Building and maintaining soil fertility
MLA & AWI July 16, 2024
Building and maintaining soil fertility is one of the most cost-effective investments than can be made in a grazing enterprise. Many Australian soils are naturally inherently low in nutrients, these nutrients can be removed in animal products (meat and wool) and hay, silage and grain.
Laboratory soil tests, plant tissue tests and the use of fertiliser test strips can all assist in making sure the right amount of the correct fertiliser is being applied to meet pasture growth objectives. The decisions relating to fertiliser application (such as the type that most suits an operation and the rates that might be used) are essentially about pasture growth and maximising profit. This module covers these decision-making processes.
Poor soil nutrient availability contributes to low pasture growth, low levels of desirable grasses and legumes and poor feed quality across most of the high-rainfall and sheep-wheat zones of Australia. Without gathering some objective information, it is difficult to assess the nutrient levels of the different soil types or management units on any given property. There are three ways to get this information:
- A laboratory soil test will identify if the macro elements (nitrogen [N], phosphorus [P], potassium [K] and sulphur[S]) are limiting pasture growth. Nitrogen is mobile in the soil and readily utilised by plants, so changes from season to season depend on plant growth and organic matter and legume breakdown. Soil tests also provide information about soil health (acidity, salinity, sodicity, etc. and whether a soil might respond to lime, gypsum or added nutrients in fertiliser). Guidelines for submitting soil samples for testing are provided in the Taking a soil sample for laboratory testing tool and the Interpreting soil tests tool offers guidelines to understanding the test results.
- Plant tissue tests are more effective than soil tests in assessing whether trace elements are limiting pasture growth. Trace elements include molybdenum (Mo), copper (Cu), zinc (Zn) and boron (B). Experienced local farmers, agronomists or veterinarians will know if trace element deficiencies are likely in their district. To assess the trace element requirements for pasture growth, take a sample of the most dominant species in the paddock during spring and submit samples to a reputable laboratory. Contact the laboratory or advisor to get the correct sampling technique for the trace element being tested.
- DIY fertiliser test strips are particularly useful if soil test results show marginal results for certain elements. Fertiliser test strips can directly assess the likely response from fertiliser applications by laying out test strips in a paddock. This approach can be used for any fertiliser type and is suited to situations where deficiency symptoms are not highly characteristic or when testing non-traditional fertilisers (e.g. chicken manure). The Fertiliser test strip tool offers guidelines for setting up and analysing your own test strips.
Identifying specific soil health problems and assessing the need for remedial action or management are addressed in MMFS Module 6 Healthy Soils.
Understanding pasture response to nutrients
Two examples of how much extra pasture can be grown, when nutrient deficiencies are corrected, are shown below for an introduced pasture at Hamilton, Victoria and a native pasture at Orange, NSW.
The effect of fertiliser on average monthly growth rates at Hamilton, Western Victoria (winter rainfall area) and Orange, Central Tablelands NSW (uniform rainfall area).
Source: Hamilton data simulated from GrassGro; Orange data from NSW PROGRAZE®.
Phosphorus and native pastures
Native pastures containing some legumes (e.g., clover, lucerne, serradella, medic) will respond to low rates of phosphorus fertiliser. However, native species can decline under competition from clover, annual grasses or broadleaf weeds if fertility levels are raised and the additional feed not utilised. For native pastures, about 20% legume content in spring is the recommended maximum level.
For native grass pastures in high conservation areas or where an increase in native species diversity is desired, do not apply fertiliser or legume seed as both will reduce the conservation value.
Phosphorus and introduced pastures
Improved (also called exotic or introduced) pasture species have a higher requirement for nutrients than native grasses so fertilising paddocks with these species will give a greater response and is essential for optimum production.
Generally, it is the legume component of the pasture which gives the immediate production and feed quality response to phosphorus fertilisers, such as superphosphate, if there are no other limiting factors such as acid soil. Legumes fix nitrogen, which becomes available to the grasses when the legume roots die off.
Nitrogen fertiliser
Some producers rely on legumes to supply nitrogen to grasses in their pasture but applying nitrogen fertiliser to supply additional nitrogen is economical in some situations. Review the Guidelines for fertiliser application to assess whether nitrogen fertiliser will be profitable in an enterprise.
To fix adequate nitrogen to support the grass component of a pasture, legumes must be adequately nodulated and be capable of fixing nitrogen in the quantities required the by grasses. The scoring system shown below can be used to assess legume nodulation for plants growing for a minimum of 12 weeks in paddock situations.
Source: Yates RJ, Abaidoo R and Howieson JG (2016) Field experiments with rhizobia in Working with rhizobia.
Legume roots with poor nodulation (few, small nodules that are white and not pink inside). Check for soil acidity and trace elements.
