Look after soil for sake of future generations
Maximising economic efficiency and protecting the environment requires the sustainable management of our Waikato soils.
A good definition of sustainable resource management is meeting the needs of the present without compromising the ability to meet the needs of future generations.
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.
Waikato Regional Council’s latest soil quality survey information has revealed several interesting issues in terms of the extent and direction of changes in soil condition.
The council measured eight primary properties to assess soil quality. Chemical and biological characteristics were assessed by total carbon (C) content, total nitrogen (N), potentially mineralisable N, phosphorus (Olsen P), soil acidity (pH), aggregate stability and derived measurements such as the C:N ratio.
Soil physical condition was assessed using dry bulk density and macroporosity (a measure of pore spaces in soil).
These soil physical measurements provided measures of soil compaction, the stability of soil aggregates, the amount of space between soil aggregates (pores) and the density of the soil.
In the past six years, including the latest 2010-11 results, of 145 sites monitored by the council:
31 met all soil quality targets (21 per cent average over six years, down from an average of 26 per cent over the five years till 2009-10).
50 failed one target (34 per cent average, the same as previous five years’ average)
64 sites failed two or more targets (44 per cent average, up from the 40 per cent average of the previous five years).
Of the 30 sites sampled in 2010-11, 13 were on dairy soils, five on cropping and horticulture soils, eight on drystock soils, three were on native vegetation and one site was supporting plantation forestry.
Compaction on dairy and drystock sites in the Waikato remains a particular concern.
Compaction reduces the number and size of pores available for water and gas movement in soil. It reduces aeration, nutrient uptake, root growth and distribution, and potentially decreases infiltration and increases runoff.
The most sensitive indicator of compaction is macroporosity.
Previous research reveals that macroporosity below 10 per cent will inhibit pasture growth.
While New Zealand’s pastoral farming systems are very efficient at generating produce, they can lead to nutrients, both nitrogen and phosphorus, degrading surface and groundwater quality.
In the Waikato, overly high fertility (N and P) still remains an issue on dairy and some drystock sites.
Wise use of fertiliser in these systems can increase agricultural yields and maintain nutrient balance. Excess fertiliser use can lead to degradation of water and air quality, biodiversity, ecosystem services and human health.
Surprisingly, about half the drystock sites were outside target ranges for macroporosity (almost all below the target range).
Additionally, about half of sites were outside target values for total N (above the target ranges) and one-third of sites were outside target values for Olsen P (generally below the target range).
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 (presumably intensively managed sites) whereas others, probably more marginal sites, are below target values, particularly for Olsen P. Production could be improved by additions of phosphatic fertiliser on land with low Olsen P but the cost of the fertiliser and spreading may be uneconomic at this time.
High phosphorus levels remain an issue for cropping and horticulture sites. While only a few sites are currently below target values for soil C, loss of soil carbon may be a more pressing issue than high fertility.
The council plans to continue to monitor closely the soil organic matter, which is a key attribute that affects many physical, chemical and biological properties that control soil productivity and resistance to degradation.
A technical report on Soil Quality in the Waikato Region is on the council’s website (www.waikatoregion.govt.nz).
Numerous management practices can threaten soil sustainability. These include overcultivation, under or overfertilisation, indiscriminate use of pesticides and other agrichemicals, clearing natural vegetation, intensive farming practices and, very importantly, failure to maintain soil organic matter levels. Also, practices which contribute to erosion can naturally cause soil problems.
The impacts of poor soil management have sometimes become so severe in many parts of the country that the adoption of sustainable soil management practices is of crucial importance for future generations.
Also, farmers – particularly dairy farmers – face a new environmental issue with recent research showing some dairy soils are losing more carbon and nitrogen than dry stock and hill country areas.
Dairy farms on certain (nonallophanic) soils 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.
Management practices that increase in-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 the protocol, 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.
Carbon concentration in New Zealand appears to be ranging from 100 to 200 tonnes/ha.
It has been estimated if soil carbon increased by 0.2 per cent on the country’s 14 million ha of grazing land that would represent a carbon trading value of $5 billion at $20 a tonne.
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.
The further development of sustainable soil management practices in this country clearly requires a multidisciplinary approach to find the best solutions.