Understanding nutrient stratification to guide crop management

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The messages for growers out of this research are:

  • nutrient stratification is a common occurrence and can potentially reduce the ability of crops to access soil nutrients and as a result significantly reduce grain production;

  • soil sampling techniques need to accurately reflect the crop-accessible nutrients in a soil profile to provide accurate fertiliser recommendations; and

  • further research should be aimed at improving our understanding of crop-accessible nutrients across varying nutrient stratification profiles.

Researchers have been studying the impact of macronutrient mobility on crop production and profitability

Mismatches between the location of roots, nutrients and water can significantly limit crop growth, research is showing.

A GRDC-funded scoping study paints a picture of macronutrient mobility through the soil and looks at how management strategies allow these nutrients to be accessed by crops. Researchers have found that nutrient stratification – a common occurrence where nutrients such as nitrogen, phosphorus, potassium and sulfur occur naturally as layers or bands of different concentrations – can reduce the ability of crops to access soil nutrients and, as a result, reduce grain production.

Photo showing the potassium and phosphorus at 10 to 15cm

While mobile nutrients such as nitrogen and sulfur can move deeper into the soil profile, immobile nutrients such as phosphorus and potassium can be concentrated in the top 10 to 15cm of soil.

PHOTO: Brad Collis

Mobile nutrients such as nitrogen and sulfur can move deeper into the soil profile, increasing stratification and creating nutrient deficiencies in the topsoil if they move past the rooting zone. Conversley, immobile nutrients such as phosphorus and potassium can be concentrated – or stratified – in the top 10 to 15 centimetres of soil.

Farming systems can exacerbate nutrient stratification. For example, reduced tillage prevents soil mixing, making banded nutrients even more distinct, either horizontally in drill rows or by vertical concentration in surface or subsurface layers.

Wider row spacings (greater than 30cm), used in northern growing regions for summer crops and increasingly in other regions to facilitate stubble retention/zero tillage, can also increase the horizontal gap between crop roots and the position of immobile nutrients such as phosphorus.

Crop rotations, narrow windrow burning, and straw handling are also expected to affect nutrient stratification, although more research needs to be undertaken in this area.

From a management perspective, stratification makes accurately assessing soil nutrients difficult because soil tests, especially from the 0 to 10cm depth, may not accurately reflect the potential response of the crop to applied fertiliser.

For example, some cropping soils have naturally high concentrations of phosphorus and potassium (and to a lesser extent nitrogen) in the topsoil (0 to 10cm) but lower concentrations in the subsoil. However, in duplex soils leaching of potassium and sulfur from the sandier topsoil means higher concentrations can be found at depth.

Crops can use significant amounts of nutrients located below the surface layer, so this should be taken into account when soil sampling to achieve more accurate predictions of nutrient availability.

For some nutrients, root uptake efficiency is maximised when the entire root surface – rather than just part of the root system – has access to nutrients (in an appropriate chemical form).

Soil sampling to depth (0 to 60cm or deeper) prior to sowing has been recommended for nitrogen for some time (and more recently for potassium and sulfur), although the actual adoption of this practice is generally low.

Soil testing to depths greater than 10cm for plant-available phosphorus is a relatively new concept and has not typically been employed to predict fertiliser responses under commercial conditions.

Surface samples (0 to 10cm) can be easily taken with a simple soil corer, but sampling to greater depths can be a challenge. For example, hydraulic soil samplers may be required to penetrate some subsoils (increasing sampling costs), while sandier soils can make keeping the soil sample intact difficult.

Previous crop and fertiliser history is not sufficient to assess nutrients at depth, as stratification deeper in the soil profile varies based on the depth of soil horizons. Greater understanding of soil characteristics would improve subsoil-sampling guidelines.

Sampling to depth (0 to 60cm) can also identify subsoil physicochemical constraints that may limit effective rooting depths, such as a low level of nutrients, salinity, sodicity, dense soils with high penetration resistance, waterlogging and ion toxicities.

Deep banding of nutrients adds another complication to subsurface sampling guidelines as the same effects seen at the surface with immobile nutrients (phosphorus, potassium) will occur in the subsurface.

Accurate assessment of crop accessibility to residual deep-placed fertiliser bands is required before effective subsurface sampling guidelines post-application of deep fertiliser can be developed.

The study has involved researchers at the University of Adelaide in partnership with the Victorian Department of Economic Development, Jobs, Transport and Resources, the South Australian Research and Development Institute and the Queensland Department of Agriculture and Fisheries.


More information:

Dr Sean Mason, University of Adelaide,
sean.mason@adelaide.edu.au

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Water–nitrogen interplay guides management

GRDC Project Code UA00155

Region North, South