The variation in the amount of fluctuation in the soil moisture data line is due to differences in clay content. Water can infiltrate faster through a sandy soil than a clay soil, with most of the water being stored in a sandy soil able to be extracted for plant use. This means that a soil moisture data line of a sandy soil will fluctuate quickly.

An example of this is the Goroke site (Figure 1), which has a surface soil Cation Exchange Capacity (CEC) of 4.9 meq/100g, which reflects low clay content, and a similar pattern of relative soil moisture for all probes in the 20-70cm depth layers. Scale is important to note, as the stacked graphs can be deceptive. At April 1 2024, all layers had between 60-75mm moisture, which decreased to between 35-55mm by March 10, 2025. It is the relative change within a line/depth, that is important to note, not the positioning of the lines on the graph.

The dark blue line (SM020) shows high fluctuation of soil moisture, with some evaporation pre sowing, and then active extraction of moisture from sowing onwards. Evapotranspiration (ET) is shown by the slight ‘wiggle’ of the blue line in April 2024 (evaporation), with stronger draw down or ‘wiggle’ from sowing through to mid-July due to the diurnal (day/night) flux of plant transpiration.

Following the various colours on Figure 1, it is easy to pick up that roots started to actively extract from the 30cm depth in mid-August, followed closely by the other depth layers in a cascading pattern, with root extraction from the 70cm zone being seen by mid-September. While there were significant rain events in late September, this didn’t result in a large water peak as it was quickly utilised, just reducing the rate of water extraction. As the wheat plants reached maturity in November and roots senesced, the lines flattened out. A rain event in December recharged the surface 20-40cm layers, with their moisture content decreasing through December (shown by down steps) as the deep layers recharged (shown by up steps).

In comparison, water can only slowly infiltrate through a clay soil, as the complex clay structures have smaller pathways for water flow than a sandy soil. However, clay soils have a greater water-holding capacity than a sandy soil, with greater amounts of water being stored, or absorbed in all the small pore spaces around the clay particles. The challenge with clay soils is that while they can hold a lot of water, most of this water is only slowly available for plant use, with a portion being unavailable, due to the high affinity of the clay particles for this water. This explains why a soil moisture data line of a clay soil will only change slowly, with a slower, and lower rate of extraction than a sandy soil.

An example of a heavy clay soil is the Natimuk 3 site (Figure 2). This soil has a surface CEC value of 33 meq/100g, reflecting a clay soil, likely a cracking clay soil, with the decreasing range of fluctuation of each soil moisture probe indicating increasing clay content with depth.

While Figures 1 and 2 are extremes, it is quite common to for soils to have a lighter surface soil, above a heavier clay subsoil, known as a duplex soil, and likely to reflect more of the red-brown earths in the Wimmera. An example is Figure 3, the Deep Lead pasture site.

The soil moisture probe data in Figure 3 shows large fluctuations in soil moisture down to about 40cm, with these fluctuations smoothing out at lower depths, indicating a sharp increase in soil texture about the 40cm depth, with increasing clay content below that. The impact of a pasture phase can be easily seen with the strong drawdown of moisture all year through all depths, albeit with less range of fluctuation at deeper depths as the clay content increases.

The Deep Lead pasture site (Figure 3) also provides an example of a common anomaly with soil moisture probes, which is preferential flow down the side of the tube. This is dramatically shown by a large spike in soil moisture on November 26, 2024, due to a high intensity rain event on dry soil, resulting in a simultaneous increase in soil moisture at all depths – not the gradual cascading of soil water down the profile over time which would be expected. This caused a reset in soil moisture at all depths at a higher level, the extent of which may or may not be ‘real’, especially at the lower depths.