3- Build and maintain soil nutrients to improve soil fertility and health in all pasture zones
Nutrients are a key element to growing good pastures. It is also important to ensure that the pasture currently being grown is suitably used by increasing stocking rate where appropriate, before growing surplus pasture.
This procedure covers:
- building and maintaining soil nutrients
- soil health
- organic matter.
Guidelines for building and maintaining soil nutrients
Pasture growth and quality depend on healthy soils and adequate nutrients
Soil nutrient availability is the factor most commonly responsible for lower than expected pasture growth and quality across much of southern Australia. The following are the most common causes:
- nutrients removed in the forms of meat and other saleable products are not replenished
- soils are inherently deficient in one or more nutrients
- nutrient imbalance limits the availability of one or more nutrients
- plants cannot access nutrients due to physical or chemical barriers
- one or more nutrients are present at toxic levels
While economic factors play a role in soil nutrient rundown, so does the perceived lack of a response from traditional applications of fertiliser. The following factors should be considered when applying nutrients to maximise the benefits of application:
- Nutrient supply – failure to supply all limiting nutrients will reduce the potential response from fertiliser applications. For example, phosphorus applications in sulfur-deficient soils will not have any effect unless sulfur is also supplied.
- Timing of application – poor timing of application can greatly reduce response. For example, applying phosphorus to wet and cold soils, as is commonly carried out in southern winters, may waste some of the phosphorus applied because it is rapidly turned into non available forms.
- Trace elements – supply of essential trace elements, such as molybdenum, may be necessary. Molybdenum is essential for root nodulation in legumes. When it is deficient, nitrogen-fixing capability is reduced, and pasture growth will slow.
- Application of lime – applying lime is the only recognised method of correcting soil acidification.
Balance major and micro-elements for productive pastures and healthy soils
Two major groups of nutrients are needed for optimum pasture production:
- major or macro-elements, such as phosphorus (P), sulfur (S), nitrogen (N), potassium (K), calcium (Ca), and magnesium (Mg)
- micro-elements, such as copper (Cu), molybdenum (Mo), boron (B) and zinc (Zn)
All these major and micro-elements are important for productive pastures and healthy soils. To get the most out of pastures and nutrients, it is necessary to identify key nutrient requirements and understand how they move through the soil, pasture and animals.
Take care with fertiliser application rates and placement to reduce product losses
Inappropriate nutrient application rates or placement of nutrient may result in:
- desirable pasture species may suffer poor vigour or death
- desirable introduced perennial grass species need high phosphorus levels to achieve their potential
- many native perennial grasses will decline under competition from clover and annual species when phosphorus levels are raised
- economic loss and environmental damage may occur through leaching of applied nutrients into aquifers (underground waterways) or through run-off with soil particles into streams, rivers and dams.
To control these risks, test the soil nutrient status of all mapped pasture zones. Carefully analyse these results to identify the range and extent of actions required to achieve the best possible soil health and nutrient status. Use Tool 2.8 as a guide to analyse soil test results.
The P-Tool - a decision support tool
The P-Tool (available on the MLA website) outlines five easy steps that allow producers to understand the value of soil testing and how to use soil test information to plan fertiliser and livestock investments. The information package provides a better framework for understanding and planning the use of phosphorus (P) fertilisers.
The tool is intended to assist producers in determining suitable levels of P-fertilisation for temperate pastures grazed by beef cattle on acid soils in southern Australia. However, fertiliser decisions are made by the user – not the tool – and the tool's purpose is primarily to support your thinking and fertiliser decisions.
Once a decision is made to apply nutrients, complete the following:
- Increase the nutrient application and stocking rate over a three- to five-year period to manage cash flow and feel confident that a higher grazing intensity is sustainable over time.
- Use the guidelines in Tool 2.9 to determine the quantity and cost of nutrients required to lift all pasture zones above minimum limits and correct the balance and any identified toxicities.
- When substantial financial capital is to be invested, establish priorities that will provide the biggest gain from the investment in nutrients. Use relative assessments of land capability, water use efficiency, current pasture composition and potential productivity to set priorities (see Procedure 1, Procedure 2 and Procedure 4 and guidelines in Tool 2.9).
