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The Role of Soil Nitrogen and Sulphur and the Importance of Subsoil Testing


Deep Nitrogen and Sulphur subsoil testing provides growers with a snapshot of what’s available in their soils and allows for more accurate fertiliser topdressing applications.


Nitrogen and sulphur are essential elements in plant nutrition, they are required for a range of plant functions and structures. The roles of these two elements are often linked, such as both being key building blocks of proteins. The major supply pathway of these elements to broadacre pastures and crops is facilitated through the soil in the root zone, where a range of nitrogen and sulphur containing compounds are often present. The proportions and concentrations of these compounds in the soil changes over time, due to a range of factors (crop residue levels, microbes, temperature, moisture/rainfall, soil type etc.). However, only a limited number of these forms of nitrogen and sulphur can be extracted from the soil by the plant in significant quantities.      


Nitrate/ammonium and sulphate are compounds of nitrogen and sulphur, respectively, and play a critical role in the take-up of these nutrients into plants. These compounds behave similarly in soil, being that they are very mobile. This common feature contributes to the ability of the plant root to take in these compounds through the soil, as it does to their tendency to move/leach down through the soil profile, particularly under wet conditions.


Particularly in the HRZ of Western Vic, leaching is one of the major ‘loss’ pathways by which nitrogen and sulphur leave the root zone and can have a significant effect on the availability of these nutrients to plants. In broadacre cropping systems, losses of these nutrients, through processes such as leaching, unless corrected can lead to nutritional deficiencies that restrict crop production/quality. The replacement of nitrogen and sulphur back into these systems is generally achieved with fertiliser applications (e.g. urea and sulphate of ammonia), however the various forms of these nutrients in these applications are all ultimately prone to the same loss pathways. While these nutrients can leach down through the soil and away from the immediate root zones of establishing crops and pastures, their accumulation deeper down the soil provides a potential source of nitrogen and sulphur for more established root systems later in the season. 


In Australian Broadacre cropping systems, deep soil samples (0-60cm cores) are collected and analysed to give an indication of mineralised nitrogen (nitrate and ammonium) and sulphur (sulphate) available to crops in the subsoil. Taking deep N+S samples during the Autumn/Winter period is critical to determining what nutrient levels are in your soils. Growers and advisors are then able to make more accurate decisions around fertiliser rates and timing in the hope of maximising yields and ROI. Speak to your Western AG agronomist today to plan your next paddock sample.  


Article produced by - Darcy Bullen, Western AG Nhill

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