Guideline Index

Chapter 10 - Keeping Nutrients on Farm

10.2 Understand nutrients in the environment

The keys to understanding nutrients are:

As summarised in the following diagram, nutrient stores, transformations and pathways are complex, especially for nitrogen. For more information on the nitrogen cycle, see Chapter 12.1.1 and the Nitrogen Cycle Animation: http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_nitrogen-cycle

Figure 10.2  Phosphorus and nitrogen stores, transformations and pathways - nitrogen specific pathways shown by dashed lines (Gourley & Weaver, 2012)
Figure 10.2 Phosphorus and nitrogen stores, transformations and pathways – nitrogen specific pathways shown by dashed lines (Gourley & Weaver, 2012)

10.2.1 Know the sources of nutrients

Nutrients on dairy farms come from inputs, such as feeds and fertilisers, redistribution or recycling, and, in the case of nitrogen, from nitrogen fixation by legumes.

10.2.1.1 Feeds, fertilisers and fixation

The relative importance of feeds and fertilisers in nutrient budgets shifts with different farming systems:

  • If imported feed is less than 20% of the feed budget, then fertiliser is usually the main source of nitrogen, phosphorus, potassium and sulphur.
  • If purchased feeds are more than 40% of the feed budget, then they tend to be the main source of nutrients.
  • On low-input farms, with little imported feed or fertiliser application, nitrogen fixation can be the biggest source of nitrogen (Department of Primary Industries Victoria (n.d.)b.)

Fertilisers are usually the main source of phosphorus and highly water soluble forms, like superphosphate and Di-Ammonium Phosphate (), are most commonly used in fertilisers. Water soluble phosphorus may be taken up immediately by plants and it may be readily transported by water, but this is dependent on the soil’s ability to adsorb phosphorus (measured by its Phosphorus Buffering Index (PBI) – seeSection 10.2.3.3.

The nutrient concentrations in feeds should be tested as they vary within and between different types. Forages and by-products are especially variable, compared to grains and pellets.

Annual fodder crops can ‘harvest’ surplus nutrients but they may require additional different nutrients and cultivation may increase the risk of losses through leaching or erosion.

10.2.1.2 Redistribution

Nutrients may be redistributed on-farm by stock in urine and dung, by recycling treated effluent and through harvesting and feeding out home-grown feeds such as silage or hay.

The Accounting for Nutrients project showed that cows on conventional farms spend approximately three-quarters of their time in paddocks (especially those close to dairies), 6% in laneways, 9% in yards and 2% in milking sheds. Changing where they spend their time can change where they redistribute nutrients, although cows tend to excrete more in response to certain stimuli, such as entering yards or crossing a creek (Davies-Colley et al, 2004).

Urine patches are sites of high nutrient concentration, especially nitrogen and potassium, which can increase pasture yield and the concentration of nitrogen and potassium in the forage. There is, however, an increased risk of ammonia loss and nitrate leaching from these patches.

Dung is another source of nutrients that can be redistributed over the farm via stock or collected and spread as fertiliser.

Recycling treated effluent onto paddocks can effectively re-use and relocate nutrients. Risks occur when effluent is applied to saturated soils resulting in nutrient-rich run-off, and when the same area continuously receives effluent resulting in high soil nutrient levels. An application of 100mm of effluent can equate to 34 kg of phosphorus and 210 kg of nitrogen (Rivers & Dougherty, 2009).

10.2.2 Understand nutrient stores and transformations

To optimise the productive value of nutrients and assess their risk to the environment, it is important to understand how different nutrients cycle through the soil, plants and environment – e.g. whether they are lost, transformed, or stored. In general, phosphorus, potassium and sulphur may be stored, but nitrogen isn’t. This section covers:

10.2.2.1 Transformations

Nutrients cycle through the environment. That is, they move through the soil, plants, animals and atmosphere; between inorganic and organic forms; and through soluble, insoluble and sometimes gaseous forms. For more information on nutrient cycles, see Chapter 3.3

The observed concentrations of nutrients in waterways may vary with ‘in-stream processes’ such as dilution by high flow rates or deposition in slow-flowing water; or through processes such as adsorption to, or desorption from sediments, and uptake by algae or plants.

Nitrogen is not generally stored in the environment and can be readily lost as water-soluble nitrate through runoff or leaching, or in gaseous forms such as nitrous oxide through denitrification or ammonia through volatilisation. This is one reason why the efficiency with which applied N is converted to produce is relatively low. Nitrogen may also be ‘immobilised’ or stored as organic nitrogen through incorporation in plants, soil organic matter, fungi and bacteria. Detailed analysis of 11 dairy farms found 50% of the nitrogen lost was via leaching and 48% via volatilisation; with 2% by denitrification (Gourley et al, 2011).

