NSW DPI and soil health

Soil is one of agriculture’s fundamental resources. Without it, agriculture would not exist as we know it. However, Australia’s soils are old and weathered, and some of our agricultural practices have degraded soils so that they become less productive and cause environmental degradation.

Soil health

In recent years there has been increasing interest in the concept of soil health, which considers all aspects of soil, that is, physical structure, chemical components and biological life, rather than looking at each of these separately. A soil does not have to be agriculturally productive to be healthy. However, many agricultural practices can make soils less healthy than they were in their natural state.

By managing structure, nutrients and biology in the soil, farmers can work soils within their capability so that the soils are able to recover from agricultural disturbance without being degraded.

To maintain and improve soil health, farmers need to plan for the long term and:

  • control erosion;
  • manage soil organic matter;
  • manage water and nutrients in the soil;
  • manage their production system so that it doesn’t degrade the soil;
  • improve soil biological life.

NSW Agriculture has a soil health research team at Wollongbar, which is looking at soil health in orchards and developing techniques for improving soil biological life. This team has:

  • undertaken a soil health survey of 15 farms in northern NSW;
  • developed composting technologies for improving soil health in macadamia orchards;
  • surveyed earthworm populations in North Coast agriculture and their benefits to sustainability;
  • carried out a collaborative project investigating soil biological constraints in vertosol soils of the Northern Grains Region, funded by the Grains Research and Development Corporation (GRDC).

In June 2000, NSW Agriculture held a soil health workshop at Wollongbar to look at all the issues involved in soil health. A copy of the proceedings is available from Wollongbar. Further soil health workshops were held in Tamworth in February 2002.

Soil issues

There are several soil issues facing agriculture today:

  • salinity
  • acidity
  • sodicity
  • structural degradation
  • soil biological decline
  • carbon emissions

Soil salinity

Salinity is a significant issue for both soil and water quality in NSW. Increasing salt concentration in soils threatens some of the most productive farmland in NSW.

Saline soils interfere with the osmotic gradient that exists between plants and the soil environment. When the concentration of salt in the soil increases, many plants are unable to take up the water they require and the nutrients that are dissolved in this water. This directly impacts on crop production and can lead to plant death and crop failure.

  • As a soil becomes more saline it will affect the plant species which can be grown in that soil.
  • As more land becomes affected by salinity, the choice of crops and the agricultural productivity declines.
  • In addition to this, increasing salt levels in the soil affect infrastructure in our towns and cities such as concrete/brick foundations, roads and footpaths.

Causes of soil salinity

Salt occurs naturally in all soils. It is the concentration of these salts in the top layers of the soil horizon that directly impacts on agriculture in NSW. Salt is concentrated in soil by water movement in the soil profile. In Australia this is mostly due to rising watertables. Watertables can rise within reach of plants’ roots and the soil surface due to the following:

  • Overclearing of deep-rooted vegetation. When deep-rooted native vegetation is removed and replaced by shallower rooted annual species, more water moves down through the soil and into the watertable. As the watertable rises it brings salt towards the soil surface and the plant root zone.
  • Over-irrigation. Excess water applied as irrigation or lost from irrigation channels, and/or not used by plants, enters the watertable, bringing it closer to the soil surface.
  • Once the watertable is within 2 m of the soil surface, evaporation and transpiration further intensify this problem, drawing more saline water to the soil surface and thus increasing the concentration of salt in the plant root zone.

Area affected

In NSW 1.3 million hectares is at risk from rising watertables. The cost of lost production and damaged infrastructure in the Murray Darling basin alone due to soil salinity is estimated to be $100 million per year.

Acid soils

When soils become acid, some nutrients such as aluminium and manganese become more available, while other nutrients such as phosphorus and molybdenum become less available. This disrupts plant growth, which directly impacts on the productivity of crops and pastures grown in agriculture. The lack of productivity also contributes to:

  • dryland salinity
  • nitrate pollution of ground water
  • phosphorus pollution of streams.

If no action is taken to slow or correct soil acidification, the acid leaches into the subsoil where it cannot be reached with current farming methods. This means the soil is permanently degraded.

Causes of acidity

Some soils are naturally acid due to either the type of rock they are derived from or the regular heavy rainfall that leaches nutrients from the soil. Soils can be made acid by:

  • using acidifying fertilisers;
  • growing legumes;
  • harvesting produce without replacing the nutrients removed in the produce;
  • adding organic matter to the soil.

Area affected

NSW has 13.7 million hectares of agricultural land seriously affected by acid soils, with another 5.7 million hectares at risk. The cost of soil acidity in lost agricultural production is estimated to be between $90 million and $225 million per year.

Acid sulfate soils

Acid sulfate soils are caused by the oxidation of coastal muds that contain iron pyrite. When exposed to air the pyrite produces sulfuric acid and iron, and these have severe impacts on soil and water quality, as well as plant and animal life.

Sodic soils

Sodic soils are the 'sleeping giant' of soil management problems facing agriculture, as almost half of NSW soils are sodic and tend to degrade under bad management.

Sodic soils are so-called because they have sodium ions attached to clay particles. When these soils are wet, the particles disperse and move away from each other, which is seen as cloudy water. When the water evaporates, these individual particles settle in a solid mass, causing waterlogging and hard-setting soil crusts.

It is difficult for roots to move through these soils. Gypsum can help ameliorate these soils, but this is not economical for large areas.

