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. Vegetation type has a very large impact on the kind of soil. However, the vegetation to a large extent reflects the climate.

Soils are generally profiled in terms of their three prominent horizons: A, B and C. The 'Aoo' layer is basically dead plant material, or mor, before it has been mixed into the soil. Whereas the 'A' proper is the upper layer of the soil itself. Soil layers 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. Bedrock, or other base material, composes the 'D' horizon. Below, are some examples of common soil orders found in eastern Canada. They are here named according to the Canadian System of Soil Classification. The FAO Unesco system is very similar to the Canadian. The USDA & Soil Survey's system is somewhat different.

Ontario’s boreal forests are typified by the Brunisolic and Podzolic soils. Brunisol is a fairly rich brown soil, that corresponds to the FAO's 'Cambisol' Order. Brown soil is commonest in the more arid west side of Ontario. The leached podzolic soils are more prevalent in the humid east. Luvisolic soils occur mostly in the Carolinian Forest zone. These are the soils with clayey bases and fairly rich humus. They have been heavily exploited as cropland. Near the Lake Erie shore the sandy Gleysolic soils are fairly common. These sandy loams can be excellent soils for crops. Most of Ontario's orchards and vinyards grow on Gleysolic soils.

Tropical soils

A few of the tropical soils are similar to soils elsewhere. The tropical brown soils are basically similar to temperate Solonetzic, Luvisolic and Gleysolic soils. The podzolic soils occur in the tropics also. However, some kinds of tropical soils are quite distinctive. One common trait of tropical soils is that the humus rots-away fairly quickly. Basically, decomposition is more rapid in warm and wet climates. One common feature of these soils is that they tend to range from red to chestnut-brown in colour. These colours are due to the soils generally being very old and leached-out.

Where there is constant rainfall, both the carbonates and finer silica clays to leach away. This makes the 'laterites' reddish, as the less-soluble sesquioxides that are left-behind are often quite reddish (Ferralsol). If there is a long dry season, the leaching may not be too extreme. The soils in drought-then-monsoon prone places do tend to retain a fair bit of clay. These are the soils that crack into hexagons and crevasses during the dry seasons. Such 'black earths' are more likely to be dark coloured, at least under the surface (Vertisol). There are in-between soils. Some of the tropical soils are leached and clayey. They tend to be reddish-brown. These soils grade from chestnut-brown to quite red versions (Nitisol). These in turn garde into the true 'red clay' soils (Acrisol). These semi-leached red clay soils are common in the tropics and in the warmer temperate zones.

Brown soils and red soils are similar to what in the USDA system are considered to be the ‘Alfisol’ Order. These soils are sometimes called ‘Nitosols’, but the proper FAO name is now ‘Nitisol’. They resemble Solonetz and Luvisol, in that they have both a clayey and carbonate laden base. They are called 'terras roxas' in parts of Brazil. In Australia they are sometimes called 'red earths'. Being among the best agricultural soils, they are now heavily exploited. They are another example of how humans have changed nature. Pristine examples of these riparian soils are becoming increasingly rare.

Ontario's Vegetation zones, as they were circa 1850

Soil Science

The first fully scientific soil classification systems were developed in the USSR. Soil scientists (pedologists) often named the great soil groups and orders in the Russian language. Dark prairie soil for example, is called chernozem, the Russian name for ‘black earth’. Parkland soils sometimes have a carbonate, or even salt, hardpan layer under the humus. This kind of soil is called solonetz, after the Russian word for ‘salty’. Even the common word podzol is a Russian term describing the ‘ashy’ colour of the lower-layers in such soils.

Podzolisation

Podzolic soils occur mostly under woodlands, in fairly moist climates. In these soils, the upper ‘A’ layers are can become quite leached out. It is largely the acidic nature of the surface humus that decelerates decay. But the ample rain allows some leaching to occur. Leaching accelerate the removal of clay particles and soluble minerals from the surface to the lower ‘C’ horizon. The less soluble sesquioxides of iron and aluminium are left behind in the ‘B’ layer. This causes the orange or reddish appearance of the ‘B’ horizon (Podzol). The 'C' layer can accummulate the leached materials. Sometimes even this layer becomes partly leached-out. Mostly podzols occur under pine forests, but other kinds of sub-humid forests can also develop podzolised soil.

Something like podzolisation can occur in the rainy tropics. In these environs, leaching can take the soluble minerals deep into the soil, even leaching the ‘C’ horizon. Often this results in ‘red clay’ soils - which still have a fair amount of clay (Acrisol). Taken to an extreme, laterites or latosols can form. These laterites are very reddish in colour. These soils even are short of clay - i.e. the finer silica particles are largely leached out. The clays of aluminium and iron sesquioxides remain. This makes the soil quite reddish (Ferralsol). Such soils can harden into a rock-like crust - if they are allowed to dry-out. These reddish soils are usually very infertile for plants. In rainforests the tree roots are largely confined to the surface layer 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 via 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).

(I) Pedalfers (Low pH): Where the rainwater supply is plentiful, water tends to leach carbonates out and away from the soil. They are driven either downstream or into the groundwater. Without carbonates the soil tends to become saturated with hydrogen ions (H⁺) and it becomes acidic. 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 are most common in sub-humid temperate forests.

(II) Pedocals (High pH): 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, arid parklands and 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'.

Tufo: Tuff or tufo is basically volcanic ash. The particles start out in assorted sizes (c 2.0-0.002 mm) and of assorted minerals. Tufo is mineral rich and it is a fertile base for agricultural soil. However, it eventual leaches and erodes into more ‘normal’ soil. In Ontario tufo has, for the most part, long ago been degraded.

