Author: Courtney Bills, Technical Agronomist, Incitec Pivot (QLD/NT)

As growers prepare for another summer season, the winter months provide a good opportunity to look deeper into the soil profile to uncover soil properties that may limit crop performance.

The alluvial soils of the cotton and grain growing regions in Queensland and New South Wales are among the country’s most productive, however, salinity, sodicity, and alkalinity can still hold yields back. While soils may appear healthy with ‘satisfactory’ levels of required nutrients, elevated salt levels can result in hidden restrictions on plant roots, reduced nutrient availability and ultimately cost yield (Tilse et. al. 2022).  

Salinity is defined as the presence of soluble salts which can dominate a specific zone in the soil (or surface layers). Salts reduce crop performance in three main ways: osmotic stress, ion toxicity, and imposed nutrient deficiencies.  

  • Osmotic stress occurs when salt concentrations increase in the soil solution, making it harder for plants to extract water. Even when moisture is present, crops can behave as though they are experiencing drought stress because they must expend more energy to access that water.
  • Specific ion toxicity occurs when salts such as sodium chloride (Cl–), carbonate (CO32-) or bicarbonate (HCO3–) accumulate to excessive levels. High levels can reduce seed germination, damage roots, impair photosynthesis, and accelerate leaf senescence, all of which reduce plant fitness and can reduce yield potential.
  • Imposed nutrient deficiencies occur when sodium, chloride and bicarbonates (alkalinity) disrupt specific nutrient interactions, limiting the plant’s ability to take up key nutrients, particularly potassium, phosphorus and nitrate nitrogen.

Salinity is often only part of the story. Many saline soils also suffer from sodicity, where high sodium levels have accumulated in the soil profile. Sodicity is one of Australia’s most common soil constraints, affecting around a third of agricultural soils and costing growers through reduced productivity and poorer soil performance (Hazelton and Murphy 2016).

Excess sodium causes soil particles to disperse, breaking down the natural structure that allows water, air and roots to move through the profile (Dang et. al. 2010). The result is a tighter, denser soil with reduced infiltration, poor drainage, lower oxygen content resulting in restricted root development – making it harder for crops to access the water and nutrients they need to reach their yield potential. In furrow-irrigated cotton systems, these structural problems can significantly reduce irrigation efficiency and the plant’s access to water.

Figure 1: Identifying Sodic soils and correcting Sodic soils with gypsum applications. Source: Kelly and Rengasamy 2006.

Where and how to measure?

Salinity and sodicity issues often occur well below the surface. Sampling the profile in commonly segmented soil sampling zones (such as 0-10cm, 10-30cm, 30–60cm and 60–90cm), helps identify where physical or chemical constraints are located and if they will stop roots from accessing nutrients.

Potential salinity issues can be identified using the following laboratory measurements:

  1. Electrical conductivity (EC) (1:5 water) provides an indication of soluble salt levels – although does not account for soil texture.
  2. EC (saturation extract) is a measure used to reflect what plants experience in the soil. It is calculated using a soil textural factor which allows known crop EC values to be used across soil types and can illustrate a more accurate assessment.
  3. Chloride and sodium soil levels (using critical values per soil texture).

Structural stability and overall soil health can be assessed with the Loveday & Pyle test. It measures both soil dispersion and slaking, helping predict how a soil will respond to physical stresses such as cultivation, irrigation, rainfall events, and traffic.  

The results provide valuable insight into the risk of structural decline, surface sealing, crusting, reduced infiltration, and poor root growth. This measure is useful to determine the soils responsiveness of gypsum applications allowing growers and agronomists to better understand and manage soil constraints.

Turning information into profit

A comprehensive deep soil test through Nutrient Advantage® Laboratory can provide valuable insights into nutrient distribution and identify whether yield limitations are being driven by nutrient supply, soil chemistry, physical constraints or a combination of all three.

Where salinity is identified in irrigation fields, alternative irrigation water may be required to leach salts down the profile or an extended fallow period to refill the soil profile with ‘non-saline’ water.  Bore and other irrigation water sources can be analysed through the Nutrient Advantage® Laboratory to provide up to date insights into water quality for irrigation and other purposes. Understanding the specific cause of a soil constraint is important, as each issue requires different management practices and presents its own economic costs and opportunities.

Practices that build organic matter, including stubble retention and utilising cover crops, can improve soil structure, water movement and biological activity in these soils – enhancing overall soil function and performance.

Gypsum can help improve soil structure in two ways. Initially, the calcium in gypsum increases soil electrolyte levels, encouraging clay particles to flocculate and improving soil structure. Over time, calcium replaces sodium on the soil exchange complex, and with adequate rainfall or irrigation, the displaced sodium is leached from the profile, resulting in longer-term improvements in soil stability and function (Figure 1).

Understanding the complete soil profile allows growers and agronomists to make more confident, targeted and profitable decisions. Knowing what’s working for your soil profile – and what’s holding it back – leads to more informed decisions and better returns from fertiliser, irrigation and in-crop investments.

Further information  

For further information on soil testing tactics, fertiliser placement, and strategies to improve subsoil constraints, please contact:  

Reference

Dang, Yash & Dalal, Ram & Buck, S. & Harms, B. & Kelly, Rob & Hochman, Z. & Schwenke, Graeme & Biggs, Andrew & Ferguson, N. & Norrish, Shane & Routley, Richard & McDonald, M. & Hall, C. & Singh, Dhananjay Kumar & Daniells, I. & Farquharson, Robert & Manning, William & Speirs, Simon & Grewal, H. & Orange, D. (2010). Diagnosis, extent, impacts, and management of subsoil constraints in the northern grains cropping region of Australia. Australian Journal of Soil Research. 48. 105-119. https://doi:10.1071/SR09074

Hazelton P, Murphy B (2016) ‘Interpreting soil test results: what do all the numbers mean?’ (CSIRO Publishing: Melbourne) https:// doi:10.1111/SR12402

Kelly, J. and Rengasamy, (2006). Diagnosis and management of soil constraints: Transient Salinity, Sodicity and Alkalinity. The University of Adelaide, South Australia.

Tilse MJ, Bishop TFA, Triantafilis J, Filippi P. (2022) Mapping the impact of subsoil constraints on soil available water capacity and potential crop yield. Crop & Pasture Science 73, 636–651. https://doi.org/10.1071/CP21627