Nutrients are a key element to growing good pastures. It is also important to ensure that the pasture currently being grown is suitably utilised by increasing stocking rate where appropriate, before growing surplus pasture.
Guidelines to building and maintaining soil 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 are unable to 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 sulphur-deficient soils will not have any effect unless sulphur 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, will waste most of the phosphorus applied because it is rapidly turned into non-available forms.
• Trace elements: A supply of essential trace elements, such as molybdenum, may need to be included. 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. However, results from liming pastures are often inconclusive, with no clear link between the application of lime and pasture composition.
Two major groups of nutrients are needed for optimum pasture production:
• Major or macro-elements, such as phosphorus (P), sulphur (S), nitrogen (N) and potassium (K); and
• Micro-elements, such as copper (Cu), molybdenum (Mo) 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 fertilisers, it is necessary to identify key nutrient requirements and understand how all these nutrients (particularly the macro-elements) move through the soil, pasture and animals.
Take care to avoid inappropriate application rates and/or placement of fertiliser as:
• 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.
Once a decision is made to apply fertiliser, complete the following:
1. Increase the fertiliser application and stocking rate over a three to five year period to manage cash flow and gain the confidence that a higher grazing intensity is sustainable over time.
2. Use Guidelines for pasture nutrient applications 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.
3. When substantial financial capital is to be invested, establish priorities that will provide the biggest gain from the investment in fertiliser. Use relative assessments of land capability, water use efficiency, current pasture composition and potential productivity (Guidelines for pasture nutrient applications ) to establish priorities.
4. 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 fertiliser application program.
5. Review herd structure and stocking numbers against investments in capital applications of fertiliser to ensure that increased pasture growth and quality is being utilised to gain the highest economic returns from the investment.
6. Once the initial ‘capital’ application has been completed, apply phosphorus and other nutrients required annually to keep nutrient levels within economically justifiable limits.
7. 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.
Soil health
The presence of soil biota (living organisms) generally improves soil health and structure.
Soil invertebrates such as earthworms play a major role in breaking down plant material into smaller pieces more prone to microbial attack, and in stabilising 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 soil-borne life (eg ants, mites, and 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
The benefits of earthworms are well established – they 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 is considered low; 15 a moderate level and more than 20 per block represents an abundant population. When soil is moist, most worms will be found near the surface in the root zone of the pasture.
What to measure and when
Complete comprehensive soil and stream water testing and analysis by National Association of Testing Authorities (NATA) accredited laboratories
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 applications of phosphorus;
• Soil acidity (pH) and aluminium level;
• Sulphur 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;
• Calcium to magnesium ratio – an indicator of soil structure stability;
• Amount of phosphorus applied as annual maintenance application (kg P/ha) – measured annually;
• Phosphorus and nitrate content in streams.