As the following graphs show, total nitrogen inputs are related directly to milk production – and to nitrogen surpluses. It cannot be concluded that all surplus N will be uniformly lost to the environment, as an array of factors will influence that including temperature, pH, moisture levels and the amount of organic matter present. However, increasing surpluses do increase the risk of nutrient losses to the environment.

Figure 10.3  Relationship of N inputs to production and surpluses for 41 contrasting dairy farms across Australia - unshaded symbols represent organic dairy farms. (Gourley & Weaver, 2012)
Figure 10.3 Relationship of N inputs to production and surpluses for 41 contrasting dairy farms across Australia – unshaded symbols represent organic dairy farms. (Gourley & Weaver, 2012)

The Accounting for Nutrients project found wide variations in nutrient conversion efficiencies. Conversion efficiencies relate to the proportion of nutrient entering the farm that is converted into saleable product such as milk or meat, or in other words, nutrient outputs divided by nutrient inputs. These wide variations as listed below, suggest that many farmers have scope for improvement:

  • Nitrogen surpluses on Australian dairy farms ranged from 47 – 601 kg N/ha/yr and Nitrogen Use Efficiency ranged between 14%-50%.
  • Nitrogen surpluses averaged 12 grams of N per litre of milk which is similar to those found overseas in Europe, the US and New Zealand.
  • Phosphorus Use Efficiencies averaged 29%, but were very variable, with some exceeding 100% if inputs failed to match losses in product (Gourley & Weaver, 2012 and Gourley et al, 2012).
10.2.2.2 Stores

Phosphorus is stored in the soil with higher concentrations in the surface layer of soil, but it may be present in forms which are inaccessible to plants. Soils differ in their ability to hold phosphorus (as measured by their Phosphorus Buffering Index – see Section 10.2.3.3) and are also affected by their acidity. In acidic soils, phosphorus may become locked up by aluminium and iron, while calcium or magnesium phosphates may form in neutral or alkaline soils.

10.2.3 Know how nutrients are lost

Nutrients are removed from dairy farms via:

  • Produce – milk, hay and livestock,
  • Atmosphere – as gases or as particulate matter (e.g. wind-blown soil),
  • Water and erosion – in surface water or ground-water, and through water erosion from paddocks or stream banks.

Taking action to reduce losses of a certain form can result in increased losses of another form – termed ‘pathway swapping’ – see Section 10.2.3.3. For example, nitrogen may be lost as nitrate, ammonia or nitrous oxides. Restricting one pathway may open another, although the losses may occur at a different rate. Managers may have to set priorities.

10.2.3.1 Produce

Livestock sales usually only account for a small amount of the nutrients exported from a dairy farm in produce. Hay can be a larger source, and it is usually high in potassium and nitrogen. In terms of produce leaving the farm, milk is the largest nutrient exporter on a dairy farm.

10.2.3.2 Atmosphere

Around half the nitrogen present in urine and faeces occurs as ammonia; a form which may be readily lost to the atmosphere. Manure, and effluent treatment, can also result in the release of nitric oxide and nitrous oxide, a potent ‘greenhouse gas’ (Birchall et al, 2008). Wet soils can also be sources of nitrous oxides.

Methane, another greenhouse gas, is produced in the rumen of cows and from the biological treatment of effluent. In terms of greenhouse impact from established dairy farms, the two main contributors are methane from the rumen and nitrous oxide from urine and nitrogenous fertilisers.

Wind erosion is not usually a big issue on dairy farms, although dust from yards and feedpads may be; and would carry nutrients with it.

10.2.3.3 Water and water erosion

Water-borne nutrients may be transported via surface water run-off or sub-surface flows (leaching). Spaces between soil particles such as macro-pores, earthworm holes and decayed roots, provide ‘preferred pathways’ for sub-surface flows, but water will also move through the soil matrix itself. Water may also erode soil particles, removing them and the nutrients within. Erosion may occur in paddocks, along tracks or from stream banks.

Understanding these pathways requires consideration of:

  • Run-off and drainage
  • Nutrient concentrations
  • Connectivity
  • Effluent ponds
  • Pugged soils
  • Sealed surfaces
  • Surface cover
  • Wetlands
  • Riparian buffers
  • Stream-bank erosion

Run-off and drainage from rainfall or irrigation are key drivers of water-borne nutrient losses. Nutrient movement is proportional to water movement. Episodic events (storms) can be a major force, with studies finding 30% and 69% of annual losses occurring in a single storm (Holz, 1997 and Nash & Halliwell, 1999).

The amount of water moving and the rate at which it moves is affected by many factors besides climate, such as soil type, slope, landform, surface cover and sub-surface geology. Different combinations of factors result in different pathways being important – and in variable losses across farms. Some parts of a farm will lose nutrients more readily than others. Areas of higher potential loss are referred to as ‘Critical Source Areas’ or ‘Hot spots’.

