The below graph represents a typical soil moisture graph. Five key interpretation points have been highlighted and are explained below the graph.
- As soil moisture drops below the stress point (irrigation trigger point), the soil moisture trace line levels out (the slope of the line becomes shallower). When soil moisture drops below stress point, the plant is using more of its energy to extract soil moisture as opposed to productive growth. This results in reduced yield.
- Spikes above field capacity indicate periods of saturation. This will have a direct effect on drainage and nutrient leaching. On heavier soils, periods of saturation will also decrease crop production as the crop becomes dormant.
- An indication of field capacity, for each sensor down the profile, is given by looking at where the first step in the trace line occurs. All the water that is drained through gravity has gone and the step effect shows the day time (plants transpire and water evaporates from the soil’s surface) and night time (plants don’t transpire and there is no evaporation from the soil) water use pattern. However, one of the easiest ways to determine field capacity is monitoring the sensor readings after a large rain event or through saturating the soil around the sensor. This works particularly well in the winter when the crop is dormant (not actively growing). In free draining stony soils field capacity typically occurs one to two days after the event, in silt or clay loams four to five days.
- As above, the stepping of the trace line indicates the daytime–nighttime plant water use pattern. During the peak of the season the stepping becomes more pronounced (vertically longer).
- A spike on the soil moisture trace below the root zone indicates a drainage event, either through rainfall or over-irrigation.