Soil testing for success in cropping and pasture

Author: Reily Menhenett, IPF Technical Agronomist

With 2025 harvest nearing completion, preparations now begin for the 2026 cropping and pasture seasons.

While there are many decisions still to be made prior to sowing, the best place to start is with a soil test. Having a solid understanding of nutrient levels and soil structural constraints is critical when planning rotations and fertiliser budgets for the upcoming year.

Whether monitoring long-term trends, shaping a nitrogen budget, or planning soil amelioration, soil testing is the first step in creating a foundation for sound decision making.

Where do I start?

Before firing up the corer, or grabbing you bucket and probe, the first step is to create a plan:

• Which paddocks will I test?
  • Paddocks going into cereals or pasture paddocks that had hay/silage cuts?
• Why am I testing?
  • To diagnose, monitor or predict?
• What am I testing?
  • For structure (e.g. slaking and dispersion), soil acidity or nutrient levels (such as deep N testing, NPKS levels)

Answering these questions will help to create a sound soil testing program, as well as assist with selecting the right test codes for analyses at the Nutrient Advantage Laboratory. You can choose from:

• standard packages which range from basic analytes to comprehensive testing.
• tailored tests such as the Soil Health Testing package that can provide a check-up for your soils.
• add-ons tests such as the pH Buffer test which can help you to choose the correct lime rate for your paddock.

If these don’t suit, the team at Nutrient Advantage can help you create a custom test code tailored to what you need to test.

Sampling techniques

There are numerous ways to sample a paddock, but the key to achieving a meaningful result is to ensure a representative sample is collected. In paddocks with distinct soil types, this may mean testing two to three zones separately (see Figure 1), as soil properties can vary widely across a short distance.

_{Figure 1: Defining soil test zones within paddocks. Suggested number of composite samples: a) 1 b) 3 c) 2 d) 5. Source: FertCare Soil Sampling Guide (Gourley and Weaver 2019).}

Avoiding areas such as stock camps, fertiliser or lime dumps and gateways is critical to avoid biasing a sample. Make sure you are sampling to the correct depth for your required enterprise. In both cropping and pasture situations, a topsoil test is to be taken from 0-10cm. Table 1 summarises the consequences on a result, where a 0-10cm soil test has been incorrectly sampled

	Correct depth	Shallow sample	Deep sample	Scuffed topsoil	Half core
Approximate depth	0-10cm	0-9cm	0-11cm	0.8-10cm	0-5cm
Variance to actual	–	+10%	-10%	-20-30%	+180%

_{Table 1: Consequence of missing correct sample depth on reported Colwell P figure. Source: Adapted from FertCare Soil Sampling Guide (Gourley and Weaver 2019).} 

Achieving a representative sample also means collecting multiple cores. FertCare guidelines suggest between at least 20-40 cores per sample to ensure variability is minimised as much as possible. Mix your samples in a clean bucket and extract a subsample to send off to the lab.

Cropping

While soil testing plans will vary by grower preference, it is worth highlighting the benefits of grid sampling and deep soil sampling in a cropping situation.

Grid sampling

This is an intensive sampling technique, which breaks paddocks into multiple smaller ‘grids’, for example two hectares or five hectares. Utilising a grid sampling strategy, it is possible to detect the natural variation in soil test levels across a paddock.

For a farm with little to no soil test history, grid sampling can be a good way to get a baseline level of soil characteristics, allowing a grower to create management zones, which can be tested in a regular fashion in subsequent years.

The power in grid sampling comes from the ability to then create variable rate maps to apply nutrients or soil amendments. This can result in a significant cost saving compared to blanket applications. Some examples are as follows:

• Phosphorus levels -> Variable rate phosphorus, either capital or at sowing
• Soil pH -> Variable rate lime
• Exchangeable Sodium Percentage (ESP) -> Variable rate gypsum
• Soil type and management zone -> In season variable rate N.

_{Figure 2: Variable rate lime map from grid sampling. Source: Precision Ag}

Deep soil sampling

Mobile nutrients, particularly nitrogen and sulphur, can often move below 10cm with rainfall. Therefore, when planning a nitrogen budget, measure the nitrogen further down the profile (often to 90cm or more). Accounting for soil type and subsoil constraints and therefore effective rooting depth can help guide depth choice for deep soil sampling.

