Campbell Scientific Sentek EnviroSCAN Access Tube Installation Guide - Manual (Page 8)

Sit e Selection
© Sent ek Pt y Lt d Page 5
Notwithstanding the care taken to delineate macro zones, some variability in soil moisture levels is inevitable.
For example: on large properties, rain events may cover only a portion of the land area, replenishing some
soil reservoirs and leaving others dry.
The aspect or orientation of sloping fields can subject the crop to more or less solar radiation, wind exposure
or water run-off – all affecting crop water use.
Soils
An understanding of how soil type influences plant-soil-water-dynamics, and hence irrigation scheduling is
important. Intrinsic soil properties are texture, structure, depth, chemistry, organic matter content, rocks and
stones and clay mineralogy. Influencing factors include compaction, salinity, water-table development,
drainage rate dynamics and topography.
Soil texture
Water storage in the soil profile and the rate it dries out, depends on the soil texture. At one end of the
spectrum, sandier soils fill up and drain quickly. Hence these soils, in general, require smaller and more
frequent irrigations. In contrast, heavier clay soils replenish and drain slowly and to a higher total water
content than lighter (sandier) soils. An infinite range of textures exist between the two extremes. Textures
often change within a profile, with the layering of different textural bands playing a large part in determining
the water holding capacity of a soil.
Soil structure
Water infiltration rates and air and water permeability within the soil profile are closely related to the size and
distribution of soil pores. Porosity in turn, is dependent upon the arrangement and aggregation (binding) of
sand, silt and clay particles (soil structure). Soil structure is as important as soil texture in governing how
much water and air move in the soil and their availability to crops. Roots penetrate more easily and rapidly in
soils that have stable aggregates than in similar soil types that have no or highly developed structures. The
effectiveness of soil moisture, air and nutrient utilization is related to the efficiency of root colonization of the
entire soil profile.
Soil depth
The effective depth of soil affects the extent of root penetration. The deeper the soil, the greater the volume
of soil that is available for gaseous exchange and water uptake. Drainage is also influenced by effective
depth.
Soil compaction
Soil compaction from farm machinery can change pore size and distribution resulting from the natural
arrangement of the sand, silt and clay particles. This can cause reductions in water infiltration rates, and air
and water permeability within the soil profile. The resultant impact upon the effectiveness of root penetration,
air exchange and water uptake affects plant growth efficiency and hence water demand.
Salinity
Salinity lowers the osmotic potential, reducing the efficiency with which water and nutrients are taken up by
the plant. The dominance of the contributing ions can result in a nutrient imbalance causing deficiencies of
essential macro and micro nutrients. The reduced plant health and vigor affect crop water use.
Water tables and drainage rate
Poor drainage can lead to the development of water tables and/or cause a temporarily saturated soil profile.
The presence of impermeable soil layers can cause the formation of perched water tables, which saturate
parts of the root zone. Efficient gaseous exchange becomes restricted and plant health and water use is
reduced.