Soil sustainability is crucial

By Bala Tikkisetty

Managing our soils sustainably is crucial for the Waikato environment and the regional economy.

Meeting the needs of the present without compromising the ability to meet the needs of future generations is a good definition of sustainable resource management.

So, broadly speaking, soil management is sustainable when it lets us get what we need today without compromising the capacity of the soil to provide for future needs.

Recent soil quality monitoring work by Environment Waikato has revealed a number of important issues.

The surveys came after the council participated in a couple of major national soil quality monitoring projects. These projects identified methods and protocols for soil quality monitoring.

Since then, we have sampled new sites and re-sampled previously sampled sites each year to determine the extent and direction of changes in soil condition.

We measured seven primary soil properties to assess soil quality. Chemical and biological characteristics were assessed by measuring the amount of carbon, nitrogen and available phosphorous (Olsen P) in the soil, how much nitrogen is potentially mineralisable by microorganisms and the soil pH. Soil physical condition was assessed from the dry bulk density and microporosity (how many pores large enough for water to infiltrate through).

There are 135 soil quality monitoring sites in the Waikato and these are sampled over a five-year rotation. Twenty-four sites were sampled in 2008/09 (eight sites on cropping and horticulture soils, five sites on dairy soils, six sites on drystock soils, four sites on indigenous vegetation and one site on plantation forestry).

However, only nine of the 24 sites (or 37.5 per cent) met all soil quality targets. It’s a reminder that maintaining soil health is something everyone involved in making Waikato agriculture successful needs to keep a close eye on.

Compaction of soil on dairy and drystock sites is a particular concern. Compaction reduces the number of pores available for water and gas movement in soil. It reduces aeration, root growth and distribution, and nutrient uptake. It also potentially decreases nutrient infiltration and increases nutrient runoff.

The most sensitive indicator of compaction is macroporosity. Previous research reports reveal that macroporosity below 10 per cent will inhibit pasture growth. There is a continuing trend of decreasing macroporosity values for dairy and drystock sites in the last few years. Surprisingly, about half the drystock sites were outside target ranges for macroporosity.

While New Zealand’s pastoral farming systems are very efficient at generating produce, leaching and run-off of nitrogen and phosphorous respectively can degrade surface and ground water quality.

High nitrogen (N) and phosphrous (P) fertility remains a problem on the dairy and some drystock sites we have tested because excessive nutrient levels increase the risk of nutrients escaping into waterways. Careful use of fertiliser in these systems can increase agricultural yields and maintain nutrient balance, while better avoiding the potential for excess nutrients to have detrimental environmental impacts.

About one third of the drystock sites were outside target values for total N (generally above the target ranges) but another one third of sites were outside target values for Olsen P (generally below the target range).

The macroporosity and fertility figures for drystock mean there appears to be a divergence in land-use management of drystock sites as the fertility measurements (total N and Olsen P) indicate some drystock sites are above target values whereas other sites are below target values, particularly for Olsen P, indicating production could be improved by additions of phosphatic fertiliser.

High P levels remain an issue for cropping and horticulture sites. Although, currently only a few sites are below target values for soil carbon, loss of this soil C may be a more pressing issue than high fertility.

Meanwhile, an emerging issue nationally is that recent research shows some dairy soils are losing more carbon than dry stock and hill country areas. Certain soils on dairy farms have lost an average of one tonne of soil carbon per hectare a year in some parts of the country.

Biologically active carbon is the most important part of the soil carbon and how it is distributed through the soil profile is equally important. Soil carbon is found primarily in organic forms which make up soil organic matter. Soil organic matter helps to maintain soil structure, retain soil moisture, prevent erosion, and can act as a reservoir for nutrients and as a source or sink for carbon. Increasing levels of SOM is likely to enhance soil biological processes and soil sustainability.

Management practices that increase soil organic carbon, such as reduced tillage, use of more organic amendments, and greater use of mixed farming may help to improve soil bio-diversity as well as soil sustainability.

The Kyoto protocol might also influence soil sustainability in the near future. Under Kyoto, agricultural soils are highlighted for possible future inclusion as a biospheric sink for carbon. If agricultural soils were to be used as carbon sinks, there would be a greater need and responsibility by farmers and land managers to increase the soil organic carbon content of their soils.

The further development of sustainable soil management practices in this country clearly requires a multidisciplinary approach to find the best solutions. As we look for those answers, it will be the wise farmer who keeps an eye on these issues given their ability to influence the bottom line.

EW itself is planning to continue closely monitoring soil organic matter, which is a key attribute that affects many physical, chemical and biological properties that control soil productivity and resistance to degradation.

Land uses that could enhance carbon sequestration and improve soil sustainability generally include best agricultural management practices such as avoiding over-cultivation and under or over-fertilisation. Other things that can hurt soil health include decreased or increased water abstraction, indiscriminate use of pesticides and other agri-chemicals, clearing natural vegetation, intensive farming practices, and, very importantly, failure to maintain soil organic matter levels. Practices which contribute to erosion can also have a negative impact.

For further reading see EW’s technical report on Soil Quality in the Waikato Region 2008 at our website www.ew.govt.nz.

Bala Tikkisetty is a sustainable agriculture coordinator for Environment Waikato, phone 0800 800 401.