GMP6: Nutrient and Cost Analysis
Reducing Phosphorus Runoff with Nick Saville-Wood
| Phosphorus management was assessed for Nick Saville-Wood’s Kaharoa farm. The analysis quantifies the estimated reduction in phosphorus runoff due to lowered soil Olsen P levels and makes qualitative comment on additional reductions in P loss from other mitigation measures. Nitrogen losses are not assessed |  |
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The subject farm has a 2016 ‘current state’ OVERSEER file prepared for BOPRC as part of a ‘Nutrient Management Plan’ or NMP. While the NMP has a primary focus on nitrogen, it also addresses P losses, noting:
- The P loss is 1.4 kg per Ha per year across the whole farm. There is no cropping as part of the long-term system. Grass to grass will be used for renewal. Soil damage is minimised as much as possible. The farm has had high historical Olsen P levels so now a lot less than maintenance is applied, but monitored over time.
- The farm has had drainage works done to slow down overland flow pathways and has been part of the P detention bund project. This work that was done in conjunction with the regional council has greatly reduced surface runoff and erosion as a result.
To assess the effect of lowering Olsen P levels over time, the farm’s OVERSEER file was reviewed. The pastoral block soil Olsen P levels were edited to assumed past and future levels. Is also assumed that the current Olsen P soil level is representative of the pastoral blocks. The OVERSEER version 6.2.3 results are as
| Past | Current | Future | |
| Olsen P, µg/ml | 60 | 46 | 25 |
| Farm P loss, kg P/ha | 1.7 | 1.4 | 0.9 |
| Comment | The property was once part of a larger dairy farm with elevated Olsen P levels. | Based on a recent soil test | Assuming the mid-point of the 20-30 agronomic optimum |
Click on the images below
The farm currently has very low P fertiliser inputs of 3 kg P/ha as part of an ongoing effort to mine down soil P levels from the previously elevated levels when it was part of a larger dairy farm. The assumed past level is 60 units (µg P/ml of soil) which is above the 40-45 optimum for a high producing dairy farm on pumice soils. However, Redding et al (2006) found that ‘More than 66% of the dairy sites sampled had Olsen P concentrations greater than the optimum range (35-45 µg P/ml)…’ based on a survey predominantly within the Lake Rotorua catchment. Further, a check of a 2001-2004 OVERSEER file for a nearby (but not adjacent) dairy farm with the same soil type also had an Olsen P level of 60 units. Therefore, the assumed past Olsen P level of 60 units is deemed relevant for comparative purposes.
The assumed future Olsen P level of 25 units is possibly conservative, when normally a range (say 25-30) would be recommended. The farm stocking rate is 14.6 RSU/ha which is higher than average for the locality but arguably reflects a combination of good rainfall, soil fertility and management. However, an Olsen P target of 25 is still towards the upper end of the 15-30 range suggested as a consensus recommendation by local farm consultants and reported in McDowell (2010).
DairyNZ (FarmFact 7-12) advises that ‘If no fertiliser P is applied the Olsen P levels will drop by 1-2 units per year’. Assuming the upper end of 2 units decrease per year, it will still take at least 10 years to reach an Olsen P target of 25 units. As DairyNZ further notes, a ‘soil testing monitoring program is required so that a maintenance P fertiliser program can be commenced once the economic optimal Olsen P range is reached’.
Critical Source Area Phosphorus mitigations
CSA mitigations include:
- Shifting stock (especially cattle) away from ephemeral waterways prior to forecast heavy rain
- Broadening ephemeral flow paths and maintaining a good pasture sward
- Two detention dams to temporarily store runoff and slowly release over 2-3 hours, thus reducing the erosive force of the downstream flow. Heavy soil particles (with adsorbed P) may settle out during this time (noting the owner’s intent to adopt the ‘detention bund’ outlet design with 2-3 days detention time, in order to improve the level of P mitigation)
- Installing multiple cut-off drains on the main farm race to divert accumulated runoff into paddocks as a succession of relatively small flows where the pasture sward can slow and possibly filter runoff
It is difficult to assess the overall efficacy of these CSA mitigations. This is expected to be possible once Ballance’s ‘MitAgator’ tool becomes available. A broader analysis by Dr Rich McDowell indicated that ‘…across 14 catchments nationally, the focus of mitigations in critical source areas (CSAs) decreased P losses by about 40% costing < 2% farm EBIT compared to “blanket” approaches that did not target CSAs and reduced P losses by 48%, but cost 12% of farm EBIT (summarised by McDowell, 2017).
