Campbell Scientific Sentek EasyAG Installation Guide v 3.0 User Manual

Page 9

Site Selection

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. This influence is predominantly in the top soil, although lamellae
(thin organic matter layers further down the profile) can be important properties.

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)

• high levels of sodium can lead to structural collapse, infiltration problems and reduced water

availability

Rocks and stones

Stones and rocks 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 full and refill 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, vigor, 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 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.

Cover crop and mulch

Cover crops provide more competition for water, but reduce evaporation and facilitate infiltration of rain and
irrigation water, reducing run-off.