Subsoil Constraints- Queensland

Key Points

Subsoil constraints are chemical, physical or biological properties in the subsoil that limit plant growth.

Poor crop growth, despite good starting moisture and adequate in-crop rainfall, may be an indicator of subsoil constraints.

Good agronomic management helps minimise the water and other physiological stresses imposed by subsoil constraints.

Background

Subsoil constraints are properties in the subsoil that limit the ability of plants to utilise soil water and nutrient resources. Subsoil constraints may be chemical, physical or biological in nature, and several constraints may occur together in some soils. Subsoil constraints have a significant impact on soil water storage and use, nutrient regimes and crop growth. The identification of the most limiting constraint and its interaction with other factors is a first step to plan for sustainable site-specific resource management.

Indicators of subsoil constraints

The presence of unused subsoil moisture and an absence of fresh roots provide a good indication of the presence of subsoil constraints. If a paddock, or parts of a paddock, show poor crop growth and yield despite good starting moisture, adequate in-crop rainfall, and an absence of any other limits to production (e.g. insect attack, frost damage), it may indicate that subsoil constraints have impacted crop growth (table 1).

Table 1:Indicators of subsoil constraints.

SYMPTOM
TYPE OF CONSTRAINT

Do the roots show honey-brown discolouration, dark lesions or knotting?
Biological constraints

Is the rooting depth resitricted by rock, a dense clay layer, hardpan or a traffic or plough pan (roots deformed or growing at a right angle)?
Physical constraints

Do the leaves or stems show characterist symptoms of nutrient imbalance?
Chemical constraints —nutrients

Are fresh roots absent from the rooting zone (less than 1 metre soil depth)? Is the subsoil wet after a dry finish?
Chemical constraints –acidity, alkalinity, salinity

Is the soil surface crusting, hard setting, sealing or waterlogged (water ponds for several days after rain)?
Chemical constraints – surface sodicity

Is subsoil structure coarse or dense? Is subsoil drainage lacking?
Chemical constraints – subsoil sodicity

Types of subsoil constraints

Chemical constraints

Nutrient deficiencies: Continuous cropping without adequate fertiliser applications have resulted in widespread soil nutrient depletion throughout Queensland cropping regions. Subsoils are generally lower in nutrients than the surface layer. This can reduce yield in dryland regions, where continued root growth and function are essential to enable crops to extract water from the subsoil once the topsoil is dried or available nutrients used up by the crop.

Acidity: In acid soils (pH < 5.5), aluminium and manganese become more soluble; and may become toxic as their concentration in the soil water rises. In the northern grains region, subsoil acidity is common in soils dominated by N2- fixing brigalows and belah (see Soil Acidity—Queensland factsheet).

Alkalinity: In alkaline soils, an abundance of anions such as carbonates and bicarbonates contribute to the high pH. At soil pH > 8.0, toxicity of carbonate and bicarbonate can reduce crop growth and yield, and/or induce nutrient deficiencies.

Salinity: A high concentration of dissolved salts in soil or water can cause ion toxicity and affect a plant’s ability to absorb water. Sodium chloride is largely responsible for inhibiting water extraction or causing toxicity in Queensland cropping regions.

Sodicity: A high proportion of sodium ions relative to other cations (calcium, magnesium and potassium) can lead to dispersion, or the disintegration of clay aggregates into individual particles, when soils become wet. In the paddock, subsoil sodicity will lead to poor drainage, and the subsoil may remain saturated for long periods.

Physical constraints

Physical subsoil constraints generally consist of gravel or compacted soil layers with high bulk density (> 1.5 g/cm3) that impede root growth and decrease oxygen and water movement in the soil. Compacted soil layers are widespread in Queensland cropping regions and may be natural or caused by heavy traffic and tillage on wet soils (see Subsurface Compaction factsheet).

Biological constraints

Biological subsoil constraints occur when there is a reduction in the activities of good soil organisms, such as earthworms and arbuscular mycorrhizae, and/or an increase in pathogens or plant-parasitic nematodes. Pathogenic fungi and nematodes often build up in the subsoils of monoculture cereal cropping systems (see Root Lesion Nematode—Queensland and Crown Rot—Queensland factsheets).

Managing subsoil constraints

Good agronomic management helps minimise the water and other physiological stresses imposed by subsoil constraints (table 2). In paddocks where subsoil constraints exist, successful cropping can be achieved by:

maximising fallow efficiency with short fallows

ensuring effective weed control

using suitable rotations that minimise disease

matching nutrients to realistic yield expectations

using appropriate species and cultivar selection

undertaking timely crop sowing.

Table 2: Some management options for soils with high chloride and sodium concentrations.

LOW CONSTRAINTS
MEDIUM CONSTRAINTS
HIGH CONSTRAINTS

<600 mg Cl/kg, <500 mg Na/kg
600-1200 mg Cl/kg, 500-1000 mg Na/kg
> 1200 mg Cl/kg, >1000 mg Na/kg

in top 1 m soil depth
in top 1 m soil depth
in top 1 m soil depth

* Cereals – legume rotation
* Grow tolerant cereals (wheat, barley, canola
* Consider alternative land use (saline forage/ pasture production, agro-forestry/forestry system).

* Consider canola if soil profile is full
* Match inputs to realistic yield
* Avoid crops or grow tolerant cereals

* Manage crown rot and nematodes
* Consider tolelant cultivars
* Match inputs to realistic yield

* Try opportunity cropping to use available water
* Manage crown rot and nematodes

* Avoid legumes and durum wheat

* Try opportunity cropping to use available water

Fact Sheets Subsoil Constraints - Qld