Soil quality

Soil quality defines whether soils are in good condition for their current use. The physical, chemical and biological characteristics of different soils vary a great deal, so that different soils are suited to different uses.

Where a soil’s characteristics match those needed for its current use, then we can say that soil is of good quality. This measure of quality relates to several aspects of a soil:

• fertility
• amount of humus (organic matter)
• physical condition (for example, whether it has become compacted)
• whether it contains beneficial soil life, such as bacteria and earthworms.

Sometimes soil quality is also referred to as ‘soil health’.

Measuring soil quality provides an early warning of how different land uses and management practices might be damaging our valuable soils. There is no single test for soil quality, because there are many things about soil that affect its quality. From the hundreds of possible measures, scientists have chosen eight key soil quality characteristics to measure in New Zealand soils.

Comparing key soil quality characteristics against guideline values tells us whether soil quality is satisfactory for different land uses in the region.

National guidelines have been developed to take into account different soil types and land use. They also take into account both production and environmental goals.

For example, when it comes to soil fertility, a low Olsen P (phosphate) value is better for soils supporting indigenous vegetation. This is because our native forests are adapted to low nutrient soils.

However, soils used for pastoral farming or horticulture and cropping need much greater Olsen P levels in order to achieve good production.

Production goals also need to be balanced with the environmental effects of high Olsen P levels in soil. Olsen P levels greater than those needed for good production will result in losses to waterways. The nutrient then feeds the growth of algae and waterweeds, reducing water clarity.

To find out more about how the national soil guidelines were developed, check out Landcare Research’s soil indicators tool (SINDI).

As well as measuring soil characteristics in the laboratory, there are simple methods to assess soil quality out in the field. For example, the Visual Soil Assessment (VSA) tool, developed by Landcare Research, provides landowners with simple tools to check their soil’s physical condition.

Waikato Regional Council began to measure the soil quality of key land use types in the Waikato region in 1996. We are building up a picture over time of how land use and management practices are affecting our soil quality.

Our monitoring shows that:

• most of the region’s soils (80 per cent) are of satisfactory quality for their current use
• around 18 per cent of our land used for pastoral farming has soil quality that is ‘of concern’. This concern mostly relates to some dairy farms which have soil compaction and excessively high soil fertility.

We are currently measuring soil quality on horticulture and cropping land. Research in other parts of New Zealand has shown that this land use can result in soil compaction and very high soil fertility. Cropping in the Waikato region has recently increased in response to demand from dairy farmers for more winter feed such as maize and maize silage.

Maintaining soil quality is not only good for the environment. It’s also likely to improve plant growth, increasing agricultural and horticultural production. This can help make your business more environmentally and financially sustainable.

There are a range of things you can do to help maintain soil quality for your current land use.

Getting fertility right

• To help maintain soil fertility levels appropriate to your production goals, carry out regular soil tests on different parts of your farm to check nutrient levels.
• To make sure you’re putting on the appropriate amount of fertiliser for your soil and production needs, do a nutrient budget. For a more comprehensive nutrient budget, contact MAF Policy, Hamilton, for a free copy of the OVERSEER^® computer package or talk to your farm consultant.

Maintaining humus (organic matter)

• To increase the organic matter in your cropping soil, grow annual cover crops between productive crops – these cover crops can then be mulched into the ground.
• You could also apply compost, organic manure or effluent to the soil. With more organic matter, soil is better able to store nutrients and water, and has improved soil structure.
• Reduce cultivation and soil disturbance to minimise the loss of organic matter – try direct drilling of seed for pasture renewal.

Looking after soil structure

Pugging and compacting your soil will reduce grass growth and impair drainage. It is also likely to produce more runoff to waterways. To protect your soil from pugging and compaction:

• Graze wetter paddocks before the wetter part of the season.
• Build pasture cover leading in to the wet season, to give better protection against pugging.
• Graze land that is at risk of pugging with light stock.
• Shift stock before daybreak.
• Use the farm bike rather than heavy tractors.
• Vary the depth of cultivation annually – to avoid forming a plough pan.
• Avoid working the soil when it is wet.

Keeping your soil alive

Fungi, bacteria and other micro-organisms in the soil make nutrients available for plant roots and recycle dead plant material. Earthworms distribute organic matter through the soil profile and improve soil structure, aeration and drainage. To help provide a healthy environment for soil life:

• Maintain a good cover of vegetation to protect soil where possible.
• Minimise cultivation and soil compaction.