Deep soil testing can also help to identify subsoil constraints such as boron accumulation or sodicity, which can limit root growth. Taking segmented tests can be a powerful way to build a three-dimensional map of your paddock’s soil, and assist with key decisions moving forward, for example, a segmented 0-60cm test:

• 0-10cm -> Phosphorus levels, Organic Matter, pH
• 10-30cm -> Soil nitrogen and sulphur, ESP
• 30-60cm -> Deep nitrogen and sulphur

From this test, an understanding of ‘recoverable’ nitrogen and sulphur can be reached, helping to shape the fertiliser budget for an upcoming crop.

Figure 3 – extracted from 279 deep soil samples over the 2024/5 summer and analysed at the Nutrient Advantage Lab – depicts the variability that can exist in deep soil nitrogen across different growing regions and sampling times.

There is a big difference to the bottom line of a fertiliser budget if deep nitrogen is not properly accounted for. The only way to know for sure is to conduct a test.

_{Figure 3: Deep nitrogen soil test levels (279 samples, 1/12/2024 – 1/3/2025). Source Nutrient Advantage}

Pasture

For pasture enterprises, considerable variability can exist based on location of paddock, and primary use (for example, sacrifice paddock, hay paddock, night paddock). This variability means it is important to define groups of paddocks, and test accordingly. Figure 4 highlights this variability in both Olsen P (Left) and Colwell K (Right).

_{Figure 4: Dairy Farm Olson P (Left) and Colwell K (Right) variation. Black dot represents Dairy. Red = Very High, Blue/Purple = High, Green = Adequate, Orange = Marginal and Yellow = Deficient. Source FertCare Soil Sampling Guide (Gourley and Weaver 2019).} 

For pastures, grid sampling can be particularly powerful in understanding soil test levels, with the inclusion of stock a major factor in the distribution of P and K across a paddock. Figure 5 depicts the within paddock variability of Colwell P, due to the movement and activities of stock.

_{Figure 5: Olsen P within a paddock. Note the accumulation of P towards the gateway and edges of the paddock. Blue dots represent water troughs; purple extremely high; red very high; orange high; yellow above optimum; green optimum; and blue, very low. Source: Cotching et al. 2019.}

Segmented sampling can also be valuable, particularly if soil acidity is of concern. Taking a 0-20cm sample, segmented in 5cm intervals can help to identify potential acid throttles that may exist and are not picked up by a traditional 0-10cm sample. If pasture renovation is planned, segmenting a test in this way may be of particular benefit, as it can help to identify the need for lime or gypsum, and the opportunity to incorporate these amendments into the subsoil. Figure 6 highlights how increasing the granularity of a soil sample can help to highlight where in the profile pH issues may exist. For example, in this pasture, an acid throttle is identified between 5-15cm, suggesting liming and potentially incorporation would be of benefit.

_{Figure 6: Stratified acidity in a pasture paddock near Wangaratta. Source: IPF Internal 2025.}

Tips and tricks

• Always sample to the correct depth
• Minimise contamination from stubble and sample between rows/away from fertiliser bands
• Make sure to take enough cores
• Use the right tools (bucket, probe, gloves)
• Choose the right analytes for your situation
• Try to sample at the same time each year to minimise temporal variability
• Don’t sample right after a rain event or fertiliser application
• Keep samples refrigerated before sending to the lab
• Follow local biosecurity measures as required.

Summary

Soil testing is a powerful tool when done right and can assist in decision making for the upcoming season. The most important step is developing a sampling plan that is aligned to your goals to ensure that your sample will provide the information that you need.

Strategic incorporation of grid sampling and deep soil sampling into your sampling program can provide additional information, which can be valuable for seasons to come.

Further Information

For more information, contact IPF Technical Agronomist Reily Menhenett on 0474 093 167 or via email: reily.menhenett@incitecpivot.com.au

References

Gourley CJP and Weaver DM (2019) A guide for fit for purpose soil sampling, Fertilizer Australia, Canberra, Australia.

Variable Rate Solutions – Precision Ag

Cotching WE, Talyor L, Corkrey RS (2019) Spatial variation of soil nutrients in dairy pasture paddocks. New Zealand Journal of Agricultural Research: Vol 40, 3.