Soil The Importance to plants 1.Substrate to anchor plants. 2.Source of nutrients. 3.Reservoir for moisture.

Soil Profile Soil – upper 1-2 meters of the regolith which has generally been subject to: a.More extreme weather, b.The activities of roots, c.The activities of invertebrates and microorganisms. Regolith – unconsolidated weathered rock and soil above bedrock. Bedrock – the rock underlying the soil.

Soil Profile The soil layer is divided into:  The solum – the A & B horizons.  The plow layer or furrow slice – the layer of soil containing considerable organic matter which is affected by plowing or tillage. This includes the A horizon and sometimes the B horizon. Figure about 18”.  The subsoil – the region below the plow layer. This would not include the A horizon. Figure 17.5 The development from a young soil consisting of a few fragments of rock particles to a deep sedentary soil is shown alongside a transported soil. A subsoil, topsoil and leaf litter layer can be identified in each soil. Simple plants such as lichens and mosses establish on rocks or fragments to be succeeded by higher plants as soil depth and organic matter levels increase.

Soil Horizons A horizon – included the surface soil; the zone in which organic matter accumulates –Zone of eluviation. This becomes the plow layer in tilled soil. E horizon – Between A & B horizon. A zone where material (clay, organic matter, oxides of iron and aluminium) accumulates from the A horizon. B horizon – The subsoil, or region below the E horizon. The zone of illuviation. Often a substantial portion of the B horizon is included in the plow layer. C horizon – is below the principle root zone of plants, and is little affected by soil forming processes and does not have properties typical of the A & B horizons.

Soil Horizons Website with great soil information

Soil Profile Variation Enger & Smith 2008

Soil Profile Houtman et al. 2013

Soil Composition

Mineral Soil - the inorganic constituents of soil. Primarily minerals resistant to weathering. a.Plus secondary minerals formed from weathering.

Soil Composition Soil Organic matter – the accumulation of partially decayed plant and animal residues. a.A transitory component, since it is constantly being broken down by microorganisms and replenished by dying organisms. b.Small component of soil volume – only 3-5% c.Major source of mineral elements, such as phosphorus and sulfur. d.Virtually the sole source of nitrogen. e.Increases water holding capacity of soils. f.Main source of energy for microorganisms, which are responsible for the majority of biochemical activities.

Soil Composition Soil Organic matter. Soil organic matter is composed of two types of material – Original organism tissue and partially decomposed tissues. Humus – the more resistant products of decomposition

Soil Composition Soil Water. a.Not all water in soils is available to plants. b.Water is held within soil pores with varying degrees of tenacity. c.When soil moisture is abundant, plants can readily absorb water, mostly from large to intermediate pore spaces between soil particles. d.When soil moisture becomes limiting, the remaining moisture is present in small pores and as thin films around soil particles.

Soil Composition Soil Air – is different than atmospheric air. a.Has a higher moisture content than the atmosphere – up to 100% under optimal soil moisture. b.CO 2 concentration much higher (from root respiration). O 2 concentration may only be 10-12% versus 20% in normal atmosphere. c.The larger the pore size of the soil, the better aerated the soil is. d.Compacted soils, or soils made of small particles are not as well aerated.

Forest Soil Development

Forest Soil Development

Forest Soil Development

How Soil is Formed Climate – probably the most influential factor. a.Determines rate of weathering. b.Temperature and precipitation exert a profound effect on chemical and physical processes. c.The rates of chemical reactions doubles for every 10  C rise in temperature. d.Climate is the essential means by which the profile (horizons) development is affected. e.Climate exerts its influence through a second soil forming process – living organisms.

How Soil is Formed Living organisms – are responsible for a.Organic matter, b.Profile mixing, c.Nutrient cycling, d.Structural stability, e.Adding nitrogen.

How Soil is Formed – effect of vegetation Figure Nutrient Cycling under broadleaf deciduous trees. (After Oberlander & Muller, 1987) The degree to which nutrients are cycled depends on the needs of the organism occupying a particular place. For instance, broadleaf, deciduous trees like oak and maple generally have high nutrient demand creating surface litter rich in nutrients when leaves die and fall to the forest floor. Decomposition of the litter releases the nutrients back into the soil for the tree to take back up. Thus soils under these kinds of forests tend to be high in soluble bases and nutrients.

How Soil is Formed – effect of vegetation Figure Nutrient cycling under pine forests (After Oberlander & Muller, 1987) Pine trees generally have low nutrient demands. The decaying litter that falls to the forest floor is poor in nutrients. As a result, little cycling of soluble nutrients like calcium, magnesium, sodium, and potassium occurs and are thus leached creating an acidic soil environment.

How Soil is Formed Parent material – even with weathering, parent material exerts a strong influence. Types of Parent Material  Igneous rocks - Hard and consolidated material containing quartz and feldspar.  Sedimentary rock  Metamorphic rock

How Soil is Formed Igneous rock -Igneous rocks are formed from the solidification of molten rock material either above or below the surface of the earth. Sedimentary rock – From material that has been transported and deposited by agents such as rivers, wind, gravity, seas and lakes. Metamorphic rock – Igneous or sedimentary rocks that have been changed under extreme heat and pressure.

How Soil is Formed The physical and chemical properties of soil are dependent upon the type of rock from which they are formed. The mineral composition of rocks differ.

How Soil is Formed Topography – can hasten or delay the process of soil formation.

How Soil is Formed Time – all process take time.

Soil Particles

Soil Particles - clay

Soil Particles and Pore Space Raven & Berg, 2006

Soil Particles - clay

Cation Exchange Capacity Cation exchange capacity –a measure of the number of negatively charged sites on soil particles that attract exchangeable cations. The higher the number, the more cations a standard amount of soil can hold.  These cations include calcium, magnesium, potassium, sodium, ammonium, aluminum, iron and hydrogen.  Contributing to soil acidity: Hydrogen and aluminum  Helping neutralize soil acidity: Calcium, magnesium, potassium, sodium, ammonium and iron.

Cation Exchange Capacity

Cation Exchange Capacity

Cation Exchange Capacity Clay, because of its layers of negatively charged micelles, has a much greater cation exchange capacity than sand or silt.

Cation Exchange Capacity of different soils

Cation Exchange Capacity - humus

Cation Exchange Capacity of different soils Humus and CEC The surface area and adsorptive capacity of humus are far in excess of those exhibited by any clay. CEC capacity of humus ranges from meq. In comparison, the CEC of clays ranges from meq. Humus has 2-30 times the CEC of mineral soil.

More on humus Humus and moisture Humus will adsorb from a saturated atmosphere, an amount of water equiv. to about % of its weight. Clay on the other hand, may be able to acquire only 15-20% of its weight.