Soil Health Monitoring Programme

Soil quality characteristic

Measure

What it tells us

Why it’s important

Fertility

Soil pH

Whether a soil is acid or alkaline.

Some plants and animals will only live in soils at a particular pH.

Olsen P (phosphate)

How much phosphate is available for plant growth.

Phosphate (P) is an essential nutrient for plants and animals. Plants get their P from phosphates in soil. Most New Zealand soils are low in phosphates and extra phosphate needs to be added for agricultural use.

Humus (organic matter)

Total C (carbon)

Organic matter content.

Organic matter helps the soil store water and release it in a steady way. Organic matter also helps a soil store nutrients and form a crumbly structure suitable for plant roots.

Total N (nitrogen)

Organic N reserves in the soil.

Nitrogen (N) is an essential nutrient for plants and animals. Nearly all the N in soil is in organic form (see mineralisable N).

Physical condition

Bulk density

Whether a soil is firm and compacted or loose and friable.

Plant roots cannot grow through compacted soil. Compacted soils also become waterlogged quickly, starving roots of oxygen. If a soil is too loose, it dries out very quickly and may be easily eroded.

Macropores

How many large pores there are in soil.

Large pores (0.03–0.3 mm diameter) are needed for air to reach roots and for water drainage. The larger pores are the first to be damaged by compaction.

Aggregate stability

How resistant soil aggregates are to breaking.

A stable ’crumbly’ texture lets water quickly soak into soil, doesn’t dry out too quickly, and allows roots to spread easily. This measure is most useful for soils used for horticulture and cropping.

Biological activity

Mineralisable N (nitrogen)

How much of the total N is available to plants through microbial activity

Mineralisable N is related to the amount and quality of organic matter and biological activity in soil. Soil life breaks down the total N in organic matter and converts it into forms plants can use.