Acidity

Acid soils are often leached of many soluble ions and are commonly deficient in major plant nutrients such as calcium, magnesium, nitrogen and phosphorus and possibly molybdenum. Phosphorus may be present in a form that is not available to plants. Metal ions may be soluble in toxic concentrations. Fungi rather than bacteria tend do dominate in acid soils.

Alkalinity

Alkaline soils are often deficient in some plant nutrients such as iron, manganese, copper or zinc.

pH 1:5 Soil:Water

This is the activity of the negative log of the hydrogen ions in a suspension of 1:5 soil:water. pH measured in 1:5 soil:water is sensitive to seasonal variations in the pH of soil solutions.

pH 1:5 Soil:0.01 mol CaCl₂

This is the activity of the negative log of hydrogen ions in a suspension of 1:5 soil:0.01 mol CaCl₂. This test method is considered to approximate average soil solution calcium and salinity levels. pH in 1:5 soil:0.01 mol CaCl₂ is not appropriate for soils rich in calcium carbonate.

Expected pH Buffer Capacity

pH buffer capacity is the amount of acid input required to reduce pH by one unit. Expected buffer capacity should not be used directly to calculate lime requirements nor speed of acidification. Instead, expected buffer capacity serves as a relative indicator of resistance to acidity increase between soil types.

Maximum Lime Requirement

This is an indication of the amount of lime or dolomite required to rectify aluminium toxicity/calcium deficiency problems. Lime is likely to be required if:

• exchangeable aluminium exceeds 5% of cation exchange capacity depending on the plants being considered;
• soil pH (1:5 soil: water) is <5.5 ; and
• exchangeable calcium is <4 me/100g .

When all these conditions are met, an indication of tonnes of lime equivalents per hectare to 10 cm soil depth is given to reduce exchangeable aluminium to 5% of effective cation exchange capacity (CEC).

This is generally considered to be the maximum amount of lime/dolomite required as many plants are tolerant of more than 5% exchangeable aluminium . Use caution to avoid adverse effects of over-liming. Dolomite may be preferred when magnesium is likely to become deficient.

Required lime t/ha/10 cm = 1.3 (Exch. Al - 0.05 * CEC), where Exch. Al & CEC are me/100 g.

Organic Carbon

Organic carbon is oxidized by potassium dichromate. The amount of dichromate not reduced is determined by titration with ferrous sulfate using Ferroin indicator. The method measures the amount of carbon in plant and animal remains including soil humus but not charcoal or coal.

Organic Matter

This is based on the assumption that soil organic matter = organic carbon * 1.755. Organic matter contents are affected by climate, drainage, biological activity and landform as well as land use. Organic matter is a key component in assessing soil fertility, stability, hydrology and land condition. Total Organic Matter can also be determined by Hydrogen Peroxide digestion.

Bray Phosphorus

Bray "available" phosphorus test involves extraction of absorbed phosphorus with HCl and NH₄F. Concentration of extracted phosphorus is determined by spectrophotometer.

Lactate Phosphorus

"Available" phosphorus is extracted by calcium lactate in dilute HCl and determined by spectrophotometer. Lactate phosphorus indicates probability of response to phosphorus fertilizer by wheat on neutral and alkaline soils.

This available phosphorus test is recommended over Bray phosphorus for neutral and alkaline soils.

A standard phosphorus solution is added to soil. After equilibration, the phosphorus remaining in solution is measured colorimetrically and the phosphorus "fixed" by the soil is the calculated difference.

High phosphorus sorption ratings indicate phosphate fixing to iron, aluminium or calcium compounds as well as certain organic-clay complexes. Large amounts of phosphate fertilizer are required for plant response to fertilizer if phosphorus sorption is high. High phosphorus sorption is often associated with high anion exchange capacity of the soil.

Electrical Conductivity

Electrical conductivity indicates the amount of soluble ions (salt) in soil. Electrical conductivity is determined on a 1:5 soil:water suspension and is prepared from the fine earth fraction of the sample. As a rule of thumb, it is considered that salinity levels where yield is affected may be increased by up to 30% before a species cannot maintain an effective ground cover.

Rust

Significant hazard for iron or steel rusting in soil occurs when:

• pH in 1:5 soil:water is <3.6 or pH in 1:5 soil:CaCl₂ is <2.9;
• clay is >35%; or
• salinity is >0.8 dS/m ECe (USDA 1983).

When any of these conditions occur, "Rust" is recorded. Other significant soil conditions contributing to rust hazard include presence of sulfides and pyrite. These properties are not normally tested by labs due to there lack of X-Ray Diffraction (XRD) equipment .Where critical steel beams and girders are exposed to soil a XRD scan which quantifies all minerals is recommended. Site conditions such as fluctuating water tables, poor drainage and intersection of several different soil materials across any iron or steel structure also contribute to rusting.

Extra care must be taken to protect buried iron or steel where a rust hazard occurs.

Concrete Corrosion

Significant hazard for corrosion of concrete in soil occurs when:

• organic matter is >12% and clay is <10% and pH (1:5 soil:water) is <5.1, or pH (1:5 soil:CaCl₂) is <4.3;
• clay is <35% and pH (1:5 soil:water) is <4.6, or pH (1:5 soil:CaCl₂) is <3.9; or
• salinity (ECe) is >15.6 dS/m ).

Other significant soil conditions contributing to concrete corrosion include significant amounts of sodium and/or magnesium sulfates. These mineral properties of soil can only be detected by XRD.

Extra care must be taken to protect buried iron or steel where a rust hazard occurs.