New Zealand's agricultural sector, a cornerstone of the economy, relies heavily on precise nutrient management to balance productivity and environmental stewardship. A recent master's thesis from Massey University has shed light on how farmers are faring in one critical area: phosphorus fertiliser balance. Titled "Analysis of the New Zealand agricultural phosphorus budget," the study by Theané de Klerk reveals that farmers across the country are largely aligning their phosphorus (P) applications with soil needs, maintaining stable fertility levels nationwide.

Phosphorus, an essential macronutrient, is vital for plant growth, root development, and energy transfer in crops and pastures. In New Zealand's pastoral-dominated landscape—where dairy, sheep, and beef farming cover millions of hectares—superphosphate has been the go-to fertiliser since the late 19th century. However, overuse can lead to soil legacy phosphorus buildup, increasing risks of runoff into waterways, contributing to algal blooms and eutrophication. Underuse, conversely, depletes reserves, threatening yields. De Klerk's work provides a timely snapshot for 2021/2022, showing a nuanced picture of balance amid evolving regulations and best management practices (BMPs).

🌱 The Study Behind the Balance

Conducted as part of a Master of AgriScience at Te Kunenga ki Pūrehuroa Massey University, de Klerk's research was funded by the Fertiliser Association of New Zealand (FANZ) and supervised by experts including Associate Professor Lucy Burkitt, Dr James Hanly, and Professor Peter Tozer from Massey's School of Agriculture and Environment, alongside Dr Katrina Macintosh from DairyNZ. This collaboration underscores Massey's pivotal role in bridging academia, industry, and on-farm application.

"While my research only represents a single-season snapshot, the findings are encouraging for farmers, showing that the shift towards sustainable nutrient management has been worthwhile," de Klerk noted. The thesis represents the most comprehensive national analysis to date, updating earlier work like Hedley et al. (2011) to reflect current land use shifts—dairy expansion (+4%) and sheep/beef contraction (-10%).

Methodology: Building a National Picture

To quantify maintenance requirements—the P needed to sustain target soil Olsen P levels (typically 20-30 mg P/L, adjusted for soil type and topography)—de Klerk developed farm typologies. For dairy (1.7 million ha), regionally representative farms incorporated DairyNZ Dairybase data on production, feeds like palm kernel expeller (PKE), effluent management, and fertiliser rates. Sheep and beef (6.9 million ha) used Beef + Lamb New Zealand (B+LNZ) benchmarking across eight classes, factoring stocking units (SU/ha) and slope.

The Overseer nutrient budgeting model (version 6.5.9) simulated balances, inputting climate (NIWA), soils (NZ Society of Soil Science, anion storage capacity), and Olsen P from 103,000 FANZ samples. Outputs included products (milk solids, meat, wool), losses (runoff dominant at 0.1-4 kg/ha, leaching minor), and pool changes (organic/inorganic). Arable (173,687 ha) and horticulture (126,803 ha) relied on applied P proxies due to data gaps.

• Dairy typologies: Effluent vs non-effluent blocks, silage, support blocks (e.g., Waikato+Bay of Plenty: 42 kg P/ha maintenance).
• Sheep/beef: Classes 2-8 by region/topography (e.g., class 4 hill: 7-13 SU/ha, low slope).
• Targets: Olsen P 25 mg/L flat, 22 rolling, 19 easy hill, 16 steep hill; higher (35-45 mg/L) for pumice/peat.

National Phosphorus Budget: A Close Match

In 2021/2022, total P fertiliser applied was 142,829 tonnes across agriculture—pastoral sectors accounting for 91% (129,974 tonnes). Maintenance needs totaled 157,259 tonnes, a modest surplus of 14,430 tonnes nationally, suggesting stable Olsen P (dairy avg 37 mg/L, sheep/beef 22-26 mg/L). Pastoral showed a slight deficit (-16,482 tonnes), offset by arable/horticulture.

This balance reflects farmers' adoption of tools like Overseer, soil testing, and variable rate application, amid regulatory pressures from regional councils and the National Policy Statement for Freshwater Management.

