New Zealand soil Olsen P levels

These soil analysis results represent the very large number of soil tests conducted to inform fertiliser advice and nutrient management decisions. This summary is based on approximately 1.17 million soil tests, with typically 100,000 soil samples being submitted by farmers and growers for analysis for Olsen P each year.

The soil test results do not necessarily represent the ‘average’ farm system, but rather those farm businesses investing in evidence-based nutrient management advice. These are often the more fertile sections of higher producing farms.

The national annual median Olsen P of soils sampled from 2012 to 2022 shows some fluctuations over time, but with a small overall increase. There has also been an increase in the number of soil samples taken.

The following graphs show median levels of Olsen P, and the 25th to 75th percentile range over ten years (2012-2022).

Figure 1: Time series of the national median value of Olsen P across all farm types, soil types and regions. The median for each year is represented by the dark solid line. The boxed area shows the 25% to 75% quantiles. The number of records per year are indicated underneath the graph.

Olsen P levels by land use and soil type

Olsen P soil analysis provides an estimate of the pool of phosphorus accessible for plant uptake as 'available’ phosphorus. Target ranges for Olsen P for combinations of soil type and pastoral land use have been developed based on pasture response trials. The identified target ranges are a guide only, but typically represent the soil fertility status which will deliver 97% of relative pasture yield across all the relevant trials.

Phosphorus is the most expensive nutrient, so where a farm should sit within the target range will be based on economic and environmental considerations. Raising soil P levels significantly above target ranges may result in only very small increases in annual pasture production and uneconomic increases in overall farm production.

The majority of soil data comes from pastoral farms, with much smaller numbers of samples from arable or horticulture land use. This is a reflection of the relative land areas in these land-use activities.

Careful consideration of where a farm sits in the target Olsen P range is required. Some consideration should also be given to the catchment context in which the farm is situated. The Code of Practice for Fertiliser Nutrient Management will help identify ways to assist with fertiliser P management. The Code is available here.

Research has shown that, where there are direct connections to surface water on farm, increasing soil P status has a correlation with instream P concentrations. Where instream P concentrations have been identified as an issue in the catchment context, a farm’s target Olsen P levels will need to take this into consideration. The key points relating to P losses from susceptible production areas are:

• P losses are generally small in well structured, well drained soils where there is no overland flow.
• Leaching of P can occur where there is drainage in soils with low anion storage capacity.
• P concentrations in overland flow increase as soil Olsen P levels increase, especially in soils with capacity retention of less than 15%.
• Greater P losses occur from heavy textured soils as there is potentially greater runoff from the compacted surface, and mole or pipe drainage allows greater transfer of dissolved, sediment and dung-P to surface waters.
• Most of the soil derived P lost from a catchment is from within 5-10 m of streams or mole or pipe drained soils.
• The use of slower release P fertilisers (e.g., serpentine super, reactive phosphate rock) which can reduce direct fertiliser losses should be considered in higher risk situations.
• Where soils have a high risk of P loss, then the lower end of the soil Olsen P target range will be more appropriate.
• Where Olsen P values exceed the target range for the particular farm production system, steps should be taken to understand the circumstances and best ways to ensure any excessive phosphorus use is reduced.

Tools like OverseerFM, can be helpful in understanding the soil phosphorus maintenance requirements. It does this by modelling the nutrient cycle and helping create a Nutrient Budget, which estimates all the inputs and outputs on an annual average basis, for the farm system.

Dairy

Phosphorus management is influenced greatly by soil type. Ash soils have high P retention and typically require higher levels of phosphorus application to raise soil fertility. Soils with low phosphorus retention have a greater risk of P loss. In 2024 we released revised advice for dairy and drystock farms.

The recommended Olsen P range for Ash and Sedimentary soils is 20-30, which represents a lower upper value for high producing dairy farms than in previous advice. For pastoral farms the recommended range for Pumice and Peat soils is 35-45. See figures 4 and 5.

Where dairy farms have high production, it may be economic to operate above the recommended target range of 20-30. Whether or not higher soil Olsen P levels will result in an economic return will be determined primarily by the relationship between the cost of applied P and returns from milk solids produced.

