OPTIMISING CROP ROOT SYSTEMS — NEW SOUTH WALES
- Maximising the abundance and depth of crop roots means plants can take up more nutrients and water.
- Maintain pH above 5.2 in the topsoil and 4.8 in the subsoil to keep nutrients available.
- Other subsoil constraints to look out for are compaction, sodicity and salinity.
- Improving soil structure can reduce both the risk and severity of root diseases.
Maximising root abundance and depth
Soil conditions affect the development of crop root systems. One way to optimise plant uptake of nutrient reserves in soil is to maximise the area crop roots can contact in the soil and ensure they can follow nutrients and water down the profile. This is especially important for leachable nutrients such as nitrogen and sulphur, and nutrients with significant reserves in the subsoil. The abundance and depth of crop roots can be increased by recognising and improving conditions that inhibit root development or performance.
Nutrient availability is optimised in soils with topsoil pH levels in the range pH 5.2–7.5 (measured in CaCl2). Maintaining pH above 4.8 in the subsurface will support root growth down into the subsoil horizons. When the pH falls below 4.8, root systems tend not to extend into the acidic subsoil, and so are not able to access water and nutrients from deeper in the soil. Agricultural lime is effective in treating soil acidity and maintaining appropriate soil pH (see ‘Soil Acidity’ and ‘Making Sense of Chemical Indicators’ fact sheets). Building soil organic matter improves buffering of pH, and so helps soil organisms and crops to cope with acid conditions.
Subsoil compaction can reduce the extent of crop root systems by slowing and restricting root penetration (figure 1). Poor aeration leads to anaerobic conditions where root growth and biological activity are restricted more often and for longer periods compared to a similar but better aerated soil. This can mean roots are unable to access nutrients such as nitrogen from the subsoil, and so poor nitrogen use efficiencies.
Figure 1: A distinct compacted layer in a sandy loam; note fractures in the hard pan through which roots preferentially grow.
Deep ripping can be an effective method of decreasing subsoil compaction when inspection of the soil profile shows compacted layers (particularly layers that are continuous and will impede root penetration, see ‘Subsurface Compaction’ fact sheet). The effect of ripping is only temporary and, unless changes in management designed to reduce ongoing compaction are introduced, compaction will occur again. Some improved practices are controlled traffic, or growing deep rooted perennials.
Sodicity is where the proportion of sodium compared to other exchangeable cations in the soil is too high. In surface soils sodicity causes dispersion and soil erosion, but the exchangeable cation balance can be corrected with applications of gypsum, lime and organic matter. Remediating sodicity is increasingly difficult with depth.
The chemistry of sodic subsoils can restrict root growth and performance by restricting the uptake of water and some nutrients while allowing other nutrients to be taken up in excess of the crop requirements. Many sodic subsoils are effectively impermeable. After rain crops are subject to both waterlogging in the surface soil and moisture stress in the subsoil due to the lack of water movement into the soil.
Deep ripping can aid infiltration and root growth in the short term, but can also cause aggregates to slump back to a worse state than before ripping. Applying gypsum with deep ripping reduces this risk.
A saline soil contains high concentrations of soluble salts that restrict the growth of most plants. A salt is any chemical compound in which a cation (a positive ion) is balanced by an anion (a negative ion). The most common salt is sodium chloride. Many soil amendments, including gypsum, lime and many fertilisers, are also salts. Salinity reduces a plant’s ability to extract water from the soil, and can cause toxicities of specific ions.
Soils develop salinity via interactions with groundwater, or irrigation water. If groundwater rises to within 2 m of the soil surface, water can rise by capillary action to the surface. When this happens the water may bring salt into the root zone and when the water evaporates at the soil surface concentrated salts are left behind (figure 2).
Figure 2: Salinity scalding. Photo: Graham Johnson © NSW government.
Salts in soil amendments are unlikely to be applied in large enough quantities to create a salinity problem, but where salinity already exists these additions of salt can be relevant to the expression of salinity problems.
The principle strategies for managing and preventing saline soils aim to keep groundwater below 2 m depth by maximising water use by plants, and to make irrigation as efficient as possible (see ‘Salinity’ fact sheet). Selecting tolerant crop varieties is important where salinity is already present.
Plant root diseases
Diseases that affect plant roots can substantially reduce a plant’s ability to access water and nutrients from the soil. Good soil structure and healthy soil biology help to reduce the risk and extent of root diseases in crops. Waterlogging or water stress encourage the development of many root diseases. Good structure allows drainage of excess water and allows oxygen into the soil profile in wet conditions, maximises plant available water in dry conditions, and provides suitable conditions for soil biology. Healthy populations of diverse soil organisms reduce the ability of disease organisms to spread and dominate within the soil, and this can reduce crop damage even when disease is present.
Crop rotations with non-host species can reduce populations of disease organisms when weeds are well controlled. Where weeds are not controlled some species can support the pathogen during the rotation break, reducing the benefit of the practise. Burning stubble can remove disease inoculates above ground but does not substantially reduce the presence of disease organisms in the soil. See ‘Fusarium’, ‘Nematodes’, ‘Rhizoctonia’, ‘Take-all Disease’, ‘Soil Bacteria and Fungi’ fact sheets.
Further reading and references
The New South Wales Department of Primary Industries has information on soils and their management (online), including sodicity and salinity, and information on crop diseases (online)
Author: Stephanie Alt (NSW Department of Primary Industries), 2013.
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.