Physics

Bulk Density

A high bulk density relates to the compaction of the soil, which can limit plants access to water and nutrients in the soil and increase surface water runoff. A low density means the soil will be very loose and could be susceptible to erosion.

Macroporosity

Large soil pores allow space for air and water transport through the soil, which is important for plant growth, drainage and aeration.

Chemistry

pH

Dictates the availability of nutrients and the nature of chemical reactions taking place. There is no "correct" pH as plants are adapted to different pH levels. Soil can naturally acidify over time but fertilisers also contribute to lowering the pH. A low pH increases the bioavailability of ions that are toxic to plant roots.

Total Carbon (C)

An important energy source for soil micro-organisms involved in nutrient cycling. It is a good indicator for biological activity, fertility and water and nutrient retention.

Total Nitrogen (N)

N is the most commonly limiting nutrient. Total N gives an indication of the N status of the soil. Inorganic N (e.g. urea) is accessible to plants for uptake and organic N (e.g. proteins) can be converted into this by micro-organisms decomposing organic matter.

C:N

Determines the release of plant-available N by micro-organisms. A low ratio means micro-organisms are C limited and so organic matter is decomposed to release N which plants can uptake. A high ratio means micro-organisms are N limited and so they outcompete plants, meaning less N is available to plants.

Hot Water Extractable Carbon (HWEC)

Higher values indicate more carbon that is readily decomposed which is related to greater levels of microbial activity and available N.

Olsen Phosphorus (P)

P is commonly the second most limiting nutrient to plant growth. Low P levels are likely to be limiting plant growth and high P levels increase the risk of P loss to waterways which can impact water quality especially when the P retention of the soil is low.

Potentially toxic elements

Arsenic (As)

Exists at trace levels but is naturally elevated in parts of Otago, particularly where schist-derived soils occur. Anthropogenic sources include CCA-treated timber, historic arsenic-based pesticides, sheep dips, and some alloys.

Cadmium (Cd)

Present at trace levels but can be elevated from long-term application of phosphorus fertilisers, which contain impurities. This has led to the Tiered Fertiliser Management System for managing soil Cd. Other sources include batteries, alloys and industrial emissions.

Chromium (Cr)

Occurs at trace levels in most Otago soils, with naturally elevated concentrations in areas influenced by mafic and ultramafic geology. Anthropogenic sources include timber treatment, metal plating, pigments, leather tanning, and industrial emissions.

Copper (Cu)

Present at trace levels but Cu is also widely added through agricultural inputs, especially copper-based fungicides, stock supplements, and pasture topdressing. Other sources include CCA-treated timber, vehicle brake wear, and industrial emissions.

Lead (Pb)

Occurs at trace levels but is elevated in many urban areas due to legacy contamination from leaded petrol, Pb-based paints, industrial emissions, and historic waste disposal.

Nickel (Ni)

Naturally present at trace levels, with higher background concentrations in soils formed from mafic or ultramafic geology. Anthropogenic sources include industrial emissions, metal plating, batteries, and waste disposal.

Zinc (Zn)

Occurs at trace levels but is commonly elevated through agricultural inputs, including Zn-containing fertilisers, stock supplements, and residues from drenches. Other sources include galvanised metal, vehicle tyre wear, industrial emissions, and urban stormwater deposition.