Agricultural Significance of Major Soils

Vertosols
Hard-Setting Red Sodosols
Hard-Setting Yellow Sodosols

The Eastern Wimmera survey has revealed a wide variety of soils most of which differ markedly in properties crucial to their agricultural land use potentials. However, a number of these soils are of only a limited occurrence. Discussed below are three main soil categories, chosen on the basis of their abundance in the survey; they are Vertosols, Hard-setting Red Sodosols and Hard-setting Yellow Sodosols.


Vertosols

Soils included in this category dominate the gently undulating gilgai plains. They also occur in low-lying areas such as flood plains, depressions and foot slopes.

They are extensively cropped for cereals, mainly wheat and barley and support many legumes and oilseed crops and sown pastures.

The agricultural significance of the main properties of these soils are as follows:

a) Soil workability

Unlike the Brown and Red Vertosols, most of the Grey Vertosols are friable and easy to work, particularly at the dry to moist stage. However, all clays become very sticky when wet.

b) Crop establishment

Crop establishment is usually better on the friable surface soils than on the non-friable compacted seed beds that are common on the brown and red clays. However, cereal crops grown on friable clays are usually more liable to cereal cyst nematode "eelworm" (Heterodera avenae) than those grown on non-friable, or massive clays.

c) Soil permeability

When dry, the friable cracking clays are usually better structured soils in that they are well aerated and more permeable to water. However, upon wetting these clays expand, reducing their total porosity which in turn affects aeration and slows water infiltration.

d) Soil moisture

Most of the clay soils have a high field moisture capacity. However, the availability of this moisture to the plant depends largely on the soils structure and consistence and, in general, the ease with which plant roots can penetrate the subsoil. In the non-friable coarse blocky subsoils, it is not uncommon to find appreciable amounts of unused ‘available’ moisture at depth.

e) Seasonal cracking

Upon rapid wetting, wide cracks in dry soils assist in rapid subsoil water recharge, which may be of crucial value to the survival of plants near wilting. Also, cracks contribute appreciably to soil aeration being the main natural passageways for air in the soil mass.

However, agricultural disadvantages of these cracks include acceleration of soil moisture evaporation, some loss of surface soil material sown into the subsoils and the protection of pests such as crickets and mice.

f) Gilgais

Amounts of carbonates and salts are usually higher in the gilgai puffs than in the depressions. Leveling of gilgai reduces the depth of the surface soils on the mounds and sometimes exposes their subsoils.

Such soil variability often significantly affects the land’s productivity and complicates its management.

g) Soil reaction (pH)

In spite of the capability of these soils to grow a wide range of plant species, their high alkalinity and the presence of appreciable concentrations of carbonates at shallow depths, reduce the soil’s potential to successfully support some crops (eg. lupins).

h) Soil salinity

Flooding at irregular intervals, helps in restricting the upward movement of higher concentrations of subsoil salts. Under irrigation, or more frequent flooding, these clays may develop salinity problems following waterlogging of subsoils or rising water-tables. In low-lying areas, soluble salts often accumulate at shallower depths in amounts harmful to the productivity of crops and pastures. In general, the subsoils of the brown and red clays contain higher amounts of soluble salts than in the grey clays.

i) Inherent fertility

Generally, the inherent fertility of these clays is moderate. The soils mineralize nitrogen readily in cultivated bare fallow and initially produce high yields of crops. However, the total nitrogen content diminishes upon cropping. On these soils, cereals and legumes including medic pasture often respond to the addition of phosphorus and zinc.

j) Root development

On these clays, deeper root development is often dependant on the density and friability of the subsoils. Commonly in the friable grey clays, roots extend to greater depths than in the non-friable brown and red clays.

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Hard-Setting Red Sodosols

These soils are widespread throughout the area surveyed with the main occurrence being on the undulating plains located in the eastern section (Donald-Charlton area).

They are extensively cropped for cereals, notably wheat and barley, and are also commonly used for sheep-grazing on native, volunteer or sown pastures.

