By Ram Dalal
A group of agencies has developed a strategy to better identify and manage subsoil constraints in the northern cropping region. These constraints affect plant nutrient uptake and balance, and lead to significant economic loss to farmers.
Preliminary analysis of some of the data gained to date is promising in identifying and mapping subsoil constraints at a paddock scale.
The agencies contributing to the project are the Queensland Department of Natural Resources and Mines, the Queensland Department of Primary Industries and Fisheries, the University of Queensland, the Agricultural Production Systems Research Unit, a unit which includes CSIRO, the NSW Department of Primary Industries, the NSW Department of Infrastructure, Planning and Natural Resources and the University of Western Sydney.
The subsoil constraints are due mainly to high levels of salinity, sodicity and chlorides in the subsoil, factors which restrict a crop"s rooting depth and make it difficult for the plants to extract water due to the high osmotic potential of the soil solution and/or chloride toxicity.
In addition, salts and sodicity alter the nutrient balance in the crop, restricting uptake of some nutrients but allowing others (and elements such as Aluminium - Al) to be taken up in excess of the crop requirements. This leads to nutrient imbalance and element toxicity in plants and further restricts soil water uptake.
High sodicity also adversely affects the physical condition of the soil, reducing water entry and water movement, aeration and porosity (high bulk density/compaction). It has also emerged that many grain cropping brigalow soils have high subsoil acidity (pH <5), which may also restrict root growth.
The northern grains cropping soils, mostly Vertosols (cracking clay soils), are generally uniform down to at least one to two metres depth. Due to summer-dominant rainfall, winter crops largely rely on water stored in the soil profile during the previous summer-autumn fallow period.
Although most soils of the region can store as much as 250 millimetres of water in their profile, the presence of salts (salinity, measured as EC), sodium (sodicity, measured as ESP) and chlorides (measured as chloride concentration) in the subsoil effectively reduces rooting depths and therefore reduces the amount of water a crop can access from the soil profile. As a result, potential yields are not realised, based on stored soil water and in-crop rainfall, leading to significant economic loss to growers. Furthermore, land and water resources are under-used.
Therefore, potential environmental damage may occur due to salts mobilised in the landscape from increased runoff and deep drainage.
To help deal with these problems, the project has concentrated on three areas - identification, management and awareness of subsoil constraints. These include:
Preliminary regional mapping of the extent and distribution of salinity is based on identifying soils with electrical conductivity of saturation extract (ECse) >4 dS/m (deci siemens per metre), and sodicity, measured as Exchangeable Sodium Percentage or ESP>15 at 60cm depths.
For example, regional maps show that in central and southern Queensland, almost 30 percent of the soils exceed the salinity values of 4dS/m, while almost 40 percent of the soils exceeded ESP of 15 or more at 60cm depth. However, there are large areas in northern NSW and Queensland where no data exists on salinity and sodicity levels. Soil data from these areas will be collected in the next three years.
Remote sensing is being used to prepare plant biomass maps near anthesis comprising Normalised Distribution Vegetation Index (NDVI) at a paddock and landscape scale. This is complemented by monitoring salinity in the top 1.8 metres of soil using EM38 near the ground level. Researchers will then be able to relate maps of paddocks" biomass with salinity levels to test whether this method can be used as a relatively cheap way to map areas with subsoil constraints. This work is being done in collaboration with projects supported under the GRDC Precision Agriculture initiative, SIP09, and preliminary analysis of data has been promising.
Preliminary model simulations of the likely impact of subsoil constraints on crop yield and return were undertaken based on the assumption that the effects of salinity and sodicity on crop growth and soil water use can be accounted for as a reduction in plant available water capacity (PAWC).
The main conclusions so far are:
Under a sorghum crop, runoff was small even with reduced PAWC over a range of sites, and differences between sites were relatively small.