The soil samples for dairy farms on Ash soils represent approximately 10% of dairy soil samples, see figure 2. Dairy farms on sedimentary soils have lower Olsen P levels, see figure 3, which typically reflects the lower P demand to maintain Olsen P levels on these soils.

The median for each year is represented by the dark solid line. The boxed area shows the 25% to 75% quantiles. The number of records per year are indicated underneath. The green band represents the target range for ‘near maximum’ pasture production.

More information about fertiliser use on New Zealand dairy farms can be found here.

Figure 2: Yearly time series of the median level of Olsen P across Ash soils for dairy farms nationally. Note: the current recommended range 20-30 for Ash and Sedimentary soils is the recommended range to achieve near maximum pasture production however for high producing farms,  there can still be an economic response to Olsen P levels higher than this.	Figure 3: Yearly time series of the median level of Olsen P across Sedimentary soils for dairy farms nationally.
Figure 4: Yearly time series of the median level of Olsen P across Pumice soils for dairy farms nationally.	Figure 5: Yearly time series of the median level of Olsen P across Peat soils for dairy farms nationally

Drystock

Sheep and beef farms will grow different amounts of pasture depending on topography, climate, pasture species, stocking rate, soil fertility and a farmer’s goals for animal production and economic return. For drystock farms, only the highly stocked (15 – 20 SU/ha) and profitable finishing farms with favourable soils, climate and terrain are likely to obtain economic benefit from achieving near-maximum pasture production. Many sheep and beef farms are breeding operations with some finishing livestock. They are often located on lower-producing hill country with seasonally variable rainfall. These farms are typically stocked at 8 – 10 SU/ha and require less than near-maximum pasture production to optimise their financial returns.

For an individual farm, econometric modelling for P, K, S and lime can be carried out to determine the economically optimal soil test ranges and the rates of fertiliser nutrients required. Whether or not higher soil Olsen P levels will result in an economic return will be determined primarily by the relationship between the cost of applied P and the gross margin per hectare. For drystock farms, the economic range maybe less than the recommend range on many farms, see figures 6 and 7. For pumice and peat soils, most drystock farms are operating well below the Olsen P target range for near-maximum pasture production, see figures 8 and 9. 

More information about fertiliser use on New Zealand sheep and beef farms can be found here.

Figure 6: Yearly time series of the median level of Olsen P across Ash soils for drystock farms, nationally.	Figure 7: Yearly time series of the median level of Olsen P across Sediment soils for drystock farms, nationally.
Figure 8: Figure 8: Yearly time series of the median level of Olsen P across Pumice soils for drystock farms, nationally.	Figure 9: Yearly time series of the median level of Olsen P across Peat soils for drystock farms, nationally.

Horticulture and orchards

Categories such as horticulture or orchard cover a wide range of crops, all with differing nutrient needs. This means there is no generic recommended Olsen P range. It also means that the soil analysis from orchard and horticulture land uses show a very wide spread of Olsen P results. Olsen P levels on horticulture and orchard land uses are generally high compared to pastoral requirements, reflecting the greater nutrient requirements of vegetables and crops such as kiwifruit. Because of the smaller number of samples, the data has not been broken down by soil type.

Figure 10: Time series of the national median value of Olsen P across all soil types for horticultural properties.

Figure 11: Time series of the national median value of Olsen P across all soil types for orchards.

For arable and forage crops Olsen P levels inform the level of fertiliser required to achieve target yields. Requirements will be different for different crops. For this reason, there is no generic target range presented.  Maintenance P applications are required to replace crop removals. Often arable and forage crops are used in a rotation with pastoral farming. This introduces the likelihood of residual P on subsequent pasture or cropping soil analysis and the need to understand land use history when interpreting soil analysis results.

Figure 12: Time series of the national median value of Olsen P across all soil types for arable and fodder crop farms.

More information about the nutrient management for dairy, sheep and beef, arable cropping, forage cropping and vegetable crops can be found in the booklets under the ‘resources’ tab of our web site.