L3 Soil components and soil formation

Why is ph important?

pH affects crop growth - species vary in tolerance Availability of the 16 essential elements is influenced by pH pH influences fate of pollutants and toxins

Soil organic matter percentages

10% fresh residue 33%-50% Decomposing OM 33%-50% Stabalized OM (humus) <5% living organisms

Grain size of sand, silt, and clay?

2 mm - 0.062 mm clay 0.062 mm - 0.002 mm < 0.002 mm

Percentage of nitrogen oxygen and carbon dioxide in atmosphere and soil air?

79 79 20.96 18 to 20.8 00.035 0.15 to 1.0 Composition of soil air is relative to that of the open atmosphere because they are mixing constantly.

What is ph?

A logarithmic method of expressing the concentration of hydrogen

why are ions important?

A lot of minerals dissolved in soil are composed of a mixture of anions and cations. Ions are important as these are the things that are available for plants and making water acidic or alkaline- How it reacts with bedrock and other clay minerals in the soil to make new clay minerals.

low ph

Acidic Associated with high OM content Causes reduced soil organism activity Causes increased aluminum to levels of toxicity.

Use of OM in soil (Physical)

Active OM is the portion available to soil organisms. Breakdown of nutrients by microorganisms - mineralization (the release of plant-available compounds such as ammonium during decomposition) Peat forms where decomposition is slowed Important - binding, water holding capacity, porosity, nutrients Keeps structure of soil stable. Produces layer on top that water can infiltrate through. But also holds some of the water- soaks up like a sponge. Keeps surface warm which is good for plant growth.

Humus

Already decomposed. Stores water very well. source of nutrients for the next set of plants often acidifies the soil when mixed with water.

What determines amount of water held in each soil state?

Amount of water held in each state depends on soil texture, structure and organic matter. Wilting point in sandy soil is low. Clay high as can hold more water. Higher field capacity for smaller grain size as can hold water. Have a higher field capacity for smaller grain size as it's harder for water to come out. Field capacity linear until it comes to clay where much smaller grain size. More difficult for water to drain through and water is less available because the pore size is so small.

high ph

Associated with sodium carbonate Phosphorus and boron become unavailable. Also where there is a bedrock that is limestone Manganese and aluminum which are important for plant growth are much less available if have an alkaline soil.

What organisms make up the food web

Bacteria, algae, fungi, and protozoa Nematodes and micro-arthropods (ants) Earthworms, insects, small vertebrates Plants

Decomposing OM

Biomass (active organic matter - organic material derived from decaying living plant or animal organisms)

isomorphous substitution

Clay minerals formed in sheets of positive charge cations surrounded by negative anions eg oxygen and hydroxyl. Clay minerals become electrically negative if cation such as Al is replaced with one with a smaller charge eg Mg when forming. Negative charge balanced by cations in water adsorbed to the clay surface by electrostatic attractions. These can then also be removed by water. More isomorphous substitution= greater negative charge on clay and greater capacity to exchange cations. Doesn't happen on sandy grain because of the type of mineral it is, the ions don't get absorbed onto the surface.

Secondary minerals

Clay minerals. These can be from glacial till, weathered parent material and precipitated minerals. Carbonates, ferric deposits etc.

How is clay different from sand and silt?

Clay particles are silicates, but they are mineralogically different.

Structure of clay

Clays are a mixture of tectohedral (4 side) sheets and octahedral sheets (8). Sheet like structure- All clays consist of mineral sheets packaged into layers. Octahedral and tectohedral layer together alternately. Naturally form platey minerals and will move and slide if wet enough.

How are clays classified?

Clays are classified as either 1:1 or 2:1. Most 1:1 clays are non-expansive (1 tectahedral and 1 octahedral) Most 2:1 clays are expansive (2 tectahedral and 1 octahedral) Illite (2:1) is a nonexpansive type of clay. Contains potassium ions, which when reacting with water are stable. However, other clays can fit water molecules within structures eg calcite. Means friction between layers are reduced so more likely to slip.

What impact do organisms have on soil?

Decompose organic compounds. e.g. manure, plant residue, and pesticides- prevents entering water and becoming pollutants. Large biochemical role. Sequester nitrogen/other nutrients that would otherwise enter groundwater. Fix nitrogen from atmosphere making available to plants. Enhance soil aggregation/porosity-increasing infiltration + reducing runoff/erosion. organisms prey on crop pests. Bacteria use organic compounds e.g root exudates or plant residues. Fungi use more complex compounds, e.g. fibrous plant residues, wood (lignin) and soil humus.

How is clay formed?

Different minerals have different susceptibility to weathering. If bedrock rich in feldspar then there is more weathering than one with lots of quartz. As minerals react with rain they produce clay. Clay minerals are formed near the earths surface, in soil or in water. Most belong to a class of phyllosilicates, which have formed from the breakdown products of other minerals.