Preliminary estimates of threshold salinity, sodicity and chloride levels under an average growing season showed the following crop species effects:
Wheat Cultivar effects:
Potential management solutions include selection of crops and cultivars that are more tolerant of subsoil constraints, and amendments and nutrients. Points to consider include:
Glasshouse experiments provided further elucidation of the effects of salinity and sodicity as they interact with the level of water stress imposed at different stages of crop growth.
Water stress imposed at anthesis reduced grain yield more than at any other stages of growth. When water was not limiting growth, yield declined by only 8 percent for the medium (0.5 g NaCl/kg soil) but 36 percent for the high salt (3g NaCl/kg soil) treatments.
Corresponding reductions in wheat yields under water stress at anthesis were 27 percent and 39 percent.
Therefore, water stress at anthesis has a large effect on yield even at low salinity levels while at high salinity levels, the crop remains under water stress even when there appears to be plenty of water available.
These experiments confirmed the observations made by growers and researchers under field conditions.
For example, wheat cultivars differed in their tolerance to salinity and sodicity. Again, chickpea was found to be most sensitive to salinity followed by wheat, barley and canola. Barley and canola showed better tolerance than chickpea and wheat, despite heavy accumulation of Na in their leaves.
A note of caution, however: it is early yet to sort out the individual and interactive effects of EC, ESP and chloride levels on yields and this requires further study.
For example, high sodicity in the subsoil alone may not limit crop yields. Also, other potential subsoil constraints should be considered.
For example, we found a high concentration of Al in chickpea in a saline poor growth area (3700mg Al/kg dry matter compared to 890mg Al/kg in a good growth area), although Al effects on plant growth is little understood under saline conditions.
Foliar tissue analysis showed relatively high Na concentrations in wheat (830mg/kg dry matter) and chickpea (1530mg/kg DM) at some of the sites near Walgett. Despite the alkaline pH of the soil profile, surprisingly high concentrations of Al and Fe (750-840mg/kg DM) were also found in chickpea foliar samples.
We observed grain yield increase of 8 percent in wheat, 39 percent in barley, 16 percent in chickpea, 52 percent in canola and 73 percent in faba bean in response to the application of 10kg P+2.5 kg Zn at Coonamble, northern NSW.
The cereals extracted more soil water than the pulses regardless of subsoil characteristics.
However, the difference in water uptake between the species declined in the subsoil with very high EC and chloride. Notably, canola was able to extract more water than the other species from a "low" EC and chloride subsoil.
In a "high" EC and chloride subsoil, there was no difference between canola and the cereals.
A benchmarking survey designed to evaluate current levels of awareness of the distribution, impact and management of soils with subsoil constraints among growers was conducted in 2003.
From a total of 421 responses, 52 percent of respondents believed that subsoil constraints were a major problem in their districts.
Almost 57 percent considered that subsoil constraints were a major limiting factor in the profitability and sustainability of their enterprises. Among subsoil constraints, 42 percent of growers perceived sodicity was the most prevalent, closely followed by nutrient deficiencies/toxicities (41 percent) and high bulk density (31 percent).
A relatively high proportion of respondents were unsure of the presence or absence of particular subsoil constraints on their properties (ranging between 30 percent and 44 percent, depending on the particular constraint).
Only 39 percent of respondents currently manage soils with subsoil constraints differently to other soils on their properties.
However, 81 to 92 percent growers were prepared to implement a wide range of management options if they could be demonstrated to be economically viable (eg change their crop rotation, apply gypsum or lime).
Eighty-seven percent of growers expressed interest in learning more about subsoil constraints. Field days/trial inspections were seen to be the most effective means of delivering knowledge and skills, followed by: via agronomists, grower training workshops, printed information, presentations at grower meetings and the Web, in that order.
The information gained from the survey will be used to design future project and communication activities and serves as a benchmark with which to gauge improvements in grower knowledge and practices later in the project.
For more information: Dr Ram Dalal, 07 3896 9895, Ram.Dalal@nrm.qld.gov.au
GRDC Research Code: DNR00004, program 4
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