Dairy Farms: Slight Over-Application and Accumulation Risks

Dairy, New Zealand's economic powerhouse (1.87 million kg milk solids), applied ~32 kg P/ha (fertiliser 24-27 kg/ha + supplements 8-13 kg/ha), against 35 kg/ha needed. National surplus: +3 kg/ha/year (organic pool +11 kg/ha, inorganic +6 kg/ha). Regional highs: Canterbury (+6 kg/ha), Otago+Southland (+7 kg/ha); Waikato+Bay of Plenty near balance (+1 kg/ha).

With Olsen P exceeding targets in most regions (e.g., 43 mg/L Waikato+Bay of Plenty), legacy P risks rise—runoff losses up to 3.3 kg/ha in wetter West Coast+Marlborough. De Klerk recommends targeted reductions to curb accumulation and waterway impacts.

Sheep and Beef: Under-Application and Depletion Concerns

Sheep/beef farms (9,165 farms, 6.9 million ha) applied ~20 kg P/ha fertiliser (11 kg/ha avg), below 18 kg/ha maintenance, yielding -2 kg/ha/year deficit. Extremes: -21 kg/ha in Marlborough+Canterbury class 8 mixed finishing. Olsen P within targets but vulnerable to mining on steeper terrains.

Production losses loom without adjustments, especially as global demand for red meat wanes. Opportunities exist to boost fertility economically, enhancing resilience.

Arable and Horticulture: Data Gaps Persist

These sectors (300,000 ha combined) applied ~27-36 kg P/ha (total ~10,800 tonnes), used as maintenance proxy due to sparse regional data. Trends: Wheat down 21.7%, horticulture up (e.g., kiwifruit, vegetables). Future studies urged for precision.

Environmental Implications: Beyond the Balance

Balanced budgets minimize losses—runoff (SoilP + FertP + EffP) dominates over leaching. Dairy hotspots risk eutrophication; sheep/beef conservation aids biodiversity. Aligns with He Waka Eke Noa and Essential Freshwater reforms, promoting BMPs like soil testing (Olsen P every 2-5 years) and precision tech.

• Runoff losses: Dairy 1.2 kg/ha avg, sheep/beef 0.5 kg/ha.
• Legacy P: Dairy soils >30 mg/L in 6/7 regions.
• Solutions: Variable application, cover crops, riparian buffers.

Economic and Productivity Perspectives

P supports NZ's $50B+ ag exports. Balance sustains yields (pasture growth 10-20 t DM/ha/year) without excess costs ($1-2/kg P fertiliser). Sheep/beef uplift could add value; dairy efficiency cuts inputs amid volatile prices (2026 forecasts steady but geopolitically sensitive).

De Klerk: "Understanding how much phosphorus our soils actually need helps farmers apply fertiliser more efficiently, supporting both farm productivity and environmental sustainability."

Massey University's Enduring Impact on Ag Research

Massey's Fertiliser and Lime Research Centre (FLRC) leads globally, with workshops since 1994 on nutrient efficiency. Supervisors like Burkitt (nutrient cycling expert) and Tozer (farm systems modelling) exemplify interdisciplinary strength. De Klerk's trajectory into industry highlights uni-farmer pipelines.

In NZ's context—70% pastoral land—Massey equips grads for roles in DairyNZ, Ravensdown, Ballance, addressing climate-smart farming amid net-zero goals.

Future Directions and Actionable Insights

Recommendations: Regional benchmarking, Olsen P 20-30 mg/L targets, supplement audits, arable/hort data expansion. Monitor trends: Dairy intensification vs sheep/beef extensification. Farmers: Test soils annually, use Overseer/Outlook models, adopt precision (GPS spreading).

• Step 1: Sample 0-7.5 cm topsoil, autumn/spring.
• Step 2: Target Olsen P by soil (e.g., 30 mg/L volcanic).
• Step 3: Adjust via single superphosphate or alternatives.
• Benefits: 10-15% yield stability, 20% loss reduction.

As de Klerk eyes sustainable careers, her work inspires: Balance is achievable, but vigilance key for prosperous, green farms.

For the full thesis, visit Massey Research Online.

Photo by Bernd 📷 Dittrich on Unsplash