Tracking Potassium Behaviour in Two Distinct Soil Types at Ballidu

Jul 14,2025 K Extension Project, Project Update Article, Technical Article, 2025, Projects, RAD2307, Technical Article,

By Kate Parker & Simon Kruger, WMG

In 2025, the West Midlands Group has launched a new potassium-focused demonstration trial just north of Ballidu with grower Corey Mincherton – an extension of the GRDC funded K Extension Project. This project aims to explore how potassium behaves across contrasting soil types and to spark conversations about potassium cycling, soil variability, and the effectiveness of traditional soil testing methods. While potassium is the headline act, the broader suite of soil tests conducted across both trial paddocks gives us a valuable early-season snapshot of the paddock variability before treatments are applied.

Site Overview

Two neighbouring paddocks are being used in this trial, with clear contrasts in soil type:

Site 1: A deep yellow sand (Figures 1 & 2)
Site 2: Sand over gravel (Figures 3 & 4)

**Figure 1.** Soil core from deep sand site (Site 1)

**Figure 2.** Soil core from deep sand site (Site 1)

**Figure 3.** Soil core from sand over gravel site (Site 2)

**Figure 4.** Soil core from sand over gravel site (Site 2)

At the time of initial soil testing, neither site had any of the planned potassium treatments (ranging from 0 to 75 kg/ha MOP) implemented yet, so any differences currently seen in soil test results are due to inherent variation, not treatment effects. This makes it a good opportunity to examine the baseline soil conditions (Site 1 Results – Figure 5)(Site 2 Results – Figure 6).

Soil Variability and Topography

The differences between Site 1 and Site 2 are stark and highlight how much topography and soil type influence nutrient availability and storage:

Site 1 (yellow sand): Characterised by low fertility, particularly in the deeper layers. Nutrient levels, organic carbon, and conductivity all decrease with depth, consistent with a leached, low-holding-capacity soil. Moisture and sulfur levels are also low and uniform.
Site 2 (sand over gravel): Displays more variation, possibly influenced by changes in depth to gravel or subsoil texture. Higher organic carbon and nutrient levels in the top 10 cm suggest more biological activity or legacy nutrition. The gravel layer likely contributes to moisture retention and higher conductivity in patches.

**Figure 5.** Soil testing results from the deep yellow sand site (Site 1)

**Figure 6.** Soil testing results from the sand over gravel site (Site 2)

Even across a relatively short distance, these paddocks (located next to each other) show how variable subsoil constraints and nutrient profiles can be. The presence of gravel or denser subsoil horizons in Site 2 likely affects both nutrient leaching and root access, particularly as depth increases.

Depth Effects on Nutrients

Both sites demonstrate a strong decline in key nutrients (nitrogen, phosphorus, potassium) with depth, though this trend is more exaggerated in Site 1. Organic carbon is sharply concentrated in the surface layer (0–10 cm) at both sites, reinforcing the importance of surface management for maintaining biological activity. Soil pH remains relatively consistent down the profile, although slightly higher at depth in Site 2, which could influence nutrient availability, particularly for potassium and phosphorus.

A Closer Look at Potassium

Potassium is the key focus of this demonstration, and the initial soil results already paint an interesting picture.

Site 1 shows a classic sandy profile with limited potassium reserves, especially below 30 cm. The total potassium Colwell values across depths are low, suggesting poor K holding capacity and likely responsiveness to fertiliser.
Site 2, by contrast, has much higher potassium levels in the top 30 cm, particularly in plots like S2T2 and S2T3. Interestingly, potassium remains present at depth in some cores, possibly indicating past redistribution or differences in soil mineralogy and gravel content. These deeper reserves may play a role in sustaining crop growth during dry periods or when surface moisture is limited.

The stacked bar charts (Figures 7 & 8) showing total potassium by treatment underscore these contrasts. Site 2 has a much greater potassium “bank” to draw from, although how accessible that K is remains to be seen.

**Figure 7.** Site 1 Potassium Colwell mg/kg by Treatment & Depth

**Figure 8.** Site 2 Potassium Colwell mg/kg by Treatment & Depth

Setting the Scene for 2025

This season’s trial won’t just test different potassium rates, it will explore how each soil type responds to the same management, with a particular focus on nutrient movement, crop uptake, and yield. As the season unfolds, it will be important to monitor how each site behaves under the same inputs, particularly in terms of:

Plant establishment and biomass
Potassium availability and plant uptake
Yield and grain quality at harvest

This is a long-term conversation, not a quick fix. With many growers in the region questioning the reliability of topsoil potassium tests and the long-term depletion of K reserves, this trial is a timely piece of the puzzle.

Let’s Keep the Conversation Going

The Ballidu site is designed to be as useful to growers as it is to researchers. As data rolls in from plant sampling, biomass cuts, and yield results, we’ll have more to share. In the meantime, take a look at your own paddocks and ask: How well do I know what’s happening with potassium across my paddocks’ soil profiles?

Stay tuned for updates as the season progresses and reach out to the WMG team if you’re keen to discuss the results, or run your own potassium checks.