The agricultural significance of the main properties of these soils are as follows:

a) Soil workability

These soils are normally not easy to manage owing, largely, to the adverse structural properties of their surfaces. Upon wetting and drying the surface soils usually set hard and become difficult to work. The range of optimum moisture content at which these soils can be easily worked is very narrow and consequently, over most of the year the soils are either too wet or too dry for field operation. Gypsum has been successfully used to ameliorate many of these hard-setting soils.

b) Crop establishment

The hard-setting nature of the surface soils is a major limiting factor to crop establishment. On these soils, crop establishment is largely dependent on the time and intensity of the first follow-up rain (ie. first rain after sowing). With adequate seedbed moisture content, seasons with no rain (or only with very light showers) received between sowing and crop emergence are favorable for crop establishment. Due to the structural instability of surface soil aggregates, heavy rains (or more frequent rainstorms) falling on these soils when cultivated, often induce surface sealing and compaction and, in turn, affect crop establishment.

c) Soil permeability

In general, these soils are moderately permeable to water and air. Soils with bleached A2 horizons are often less permeable than those which are either without A2 horizons or with non-bleached A2 horizons.

Permeability through these hard setting soils is primarily restricted by the compaction of the surface layers. Also in the dense clay subsoils, permeability is generally low and tends to decrease as sodicity increases. The degree of a profile’s permeability, is generally indicated by the amount of bleaching in the A2 horizon and the extent of subsoil mottling. However, improved permeability has been obtained by the application of gypsum.

d) Soil moisture

As a rule, the texture of a soil significantly affects its capacity to store water and to supply this water to the plant. Therefore, on these texture contrast soils the field moisture capacity of the surface horizons varies greatly depending on their textures. The clayey subsoils usually have moderate capacities to store available moisture.

e) Soil salinity

Soil salinity usually increases with depth. In the clayey subsoils, a moderate to high accumulation of soluble salts commonly occurs. Improvement in profile drainage is essential for decreasing soil salinity.

f) Inherent fertility

Most of these soils have low to very low phosphorus and nitrogen contents, moderate levels of potassium and low zinc availability. Generally they respond well to fertilizers.

g) Root development

On these soils, structural properties and depth of the soil profile are major factors affecting root development. Coarsely structured tight clay subsoils offer high resistance to root penetration. Cemented subsurface horizons also decrease the rate of root growth particularly in the early stages of crop development.

h) Soil erosion

The structural instability of these soils greatly contributes to their erosion. When cultivated dry, most of the surface material breaks to very fine ‘powdery’ aggregates and become wind erodable. Upon heavy rain, unprotected surfaces are also usually prone to water erosion and the sodic subsoils often develop tunneling and eroded gullies.


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Hard-Setting Yellow Sodosols

In general, the hard-setting Yellow Sodosols are associated with the undulating valley plains and hilly terrain. In the area investigated in this report, these soils commonly occur in the southern and eastern sections occupying mainly the poorly-drained positions of landforms including flood plains, drainage lines and outwash slopes.

In the study area, the main agricultural land use of these soils is sheep-grazing on native, volunteer and sown pastures. However, in some dryer areas these soils are utilized for cereal cropping.

The agricultural significance of the main properties of these soils are as follows:

a) Soil workability

In general, these soils are considered to be difficult to manage due to their narrow range of moisture contents at which optimum workability can be achieved. When wet, the surface soils become waterlogged and their aggregates readily disperse resulting in a compacted hard setting cultivation layer upon drying.

b) Crop establishment

Crops are usually poorly established on these soils due to the hard setting condition of the surface horizons. Also on these soils, periodic waterlogging and the presence of perched water-tables at shallow depths are major limiting factors to crop establishment.

c) Soil permeability

Water infiltration rate is commonly low to moderate in the surface soils (depending on soil texture and degree of compaction) and is essentially low to very low in the subsoils. This is usually indicated by the degree of subsurface bleaching and/or subsoil mottling.

Low soil permeability often results in:

• Waterlogging, intermittent perched water-tables in the subsurface horizon (A2) and partial saturation of the upper B horizon.
• Loss of surface moisture through runoff and/or evaporation.