level changes, backfill storage or other soil level raisings should not be allowed within the drip-line of a tree. Most municipal forestry services have explicit prohibitions of grade modifications in or around city trees. I have seen trees die because of as little as six centimetres of clayey luvisolic soil piled over their root-systems. Even the standard practice of raising grades with light sand does not always work. Sometimes trees do not survive even the best planned grade modifications.

Fertiliser Excesses

Fertiliers should only be applied if they are actually required. It is important to diagnose soil nutrient problems correctly before applying fertilisers. When trees flush in the spring they tend to use as much of their stored carbohydrates for shoot elongation as they can afford. If the soil is richly endowed with nutrients, a tree can act as if these nutrients surpluses are going to persist throughout the season. When the nutrient surplus in the soil is exhausted, the tree suddenly has to revert to normal rates of growth. Excessive fertilisation can raise the proportion of shoot growth, at the expense of the root growth. Excessive shoot elongation can also come at the expense of the production of protective chemicals. In other words, too much spring fertiliser can cause trees to be less drought-hardy, and less resistant to parasites and herbivores.

Some studies seem to show some problems with autumn fertilisation. Normally, autumn is a good time for slow-release fertiliser application. However, it seems that some species have root-flushes after these treatments. This can delay or inhibit the winter hardening process. The problems caused by autumn fertilisation do not seem to be as extreme as those of spring application.

Studies are quite unambiguous with regard to improving soil quality around planted root-balls. Quality fill around the root-ball of a transplanted tree increases its survival and hardiness. Although, there may be a period of shock when the roots outgrow the fill years later.

References

Atkins, P.W. 1995. The Periodic Kingdom. BasicBooks. New York.

Daniel, W.D., Helms, J.A. and Baker, F.S. 1979. Principles of Silviculture. McGraw-Hill Book Company. New York.

Emsley, John. 2001. Nature's Building Blocks - an A-Z guide to the elements. Oxford University Press. Oxford.

Morrison, R.T. and Boyd, R.N. 1980. Organic Chemistry. Allyn and Bacon Inc. Boston.

Scharenbroch, B.C. and Lloyd, J.E. 2004. A literature review of nitrogen availability indices for use in urban landscapes. Journal of Arboriculture. 30(4): 214-230.

Smiley, E.T. and Shirazi, A.M. 2003. Fall fertilization and cold hardiness in landscape trees. Journal of Arboriculture. 29(6): 342-346.

Struve, Daniel K. 2002. A review of shade tree nitrogen fertilization research in the United States. Journal of Arboriculture. 28(6): 252-263.

Watson, G.W. 2002. Soil replacement: long-term results. Journal of Arboriculture. 28(5): 229-230.

Drought Stress

Drought became a North American problem the summer of 2001. July and August were unusually dry for most of North America. For more than one month there was insignificant rain.

When a tree is drought stressed they first conserve water by holding it back from their leaves. This causes semi-wilting, and eventually dying of the leaf margins. Water translocation is more difficult to outer twigs. Thus drought stress is first manifest in the upper crowns. As the stress gets more severe upper leaves may fall, and buds set, similar to the physiological preparations for autumn. Early leaf fall may means that there are too few hours of photosynthesis. Photosynthates stored in the roots and stem may not be enough to survive the winter. More importantly, if the water reserves in the soil are too low to last the winter, the tree may be stressed again in the spring. Consequently, the tree may die from the water stress disorder, and its ramifications.

If one can, trees showing signs of drought stress should be watered. Beware, it is possible to drown roots. It is not a good idea to let the water run all night. Nor should one continue watering if the soil is saturated to the point of being muddy, or if it is pooling.

Conserve water, water only the trees showing obvious signs of drought stress. The stressed tree is probably stressed because of local drainage conditions, or because it is a sensitive species. Neighbouring trees may have slightly better access to water, or they may be more tolerant species.

Bark Inclusion

The cambium of dicot trees usually grows in two directions. The inner layer produces xylem (wood), the outer produces layer phloem (bark). Thus wood is oldest on the inside, and bark oldest on the outside. Stems that grow at small angles to each other can produce bark surfaces that press against each other. Such areas of apressed bark often occur in narrow branch crotches. Bark inclusion can also occur between two trees which are closely appressed together. In both cases the two bark layers can prevent the two stems from ever fusing together. The seam that forms along this bark inclusion is structurally weaker than normal wood.

Large branches that break during windstorms often break along the seam of bark inclusion. Likewise two individual trees with a seam of included bark between them can be very dangerous. Sometimes such trees have two totally independent and lopsided root systems. Such half-circle root spreads are very unstable. It matters little whether the two trees were always separate individuals or if they were clones. If their roots are disconnected they are risk trees. In most instances, that I have investigated, where trees have fallen and damaged property or injured people, bark inclusion was a factor. Trees with a seam of included bark that start at ground level are especially hazardous. In public places one or the other of the twin stems should be removed before the trees become too large. Likewise bark inclusion in the narrow crotch of a tree is a potential risk. Such trees should be monitored, or an arborist's appraisal should be sought. Sometimes cable bracing can be installed to prevent the crotch from splitting.

Root Girdling

Too often trees planted from nursery stock develop 'root girdling'. This girdling is caused by roots growing tangentially to the trunk. Eventually the 'sideways' growing roots can conflict with the outward growing trunk. With time this can strangle the xylem cutting off sap flow. This manifests itself as a generalised decline with branch dieback and necrotic shards in the trunk.. Strangely, root girdling can manifest itself decades after planting. Sometimes a whole set of trees from the same nursery succumb to girdling within a short span of a decade or so.

At one time girdling was blamed on poorly potted nursery stock. However, container-grown bare-root and burlap-wrapped tree stock are also prone to girdling. Girdling is actually caused by fine roots growing tangentially along a container's inner wall. When these rootlets enlarge, they can become girdling roots. Sometimes transplanted trees become girdled, even if the root-ball never was containerised. This is probably caused by the severing of large roots during the digging-up process. Severed roots tend to sprout many new rootlets when the recover.

Girdling is often the result of nursery trees being left too long in their growing container. Sometimes the offending root can be seen bulging up near the base of the tree. One remedy is to dig up the soil around the root, if it is visible, and cut-out the girdling section. The bark and cambium of the trunk must not be harmed. This solution has a chance of efficacy if only a few of the tree's major roots are causing the girdling. Still, the tree may die from root loss. (This cheapo solution is worth a try. In my humble opinion.)

References

Buszacki, Stefan and Harris, Keith. 1998. Pest, Diseases & Disorders of Garden Plants. Harper Collins Publishers. London. 598-599.