Cost analysis
The agronomic optimum of an Olsen P level of 25 for the pumice soils on the property assumes that current product prices and input costs, there is no economic benefit from maintaining a higher level of soil P in order to grow more pasture, increase carrying capacity and lift profits. The actual agronomic optimum will of course vary between location, farm type and production systems.
On this basis, the mining of soil Olsen P levels will have two economic benefits: (a) the saving in fertiliser costs during the period in which reduction in soil Olsen P is assumed to occur and (b) the ongoing savings in fertiliser from a lower maintenance fertiliser requirement at the lower levels.
The OVERSEER 6.2.3 analysis for the property indicates that at the a soil Olsen P level of 46, the main 16.8ha pastoral area property has a maintenance P requirement of 50kg P/ha/year. This could be supplied by an annual application of 555kg/ha of super phosphate (NPKS of 0 9 0 10.5). However, at the target soil Olsen P level of 25, the maintenance P requirement is only 39kg P/ha/year – a reduction of 11kg P/ha/year. At the current price of superphosphate from Ballance at $309/t, plus the cost of cartage and spreading, this equates to an annual reduction in superphosphate of 122kg P/ha/year and an annual cost saving of $46/ha/year. Based on the latest Beef + Lamb New Zealand Survey data for Class 5 (Intensive Finishing) farms in the Northern North Island, adjusted for managerial salaries, this saving would represent a 9% increase in annual operating profit (“EBIT”), currently forecast at $521/ha for the 2016/17 year.
In the “mining” phase, the economic benefit is essentially the difference between what is being applied (currently 3kg P/ha/year) and the maintenance requirement of the target soil Olsen P level (39kg P/ha/year). At the current price of applied superphosphate, the annual cost saving while P levels are being mined for the property is $149/ha/year.
On the basis of the cost efficacy of addressing CSAs on farms presented by McDowell (2010), which incorporates the economic impact of mining elevated soil Olsen P levels, the net cost of addressing CSAs on a property like the Saville-Wood’s would be expected to be in the vicinity of $10.40/ha/year (2% of $521/ha).
Key risks
There are production risks related to undershooting the optimum soil Olsen P levels. This risk can be reduced by regular and consistent soil testing, and reviewing the farm’s P fertiliser inputs and production levels (actual and targeted). Although of limited value on a small farm, consideration could be given to differentiating the OVERSEER block set up based on P loss risk, particularly slope, rather than N management (as is usual).
Some of the CSA mitigations require ongoing management (e.g. pre-emptive shifting of stock away from the ephemeral watercourse) and/or ongoing maintenance (e.g. cut-off drains). Without ongoing vigilance, the efficacy of these CSAs would diminish.
OVERSEER P mitigation comment
It is noted that OVERSEER is limited in its ability to assess P loss from critical source areas – reasons include: (i) OVERSEER is not spatially explicit; (ii) most CSA mitigations are not specifically modelled within OVERSEER although some may be indirectly captured by OVERSEER’s assumptions of GMPs being followed; (iii) much runoff is generated on neighbouring properties and therefore outside the scope of OVERSEER. Despite these limitations, a review of the OVERSEER P model (Gray et al., 2016) found it could predict pastoral P losses reasonably well, although numerous recommendations for improvements were made.
Resources
McDowell 2010 P report:This includes an expert consensus recommendation for Lake Rotorua catchment farms of economic optimum Olsen P levels:
- Dairy farms: flat to rolling land is 40-45 mg/L and for steeper areas 35-40
- Drystock farms: range varies depending on production targets but generally 15-30.