- Complete partial budgets or a whole farm analysis to determine the marginal return on investment; then revise the priority list based on the value and economic benefit from an investment in a nutrient application program.
- Review herd structure and stocking numbers against investments in capital applications of nutrients to ensure that increased pasture growth and quality is being used to gain the highest economic returns from the investment.
- Once the initial ‘capital’ application has been completed, apply phosphorus and other nutrients required annually to keep nutrient levels within economically justifiable limits (see Tool 2.9).
- Monitor nutrient levels to assess the need for future ‘capital’ applications in an annual maintenance program. Re-test every two to three years at the same time of year and avoid collecting samples when soil is likely to be cold and waterlogged.
The presence of soil biota (living organisms) generally improves soil health and structure.
Soil invertebrates, such as earthworms, play a major role to break down plant material into smaller pieces more prone to microbial attack, and to stabilise soil structure by building networks of soil macro pores. Earthworms are a well recognised indicator of soil condition. They move up to the soil surface when organic matter (their food source) and moisture are plentiful. Earthworms are a primary incorporator and degrader of decaying pasture and dung.
Other soilborne life (eg ants, mites, other insects and fungi) may be more prevalent in some soil types due to the harshness of the environment and may have a similar function to earthworms. In general, the more abundant and diverse the soil biota, the more fertile the soil.
Earthworms can substantially improve the quality and quantity of pasture production. Feeding and burrowing activities increase the cycling of soil nutrients and organic matter and are beneficial to soil structure.
Earthworms are counted in spadefuls of soil taken from the top 10cm of a 25cm2 block. Samples should only be collected when soil moisture is high (generally in late winter or early spring) as worms will retreat to greater depths when surface drying has commenced. Separate soil carefully to avoid splitting the worms and always count the worms twice to confirm the number.
Less than 10 earthworms per block is considered low, 15 moderate, and more than 20 represents an abundant population. When soil is moist, most worms will be found near the surface in the root zone of the pasture.
For further information see the MLA Tip & Tool: Increasing earthworms in pastures.
Organic matter contributes to a variety of biological, chemical and physical properties of soil. It is essential for good soil health and plays a role in:
- providing nutrients and habitats for organisms living in the soil
- providing energy for biological processes
- contributing to soil resilience
- binding soil particles into aggregates improving soil structural stability
- enhancing water holding capacity of the soil
- moderating changes in soil temperature
- providing resilience against pH change
- storing many of the key nutrients, especially nitrogen and potassium.
The amount of carbon (the measure of organic matter) in a soil depends on a range of factors:
- climate – in similar soils under similar management, carbon is greater in areas of higher rainfall and lower in areas of higher temperatures
- soil type – clay helps protect organic matter from breakdown; clay soils in the same area under the same management tend to retain more carbon than sandy soils
- vegetative growth – the more vegetative production, the greater the inputs of carbon
- topography – soils at the bottom of slopes generally have higher levels of carbon because these areas are generally wetter with higher clay contents; poorly drained areas have much slower rates of carbon breakdown
- tillage – tillage will increase carbon breakdown.
What to measure and when
Monitor the following where applicable:
- extractable soil phosphorus status (Olsen, Bray or Colwell)
- phosphorus buffering index (PBI) – used to evaluate Colwell test results and guide level of maintenance and capital phosphorus applications
- soil acidity (pH) and aluminium level
- sulfur and potassium levels
- exchangeable sodium percentage
- electrical conductivity (EC) – a measure of soil salinity
- molybdenum to copper ratio
- cation exchange capacity – an indicator of the likely response to liming
- amount of phosphorus applied as annual maintenance application (kg P/ha) – measured annually
- phosphorus and nitrate content in streams.
- The following MLA publications are available from www.mla.com.au/publications or by phoning 1800 023 100:
- Towards Sustainable Grazing – The professional producer’s guide
- Tips & Tools: Managing soils to keep them healthy and productive
- Tips & Tools: Managing deep drainage
- Tips & Tools: Managing ground cover to reduce run-off and water loss
- Tips & Tools: Increasing earthworms in pastures
- Pasture health kit
- Why soil organic matter matters – CSIRO www.csiro.au/resources/soil-organic-matter.html
- Northern Rivers Soil Health Card www.dpi.nsw.gov.au/agriculture/resources/soils/testing/health-card