Glossary of Terms

Daily Water Use: the amount of water used by the plant in a 24 hour period. This parameter is generally determined by measuring the volumetric soil water content of the soil at the same point, and at the same time each day. The current value is then subtracted from the previous day's value to calculate the difference in soil moisture content. It is important to mention that this calculation does not differentiate between water lost through plant transpiration, soil evaporation, through drainage, and runoff. If you are specifically trying to identify the amount of water used only by the plant itself such as in a situation where a crop may be accessing water from a perched water table you must measure plant sapflow.

Deficit: an amount of water held within the soil between the refill point and full points of a plant's rootzone. This water is readily available to the plant and indicates the amount of water that is required to be added to the soil profile to replenish the reserve that has been lost through plant water use, evaporation and or drainage.

Electrical Conductivity: When a voltage is applied across a substance, an electric current will only flow if the substance conducts electricity. When salts dissolve in water, ions are formed and the solution will conduct electricity. As a general rule, the higher the concentration of ions in solution, the better the solution conducts electricity; in other words, its electrical conductivity increases.

One way of measuring salinity at ground level is by determining the electrical conductivity of a soil 'saturation extract' (a solution obtained by saturating a soil sample with water). The electrical conductivity increases with salinity because of the increasing presence of ions (usually sodium and chlorine ions). Electrical conductivity is often expressed in units such as deciSiemens per metre (dS/m). Rain water, for example, has a conductivity of 0.02-0.05 dS/m, while sea water has a conductivity of 50-60 dS/m. Ground water becomes saline at about 6-8 dS/m.

Full Point: the moisture content of the soil where the soil is holding the maximum amount of moisture not yet in an anaerobic state, i.e. the soil contains organic/mineral content, water and air. The full point is generally determined using a logging soil moisture device such as an ECH2O to record the soil moisture content at hourly intervals (more frequently in light sandy soils) during a wetting and drying cycle either as a result of a saturating rainfall event or irrigation.

Refill Point: the volumetric soil moisture content at which the plant can no longer freely extract its daily water use requirements. Many factors can affect this moisture content such as soil type, crop type, and the effect of cultural practices specific to each individual field such as compaction from trafficking during wet periods. For this reason the soil moisture content must be measured on each site or block to determine the correct refill point for each crop.

Rootzone: the integrated soil moisture content from which the plant is extracting moisture from the soil. The rootzone is used the calculate the amount of water in mm which is required to be applied to the crop to maintain optimum productivity.

Soil Profile: Where soil has been cut through vertically, such as along a roadside embankment, you may see that it has various layers of different textures and shades. This is called the soil profile. The top layer, called the A horizon, is the one where most roots are, where most biological activity occurs and where organic matter accumulates. Water washes clay particles down out of this horizon.

In the next layer, the B horizon, clay particles and soluble substances washed down from above tend to accumulate. Below that is the C horizon, or parent rock. The type of parent rock can affect the fertility and structure of the soil that develops above it.

Volumetric Soil Water Percent (VSW%): the percentage of water held in a cubic metre of soil. Typically the maximum VSW% possible in soil is approx 55%. The remaining 45% consists of soil and air.