Soil testing guide for New Zealand farmers
Last updated: 18 June 2026
Soil testing is the foundation of effective fertiliser management in New Zealand farming systems. Understanding the levels of nutrients in your soil allows you to apply the right fertiliser at the right rate, improving or maintaining pasture production while avoiding unnecessary input costs.
This guide explains how soil testing supports fertiliser decisions in New Zealand farming systems. We cover what soil tests measure, how key tests (such as Olsen P) are used to assess soil fertility, how results translate into fertiliser decisions, and when and how often to test your soil to maintain productivity over time.
Contents
- What is soil testing?
- What do soil tests measure?
- How do soil test results guide fertiliser decisions?
- How often should you soil test?
- What is Olsen P and how is it used?
- How should soil samples be taken?
- What happens if you don’t soil test?
What is soil testing?
Soil testing measures the major nutrient and pH status of your soil, providing the data needed to make fertiliser and lime decisions.
In New Zealand grazed pasture farming systems, soil tests are used to determine whether nutrients are deficient, sufficient or in excess, and to guide fertiliser programmes based on expected pasture response.
Without calibration to New Zealand conditions, a soil test is just a number - it doesn’t show how results relate to pasture production or what action should be taken. That’s why local calibration is essential. It’s also important to use an accredited laboratory to ensure the results are accurate and reliable.
What do soil tests measure?
Soil tests measure specific nutrients and soil properties, including pH, phosphorus (Olsen P), potassium, sulphur and nitrogen, which are used to assess soil fertility.
The table below outlines commonly used soil test suites in New Zealand and what they are used for.
| Test suite | What it measures | When to use it | What it tells you |
|---|---|---|---|
| Basic soil analysis | pH, Olsen P, Ca, Mg, K, Na, CEC | For routine monitoring of soil fertility in pastoral and cropping systems. | General soil fertility, including phosphorus, potassium, magnesium and pH. |
| Pasture soil analysis | Above + sulphur | When an indication of sulphur fertility is required. | General soil fertility plus sulphur availability for pasture and crop production. |
| Nitrogen tests | Mineral N, PMN | Cropping systems. | Soil nitrogen supply. |
How do soil test results guide fertiliser decisions?
Soil test results are used to determine whether nutrients need to be added to support pasture production.
As soil fertility changes, so does the response to fertiliser:
- At low fertility levels, nutrient or fertiliser additions produce large increases in pasture growth
- As soil fertility approaches optimal levels, the plant response to fertiliser is more moderate
- Above optimal levels, additional fertiliser produces little to no extra plant growth as the nutrient is no longer limiting
Fertiliser programmes are then designed to either:
- Build soil fertility towards an optimal level (capital fertiliser), or
- Maintain fertility by replacing nutrients removed in products and losses (maintenance fertiliser)
How often should you soil test?
Regular soil testing is needed to monitor nutrient levels, guide fertiliser recommendations, and check that the correct fertiliser rates are being applied.
For most pastoral farming systems, testing every 2–3 years is sufficient, provided it is done consistently at the same time and location to track trends over time.
Crop paddocks are often tested annually.
Regular testing allows you to:
- Identify deficiencies or excess nutrients early
- Adjust fertiliser inputs based on actual need
- Track changes in soil fertility
- Monitor the effectiveness of fertiliser programmes
What is Olsen P and how is it used?
Olsen P is the standard soil test used in New Zealand to measure plant-available phosphorus.
Olsen P, along with the other soil tests in the pasture suite (e.g. pH, QTMg, QTK, sulphate-S and organic-S), has been calibrated over decades of research across a wide range of soils and environments in New Zealand, linking soil test levels directly to pasture production. This allows fertiliser decisions to be based on expected response rather than guesswork.
With this calibration, we can interpret results with confidence. For example, low Olsen P levels indicate phosphorus deficiency and a strong production response to fertiliser, while higher levels indicate lower response as soil fertility approaches optimal levels.
Dr Ants Roberts explains why soil test results must be calibrated to New Zealand conditions and how Olsen P is used to guide fertiliser decisions.
How nitrogen supply is assessed
Nitrogen testing is used in cropping systems to estimate how much nitrogen the soil can supply.
By measuring mineral nitrogen and potentially mineralisable nitrogen (PMN), it is possible to estimate both the nitrogen available now and the amount likely to be released over the growing season.
This allows nitrogen fertiliser to be applied to fill the gap between soil nitrogen supply and crop nitrogen demand, if target yields are to be achieved.
How should soil samples be taken?
Accurate soil testing depends on taking representative samples.
Sample areas should have similar soil type, fertiliser history, productivity and land use. Areas such as gateways, fence lines and manure/urine patches should be avoided, as they are not representative of majority of the paddock.
Multiple soil cores are taken and combined into a single sample to represent the area being tested.
Dr Ants Roberts explains why pasture soils are typically sampled at 7.5cm and cropping soils at 15cm.
What happens if you don’t soil test?
Without soil testing, fertiliser decisions are based on assumption rather than measured data.
When fertiliser inputs are reduced or withheld, the impact depends on existing soil fertility:
- In the first year, there may be little visible change
- By the second year, pasture production can begin to decline
- Over time, production losses can increase significantly
- Pasture quality may decline, with increased weed content and reduced clover
Long-term, failing to monitor soil fertility and apply the required nutrients can lead to reduced productivity and lower farm performance.
Know what your pasture needs before applying fertiliser
Understanding pasture conditions and feed demand is key to getting a strong response. Soil testing gives you the information to make confident fertiliser decisions for your farm.
