Soil
Soils are a non-renewable resource and are vital to Southland's productive capacity. They sustain plant and animal production, can maintain or enhance water and air quality and support human health and habitation. When the physical, chemical and biological performance of soils are optimised, they are able to soak up rain and contaminants. Even if the soil structure is good, if not well managed, it threatens agricultural productivity, air and water quality.
Soil structure refers to the arrangement of the aggregates and pores within the soil. The structure of soil determines the porosity, strength and stability of a soil which influences:
- Water movement and storage
- The roots ability to penetrate, grow and withdraw water and nutrients
- The ability of a soil to resist erosion
Good Soil Structure
A well-structured soil has many stable aggregates in a wide range of sizes. Aggregate size of 2-5 mm diameter is considered to be the best and most wind erosion resistant. Good soils also have a large number of pores within and between these aggregates. These pores maintain the correct balance of air and water in the soil and allow easy seedling emergence and plant root growth.
Poor Soil Structure
Poorly structured soil either does not have aggregates of many different sizes or the aggregates are packed tightly together with few pores. In light textured soils the soil appears fine and powdery and has unstable aggregates. Poor soil structure can markedly increase the risk of wind erosion.
The major soil types found in the Waituna catchment are Organic, Podzol, Gley and Brown. Overall, the soils in the Waituna catchment tend to have poor soil structure.
Soils in the Waituna Catchment generally are poorly drained due to the fine sediment (e.g. clays and silts) in the soil which reflects the mudstone, siltstone and claybound gravels that these soils sit above. The soil type, along with the relatively low-lying topography, means that much of the developed land in the Waituna catchment is likely to include extensive artificial drainage (mole, tile and surface drains).
Originally, groundwater and extensive wetland areas (like the Awarua Plains) stored and slowly released excess rainfall to surface waterways and in a sense acted like a natural water quality filter and sponge. However, with the onset of artificial drainage, water now flows much more rapidly to streams, thereby reducing summer stream flows and reducing the opportunity for natural biochemical processes to improve water quality.
The distinct line between Brown soils (upper catchment) and others (lower catchment) clearly shows the shoreline from 70,000 to 130,000 years ago. The key difference in soil characteristics across this boundary is drainage whereby the Brown soils are generally more well drained compared to the Organic, Gley and Podzol soils which are all poorly drained and can often become waterlogged.
As the Waituna Lagoon fills with water, the relatively flat nature of the old marine terrace that marks the lower half of the catchment causes drainage water to bank upstream which also waterlogs the surrounding soils. The different soil characteristics between the northern half and southern half of the catchment can result in different environmental impacts for the same landuse activity.
Guidelines for Land and Stock Management Adjacent to Waterways
Drainage Maintenance
Environment Southland is the brand name of Southland Regional Council.