The Importance of Deep Nitrogen and Sulphur Testing for Crop Nutrition Management

Deep Nitrogen and Sulphur subsoil tests provide growers with a snapshot of the availability of these nutrients in their soils and is used as a tool to create a more informed fertiliser topdressing decision.

 

Nitrogen and sulphur are essential elements in plant nutrition, being required for a range of plant functions and structures. 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.

 

Soil moisture and temperature are two factors that tend to indirectly affect the composition of the soil nitrogen ‘pool’, largely due to the direct effect of these factors on soil microbes associated with the nitrogen cycle. The significant 2019/20 summer rainfall events across many districts in South East Australia along with periods of relatively mild summer temperatures have led to warm and moist soils for considerable periods of time. Warm, moist soils under the right conditions are known to stimulate the activity of nitrogen-mineralising bacteria and so there is likely to be significant net nitrogen mineralisation during this period. The continuation of warm, moist soils into the Autumn in areas would have likely increased this potential even further. Consequently, it is likely that there will be increased reserves of plant-available (mineralised) nitrogen leading into the 2020 Winter cropping season, however the extent of this will vary from paddock to paddock.      

 

Nitrate, ammonium and sulphate are inorganic 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 solution as it does to their tendency to move/leach down through the soil profile, especially 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 (typically down to 60cm in the soil profile) soil samples 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 return on investment. Western AG is currently collaborating with Precision Agriculture in the implementation of deep N+S soil sampling programs, on behalf of Western AG clients. If you would like to know more about deep N+S soil sampling or to arrange a paddock sampling program, please feel free to contact your local Western AG Agronomist.

Article produced by - Darcy Bullen, Western AG Nhill

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