Structural degradation

Soil structure is the arrangement of individual soil particles - sand, silt, clay and organic matter - into aggregates or peds. Peds store moisture and nutrients, while the spaces between them allow water and air to flow through the soil. Soil structure can be affected by:

  • cultivation, which breaks up the peds;
  • compaction, where the weight of animals and machinery presses the peds together and stops air and water flow;
  • heavy rain, which can smash the surface soil causing soil crusts and erosion.

Structural degradation in the Murray–Darling Basin is estimated to cost $145 million annually in lost production.

NSW Agriculture has worked on ways to reduce structural degradation, including:

  • conservation tillage, where the soil is not cultivated;
  • crop–pasture rotations;
  • more suitable tillage machinery;
  • precision agriculture systems;
  • the use of machinery tracks on cropping soils so that only a small area of the paddock is compacted;
  • the return of organic matter to the soil.

NSW Agriculture has produced several SOILpak advisory guides to help farmers minimise soil degradation and improve soil structure.

Soil biology

Compared with other soil facets, little is known about soil organisms and how their functions affect the nature of soil. However, it is recognised that they play a vital role in maintaining soil health and determining many soil characteristics. These organisms include earthworms, nematodes, protozoa, fungi and bacteria.

Living soil organisms will:

  • release nutrients required for plant growth;
  • retain nutrients;
  • suppress soil diseases and pests;
  • improve and maintain soil structure;
  • degrade harmful chemicals;
  • promote plant growth.

The linkages and connections between organisms in the soil are thought to be incredibly complex. The 'food web' should be considered as one part of a larger ecological food web covering the whole landscape. The interconnectedness and complexity of the soil 'food web' means any disruption to one of the functional groups or to a species within the group will have far-reaching and significant effects, even beyond the soil sphere itself.

Any appraisal of the health of a soil must take into account the wellbeing and interactions of the living communities that exist within it.

Nutrients

Soil organisms break down organic matter, releasing nutrients for uptake by plants and other organisms. The nutrients stored in the bodies of soil organisms are also released when the organisms die or are preyed upon by other organisms, thus preventing nutrient loss by leaching.

Soil organisms may also excrete nutrients in plant-available forms. Some organisms, such as vesicular arbuscular mycorrhiza (VAM), actively help plants to take up nutrients such as phosphorus and zinc.

Diseases and pests

Soil organisms can protect plants from diseases and pests in several ways:

  • Healthy populations of microbes compete with pathogens for nutrients and can suppress the severity of plant disease.
  • There are predatory organisms that will keep pest species such as nematodes and fungi in check; protozoa engulf fungi and bacteria, while predatory nematodes eat root-feeding nematodes.
  • Beneficial fungi provide a physical barrier to root-feeding pests by wrapping the roots in a network of threads (hyphae).
  • Other soil organisms secrete chemicals that ‘hide’ plant roots from their attackers.

Soil structure

Soil organisms help to maintain the stability of soils as well as enabling proper aeration and drainage. For example, excretions from microbes act as ‘binding agents’, which maintain soil structure, and earthworms are important in soil mixing. These help to prevent erosion, waterlogging and compaction problems.

Plant growth

In addition to the previous points, which will result in better plant growth, some soil living organisms secrete substances that act as growth promotants.

Agriculture and soil biology

The following factors are important in ensuring the maximum benefit for the organisms that live in the soil:

  • minimising habitat disruption;
  • maintaining adequate food (organic matter);
  • aeration;
  • water;
  • soil pH between 5 and 8.

Many agricultural practices can harm the organisms that live in the soil. There are management actions that can be employed to minimise harmful practices:

  • Using rotations will provide a more diverse food source and thus a more diverse group of soil organisms. It can also break an existing pest/disease cycle.
  • Reducing tillage will minimise habitat disturbance, maintain soil structure and increase organic matter (food) for soil organisms and encourage a more diverse group of soil organisms.
  • Liming to keep the soil pH in a range favoured by plants will benefit soil organisms too. Be careful when using fertilisers that have an acidifying effect, as this will decrease soil organism numbers and diversity.
  • Retaining stubble will provide organic matter for soil organisms and encourage diversity. The fungi that benefit from stubble left on the soil surface will prey on nematodes. Incorporating stubble will increase its breakdown and may reduce habitat for some pathogens.
  • The non-target effect of various agricultural chemicals is hard to determine, as there have been so few studies on the organisms that live in the soil. Obviously fungi, including beneficial ones like VAM, will be susceptible to fungicides, and there is growing concern over the effects of copper-based products on soil organisms such as earthworms.

Carbon emissions

The cultivation of agricultural soils releases carbon dioxide into the atmosphere, contributing to the world’s greenhouse gas problems. Agriculture may be able to play a part in reducing greenhouse gas emissions by storing carbon so that it is not released into the atmosphere. This process is known as carbon sequestration and can occur in several ways:

  • Retaining stubble rather than burning it can sequester 70–90 kg carbon/ha/year.
  • Not cultivating soil can sequester up to 135 kg carbon/ha/year in high-rainfall areas.
  • Using grain legumes in rotations can sequester up to 150 kg carbon/ha/year.
  • Using pasture rotations, particularly clover, can sequester 250 kg carbon/ha/year more than cropping.

Concerns

However, there are several concerns about sequestering carbon, including:

  • disease build-up in stubbles;
  • cost of conversion of farm equipment to handle standing stubble;
  • low returns from pasture-fed animals compared with returns from crops.