Source: photo on right – Jo Powell, NSW LLS
Protect the environment
Ensure the proposed fertiliser application will not adversely impact on the environment. Given the prevailing conditions in a paddock (e.g., proximity to drainage lines, amount of bare ground, etc.) nutrients can be easily lost to erosion, run-off or leaching beyond the root zone.
The most common issues caused by poor or over-application of fertilisers are:
- Groundwater pollution: Nitrate leaching through the soil can present a serious health hazard and contributes to soil acidification. When high rates of nitrogen are used or where clover–grass pastures fix substantial nitrogen, especially on sandy or permeable soils, inevitably some nitrate is leached and may enter groundwater if there is a water table. If this groundwater is used for domestic supplies, the leaching presents a serious health hazard. To reduce the risk of leaching, apply nitrogen fertiliser in small amounts frequently rather than all at once.
- Eutrophication: The enrichment of water by the addition of nutrients. The extra nutrients encourage the growth of algal blooms, particularly in stagnant water. Blue–green algae can produce toxins poisonous to animals, including humans. For algae to grow, phosphorus must be present in the water above a certain level. Phosphorus can be introduced into waterways in run-off from pasture, forests and fertilised land, and in drainage from irrigated land and urban areas. These sources, representing most of the total run-off, normally contribute low concentrations of phosphorus and are referred to as diffuse or non-point sources. Point sources, such as sewage effluent and drainage from dairies and feedlots, contribute smaller flows but contain much higher concentrations of phosphorus. These are frequently found to be the sources for most of the phosphorus found in waterways. To reduce the risk of contributing to phosphorus run-off, don’t top dress dams, streams or swampy areas, or bare ground, and avoid topdressing when heavy rain is expected. Maintain good groundcover around dams and streams.
- Soil acidity: There are three major acidifying processes in agricultural systems:
- addition of nitrogen to the soil by fertiliser or fixation of atmospheric nitrogen, followed by loss of nitrate from the soil due to leaching or run-off
- production of organic acids from decomposing organic matter
- removal of alkaline products such as hay from the soil.
Several actions can be taken to lower acidification rates:
- Use the least acidifying fertiliser available (e.g. use urea rather than ammonium sulphate).
- Incorporate stubbles into fallow to minimise net nitrification.
- Sow early to maximise the opportunity of the crop to recover soil nitrate.
- Use perennial, deep-rooted plants that can rapidly absorb mineralised nitrate at the start of the growing season and maintain low soil nitrate levels throughout the year.
- Use deep-rooted crops.
- Minimise water percolation below the root zone.
- Avoid excessive irrigation.
- Minimise removal of product from the soil. To prevent acidification, apply 55–60 kg of lime for every tonne of lucerne or clover hay removed; 35 kg of lime per tonne of grass hay removed; 22 kg of lime per tonne of cereal hay removed; and 3kg of lime per tonne of cereal grain removed.
- Minimise manure removal from pastures, preferably leaving manure where the animals graze.
- Feed hay on the paddocks where it is cut.
- Use cropping rotations to minimise excessive accumulations of soil organic matter under pasture.
4. Cadmium (Cd) build-up: Cadmium is present in tiny amounts (less than 0.5mg/L) in the soil, and in larger amounts in rock phosphate. Plant uptake of cadmium is small, but when livestock graze plants containing cadmium, the cadmium accumulates in offal and may reach high concentrations. This is a severe problem on the sandy grazing soils of South Australia and Western Australia but is not a problem on soils with even a low clay content.
Use the Guidelines for fertiliser application to quickly assess any potential nutrient losses.
Economics of fertilising pastures
More than 100 producers across Victoria, NSW, Tasmania and SA who participated in the Grasslands Productivity Program (GPP) from 1993 –1997 evaluated the economic benefit of correcting nutrient deficiencies (see table 7.1).
The economic benefit of correcting nutrient deficiencies for producers who participated in the Grassland Productivity Project (GPP) from 1993 – 1997.
Parameter
GPP (Vic, NSW, Tas, SA)
Victorian GPP sites
NSW GPP sites
Change in stocking rate
Data unavailable
+28
+34
Change in gross margin (%)
+25
+25
+30
$ Return per $ spent
1.89
2.17
2.28
Source: Grasslands Productivity Program – final report to members (1998) prepared by J. Court. The Grasslands Society of Southern Australia.
Pasture types ranged from improved perennial to annual to native. The average return was $1.89 for every dollar spent on fertiliser and livestock. Although livestock gross margins and fertiliser prices have changed since then, the response to fertiliser and resulting ability to increase stocking rate from the program will still hold true. Gross margins from 2016 – 2020 averaging approximately $35/DSE for wool sheep (source: Victorian Farm Monitor Program) make investing in fertiliser attractive.