Use of OM in soil (Chemical)

Exchanges the minerals dissolved in the water (acts as a Cation exchange). Changes ph. - if a lot of organic matter not decaying fast then get an acidic soil. Make nutrients available by binding soil organic matter to soil minerals to create an interaction.

Aeration

Extent of soil air = ventilation of soil air; supplying oxygen and removal of co2 from soil atmosphere.

Dead plant material includes

Fresh residue, decomposing OM, Stabilized organic matter.

Different types of soil water?

Gravitational water (free water that moves through the soil due to the force of gravity), 2-mm pores. Capillary water (water in the micropores). Gravity less important than capillary tension. Capillary water moves because something is making it get dragged through. Evaporation at the top, plants draw water from groundwater. Hygroscopic water- remaining water adheres to soil particles and is not available to the plant.

Why is soil air important for plants?

Important, otherwise it is waterlogged constantly - some plants roots prefer to have air around.

What processes result in hydrogen and aluminium becoming predominant cations in the soil?

Leaching of base cations in wet climates Carbon dioxide from respiration dissolves into carbonic acids Acid rain Nitrification of ammonium-based fertilisers releases nitrate and H+ Crop harvest removes base cations taken up by plants

why is soil air different to atmospheric?

Less oxygen and more carbon dioxide in soil air than in the atmosphere because of the respiration of microorganisms in the soil which uses oxygen and produces carbon dioxide.

What does fresh residue contain?

Litter i.e. Trees and leafs resting on top surface that have fallen. Dead plant material including lignin.

importance of clay in drainage?

Mineral grain are 2 micrometres (very small) If lots of these in a soil profile or bedrock then drainage is reduced and water wont be able to percolate through the soil as easy

Anions

Negatively charged ions.

Cation exchange capacity

Overall net negative charge of a clay mineral per unit mass of soil The more isomorphous substitution the greater capacity to exchange cations Different cation exchange activity if clay soils and sandy soils Base cations (weak acidic) and acid cations (proton acceptor/proton donor)

Give an example of importance of food web?

Plants photosynthesize and other creatures share an interlinked food web. It is thought that decomposition in Chernobyl was reduced as smaller animals were killed.

name two important differences between sand, silt?

Porosity: their relative capacity to hold water that is available for uptake by plants. Permeability: Their effect on soil drainage.

Cations

Positively charged ions

What are primary minerals of soils?

Primary source of rock particles in soils are from the parent material. May also be a contribution from windblown particles or from rivers from an area with a different rock type. Examples of rocks in soils include sand and silt.

Use of OM in soil (biological)

Produces energy for bacteria and source of energy for plants growing there. soil plant system resilience. Part of growing produces funghi in the soils, keeps bacteria in the soils, as well as water and nutrients.... Supports next lot of growth.

Why are living organisms important for decay process?

Root exudates are substances secreted by a plant (e.g. soluble sugars and amino acids). Sugars and acids will dissolve in the soil water and change water chemistry and start reacting with dead plant material (form part of decay process) but also interact with parent material underneath.

Different soil states

Saturation (pores filled with water). Field capacity- water content after excess water has drained away. Smaller pores filled with water, larger with air. Wilting point: soil moisture when plants cannot recover from wilting. Prolonged dry period and water table goes down-plant no longer able to access water.

Common rock forming minerals?

Silicates, such as quartz and feldspar. Carbonates such as calcite.

Cation exchange

Soils with a large negative charge are more fertile because they retian more cations. A lot of exchangeable cations are nutrients used by plants eg potassium and calcium. exchange between cations of an equal charge between solution and soil colloid. soil colloid is organic gloop that wraps around grains.

Importance of soil water?

Stores and supplies nutrients. All nutrients get dissolved in water and they are then available for plants.

how can minerals be dissolved in water?

Water is a polar molecule. Oxygen has a small negative charge and hydrogen has a small positive charge. positive end will therefore be attracted to negative and vice versa. Minerals also are ionic and have polar features-therefore minerals can be dissolved in water. Salt completely dissolves in water as water molecules keep the sodium (Na+) and chloride (Cl-) ions apart and stops them from reforming the solid salt.

Problems with 2:1 clays?

When it shrinks it cracks. If water is added to it the clay will expand but not necessarily to the extent it was. This changes the soil profile. Once it has shrunk there are pathways in which rainfall goes through (cracks) and get erosion. Problem for locations with a lot of clay where people are building, subsidence with clay shrinking. Don't want to be growing something where if you have a dry summer it takes a long time for soil profile back to a situation where it is draining and responding properly to the water table. Heavy rainfall could erode and damage integrity of soil.