Soil health

Soil health is key to your farm's profit and production. It underpins the fertility and crop production of a farming enterprise, providing the cotton plant with support and access to water, oxygen and nutrients.

An understanding how modern farming practices impact on the physical, chemical and biological properties of the soil is critical in making optimal soil management decisions.

The key components to soil heath are:

Soil organic matter and soil carbon  

Soil organic matter is composed of dead and decomposing plant material, soil biota (fungi, bacteria, worms etc.) and animal waste. Soil organic carbon is a component of the soil organic matter and the role this carbon plays is important for all three aspects of soil fertility:

• biological function: supplying energy for plant and microbial activity.
• physical function: stabilising soil structure, improving soil aggregation and water movement.
• chemical function: increasing cation exchange capacity (CEC), buffering pH and reducing the effect of salinity and sodicity.

Subsoil constraints

Subsoil constraints are soil properties that limit or restrict the cotton plant from meeting its water and nutrient requirements. 

An ideal soil for cotton production has good infiltration and internal drainage, high plant available water capacity (PAWC), good soil structure for root growth and development, optimum pH, low salinity, balanced nutrient availability, low sodicity and adequate soil biota.

Problems associated with subsoil constraints include compaction, soil dispersion, high or low pH and waterlogging. These problems can result in poor plant growth, loss of bolls and poor boll set, reduced yields, erosion, increased land management costs and other management issues.

Sodic soils

Sodic soils have too high a proportion of sodium. This excess of sodium in soil reduces the strength of bonds that hold clay particles together within aggregates. The sodium also attracts water molecules more readily than other cations, which forces the clay particles apart, causing what is known as dispersion, which causes the soil structure to collapse.

Many of the soils used for cotton production in Australia, are sodic or strongly sodic below a depth of 0.5m. This affects root growth and uptake of water and nutrients. Groundwater used for irrigation can cause sodicity problems particularly when the water contains high sodium levels relative to calcium.

Saline soils

Saline soils contain excess salts in the soil solution (the liquid in soils held between the soil aggregates). When the concentration of salts in the soil solution exceeds that found in the plant roots, water flows from the roots back into the soil, so that the plant is unable to obtain sufficient water even though the soil is moist. 

Compaction

Compaction restricts root growth, reducing the availability of nutrients and water to the cotton plant. Soil compaction can also increase denitrification, further reducing the availability of nitrogen. Some compaction is an inevitable consequence of using heavy machinery on soils, but impacts can be minimised by implementing good management practices, minimum tillage, controlled traffic and guidance systems.

Waterlogging

Waterlogging, particularly following surface irrigation, can impact significantly on cotton production, causing problems such as denitrification, boll shed and reduced boll set.

Soil pH

Soil pH is a measure of the acidity, neutrality, or alkalinity of the soil solution. pH directly influences the availability of soil nutrients to the cotton plant. Most cracking clay soils are alkaline (pH 8.0 to 8.5), which affects the availability of many micronutrients.  

Soil mycorrhiza

Soil mycorrhiza (also referred to as AM) are soil-borne fungi that attach themselves to the growing roots of crops. They can allow roots to scavenge more effectively for nutrients, particularly phosphorous and zinc, which are immobile in the soil.

AM fungi survive well in dry soils, but can decline if the soil undergoes cycles of wetting and drying without a host plant being present. AM can also be reduced in some systems following non-mycorrhizal crops like canola. Low AM colonisation is a symptom of cotton long fallow disorder, but is not a cause.

• Conduct soil sampling to determine your soil’s physical and chemical properties. 
• Prevent or minimise erosion in susceptible areas and establish a monitoring plan to track progress.
• Monitor soils for structural issues like compaction, hard setting, salinity and sodicity to minimise potential problems.
• Use soil tests and field history information to determine nutrient input requirements.
• Use the most appropriate and efficient nutrient application methods and timing.