Nutrient concentrations are also important. The application of fertilisers, defecation by grazing stock and recycling effluent in paddocks, all immediately increase the concentration of nutrients. Run-off or infiltration soon after will consequently have high concentrations. However, the concentrations measured in paddocks decrease rapidly with time. Phosphorus from most fertilisers has a ‘half-life ’ (the time for concentrations to halve) of less than 10 days (Nash & Halliwell, 1999) and for N it is less than 2 days (Barlow et al, 2007).

Nutrient concentrations due to effluent recycling will be influenced by the ‘depth’ or volume of effluent and the rate and frequency of application.

Trade-offs: pathway swapping.

Applying nitrogenous fertilisers after rain will reduce the risk of N loss as water-borne nitrate. However, to reduce losses of N as gaseous ammonia from volatilisation, it is recommended that fertilisers be applied within a day before expected rainfall or irrigation (Barlow et al, 2007). Reducing atmospheric losses can require a trade-off; increasing the risk of water-borne losses.

The concentration of phosphorus in surface run-off is proportional to the concentration of phosphorus in the surface layers of soil with which it is in contact – and inversely related to the Phosphorus Buffering Index (PBI) of the soil. That is, soils that are high in available phosphorus and have little capacity to hold additional (or ‘buffer ’) phosphorus (low PBI), are most likely to result in high concentrations of phosphorus in surface run-off, as shown in the following graph. The risk of losses occurring increases as soil phosphorus concentrations increase and soil is saturated with phosphorus (Dougherty et al, 2010; Weaver & Wong, 2011; Bolland & Russell, 2010).

Figure 10.4  Runoff P concentrations (Dougherty et al, 2010)
Figure 10.4 Runoff P concentrations (Dougherty et al, 2010)

Critical concentrations of phosphorus for maximum pasture production also vary with the Phosphorus Buffering Index of soils, as demonstrated by the following table.

Table 10.1  Critical P levels (DPIV, 2011)
Table 10.1 Critical P levels (DPIV, 2011)

With regard to environmental impacts, ‘connectivity ’ is another important factor. If nutrient-carrying water flows from a critical source area directly to a water body, then more nutrients will enter that water body, than if the ‘connection’ was via considerable overland flow where sedimentation would remove nutrients. However, connectivity is not just a factor of physical location. Soils that are saturated will have stronger connectivity with adjacent sites than will dry soils that hold water with little run-off. Artificial drainage can increase connectivity and larger storms will have more connectivity than small ones.

Inadequate or poorly maintained effluent ponds can be sources of direct loss of water-borne nutrients to the environment. Problems can include insufficient free-board to handle input surges or insufficient storage capacity to hold effluent when conditions dictate against it being recycled in paddocks (e.g. saturated soils not suitable for irrigation).

Pugged (saturated and compacted) soils have low infiltration rates and hence high run-off. They are also sites of increased nitrogen loss through denitrification and can be liable to erode. If they occur in low-lying areas, where water and nutrients from adjacent paddocks accumulate, they can be critical source areas for nutrients. Draining such areas will transport nutrients as well as water – making management of the drainage water crucial if environmental risks are to be managed.

Sites of heavy stock use which are compacted or sealed (e.g. laneways near milking sheds, yards, feedpads, troughs and gates) have high concentrations of nutrients and run-off rates. Although loaded with nutrients from manure and urine, they are often only a small area of a dairy farm, hence their total contribution to losses may not be as significant as appearances may suggest. Their ‘connectivity’ with vulnerable environments will be another factor in assessing their risk. For more information see; Monaghan & Smith (2012), Lucci et al (2012) and Dougherty & Hossain (in-print).

Cultivated paddocks or those with little surface cover such as emerging fodder crops, are potential sites for soil erosion, subject to features like slope, length of run, rainfall or irrigation intensity and soil type.

Wetlands can be very effective in ‘mopping up’ excess nutrients and preventing them from leaving a farm, but there are a number of caveats about that. Wetlands can be considered as sinks which work well until they are full or saturated, at which time they can then become sources of nutrient. Constructed wetlands are made in a way to permit their periodic cleaning out to avoid that problem. The other main challenge for wetlands is their ability to take flood waters. If floods (when most nutrient movement occurs) simply flow straight over a wetland or run through it very quickly, then they have minimal impact on reducing peak loss events.

Riparian buffers (‘filter strips’ adjacent to waterways) play a similar role to wetlands and face the same performance issues.

If the riparian areas include vegetated stream banks, then that can help stabilise the banks and reduce stream-bank erosion – which can be significant in some soil types. Stream flow-rate is another key factor in stream-bank erosion. Although it is driven by flows and run-off in upstream areas, slowing the flow of water as it leaves farms is one way to help reduce flow-rates in streams; and, in suitable circumstances, reduce stream-bank erosion – see Section 10.5.2.