Loess: Loess is a kind of fine sand and clay mixture. It is composed of very fine particles of quartz, mica and other common minerals. Loess makes a good base material for fertile loam soils. These deposits originated from windborne dust, i.e. they are basically aeolian dust. The dust came mostly from the exposed barrens left after the retreat of the glaciers - after the last Ice Age. Thick loess deposits occur mostly south of the Great Lakes, and not so much in Ontario.

Till: Till is composed of assorted rubble, varying from erratic boulders, to gravel, to sand and even silt. Drumlins and moraines are composed of this ‘glacial till’. In Ontario till was mostly produced by glacial action during the last Ice Age. Till is common around the southern rim of the Canadian Shield.

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.

References

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

Gourou, Pierre. 1982. Terres de bonne espérance - le monde tropical. Édition Plon. Paris. pp 89-97, 192-196.

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.

Ontario's Vegetation, as it was in circa 1850.

Oak openings are a kind of the ‘parkland’ or ‘forest-steppe’ vegetation. Oak openings grade into aspen parkland in the north. To the south, the oak openings tend to grade into pinewoods or flatwoods. In these parklands the tree roots spread broadly, and the root and crown competition tends to suppress the smaller groundcover plants. In many parkland areas ground-fires were once fairly common. These fires also helped to clear the groundcover.

In the American Midwest, burloak (Quercus macrocarpa) and chinkapin oak (Q. muhlenbergii) can dominate the oak openings. In the eastern Great Lakes area, it is black oak (Q. velutina) that is most dominant. There are oak openings on the margins of the palouse prairies in the Plateau Country of the western USA. Garry oak (Q. garryana) is common in these parklands. In the mid-latitudes of Eurasia, the pollardo oak (Q. robur) and durmast oak (Q. petraea) are very common forest-steppe species.

Tallgrass

Believe it or not, outliers of prairie once extended into southern Ontario. They differ from the full-prairies in that Ontario is rainier. The Great Plains can have between 100 mm and 500 mm of rain per year Whereas, the driest corner of southern Ontario usually has at least 600 mm of rain per year. These outlier grasslands usually have light-brown soils, and not the dark-brown soils of the Great Plains. It is the sandy soils that seem to encourage the tallgrass - not the climate per se. A few remnants of this tallgrass ‘prairie’ still exist in the Windsor-Essex to Pelee corridor. A mixture of tallgrass and savannah also used to stretch between Long Point and Brantford. Similar vegetation could be found from Toronto's lakeshore to the environs of Rice Lake. Oak openings were most common near these patches of grassland. Human activities, of course, have vastly reduced the oak openings as well as the tallgrass prairies. By some estimates, less than 0.1 percent of this forest-stepe vegetation still exists in Ontario.

The best examples of tallgrass prairies occur near Lake Huron and Lake Erie. An especially intact example of this grassland occurs in the Pinery Provincial Park, on the Huron shore. These areas are dominated by robust grasses with wide spreading mats of rhizomes. These tallgrasses can grow upwards of two metres tall. The coastal sand dunes are often colonised by switch grass (Panicum virgatum). Indian grass (Sorghastrum nutans) occurs on more established sandy areas. Little-bluestem (Andropogon scoparius) and big-bluestem (A. gerardii) grasses are also fairly common. These species also occur in the eastern side of the Great Plains. Though, of course, the real prairies have a broader range of species.

Pine Barrens

In sundry parts of the world one can find forest tracts wherein most of the canopy trees are pines (Pinus). Enigmatically, sometimes these forests seem to have little in common, except for the fact that pines are one of the dominant plants. The hard needled pines are pre-adapted to periods of water-stress. This is why pines are especially common on barren-lands. They can retain their water very well during droughts. Furthermore, being evergreen they can also take advantage of short growing seasons. This is why pines dominate in both the high boreal forests, and in the dry parklands to the south. Paradoxically, high alpine forests can also be pine dominated. This is because of the rapid drainage and the short growing-hours during the day.

In the highlands of Mesoamerica and in the Greater Antilles pinewoods are common (eg. P. caribaea). Oaks and palms often occur along with these pines. From Alberta to Quebec, jack pine (P. banksiana) parklands occur in the dry barrens of the boreal forest. Southern Ontario has a few white pine (P. strobus) dominated stands in the sandy barrens along the Lake Huron shoreline. In the southern USA the ‘southern pines’ are a common sight (eg. P. palustris). In northern Florida these pine-parklands are called the ‘flatwoods’. Similarly, open forests dominated by Aleppo pine (P. halepensis) can occur in the drier parts of Mediterranean Europe. These pinewoods are most common on limestone alvars or on sandy barrens.

The secret of all of these pinewoods is the pines themselves. Pines are, in general, tolerant of water-stress and they are tolerant of short growing-periods. Pines have some water-retaining and drought-resisting traits in common with succulents. Thus it is not surprising that semi-arid forests and parklands tend to support pines. At the same time, very cold locals can also become dominated by pines. This is true in the far north, with its short growing season. It is also true in alpine locals, where the growing-period of the day is short. Cold air can also desiccate plants. Both arctic and alpine plants need to be tolerant of water-stress. Pines are paradoxical plants.

References

Easterly, Nathan William. 1979. Rare and Infrequent Plant Species In the Oak Openings of Northwestern Ohio. Ohio J. Sci. 79(2): 51-79.

Eyre, S.R. 1968. Vegetation and Soils a world picture. 2nd Edition. Aldine Publishing Company. Chicago.pp 118-121.

Tallgrass Ontario. 2009. Ontario Tallgrass. The Prairie and Savanna Association: www.TallgrassOntario.org.

Szeicz, J. M. and MacDonald, G. M. 1991. Postglacial vegetation history of oak savanna in southern Ontario. Can. J. Bot. 69(7): 1507–1519.