Campbell Scientific Sentek EnviroSCAN Access Tube Installation Guide - Manual (Page 9)

Sit e Selection
© Sent ek Pt y Lt d Page 6
Organic Matter
The presence of organic matter and humus increases the cation exchange capacity (CEC), water holding
capacity and structural stability of soils.
Soil chemistry
Acid, alkaline, sodic (soils characterized by a dominance of sodium ions) or nutrient deficient conditions
impact on expected soil chemical properties. For example:
• pH conditions change CEC and the availability of nutrients (by changing their form). Nutrient
deficient plants have a lower water uptake rate
• high levels of sodium can lead to structural collapse, infiltration problems and reduced water
availability
Rocks and stones
Stones and rocks and other coarse fragments within a soil profile occupy part of the soil volume and hence
reduce the soil water storage capacity. Very stony soils have a substantially lower water holding capacity
than soils of the same texture that are free of stones.
Topography
Topography relates to the configuration of the land surface and is described in terms of differences in aspect,
elevation and slope. This has an impact on plant-soil-water dynamics via influencing climatic conditions
including:
• rain shadows and sunshine hours
• rainfall and temperature patterns up slopes
• elluviation (washing-out) of clays from higher elevations and illuviation (washing-in and
accumulation) of clays at lower elevations
• relatively poorer drainage in low lying areas
Crop
Crop differences have an impact on crop water use and irrigation scheduling requirements. While all require
management between field capacity and wilting point at most times, the depth of root extraction varies, as do
specialized requirements, e.g. the deliberate stressing of wine grapes.
Most plant tissues contain about 90% water and the rate of uptake of water from the soil solution by plant
roots is largely controlled by the rate of water loss through transpiration. Plant characteristics such as crop
type, size, age, vigour, variety, rootstock, development stage, leaf area, nutritional status, crop load and
harvest all affect crop water use. Specialized advice should be sought in this regard. A rough guide to water
use can be obtained from crop coefficients, which are widely available in the literature for different growth
stages of most crops. These express evapotranspiration as a ratio of reference evaporation.
Cultural Management
The impacts of cultural management (agronomic/horticultural practices) also need to be understood for
proper irrigation scheduling.
Soil preparation
Cultivation increases evaporation from the topsoil, reducing the amount of soil water available to the plant. It
may also reduce water run-off and improve the infiltration of rain and irrigation water, improving plant water
availability.