Brett Rosenzweig – Industry Development Officer
Soil salinization is a problem of increasing concern in Australian agriculture. Saline soils are characterised by high concentrations of salts which are present in quantities sufficient to restrict the growth of salt sensitive plants. Sodium chloride, common salt, is the major offender in the development of saline soils in Australia, and usually contributes about 30% or more to the water-soluble salts in saline salts.’ The above text is taken from the Australian Soil Fertility Manual published by CSIRO. What image comes to mind when reading the text? An image of dryland salinity with lakes of salty water caused by rising water tables as a result of land clearing? Maybe a perched water table that’s come to the surface as a direct result of over irrigation or flooding rain events? The reality is that while those two scenarios are very visible examples of soil salinization, not all salinity induced problems are visible.
Components of Salinity Stress
Salinity stress in a tree can be caused by a number of factors (Figure 1). Most commonly stress is caused by the toxicity of ions, usually sodium, chloride and boron, which can affect the uptake of positively charged ions i.e. potassium and calcium. In addition, there is also an osmotic effect whereby if the concentration of toxic ions is high, uptake occurs more readily resulting in leaf tissue damage. The symptoms are usually noticed on the margins of the leaf and include necrotic spots, leaf bronzing and possibly defoliation. The most common observation of salt damage in the orchard is the ‘typical leaf-tip burn’ (Figure 2). Both sodium and chloride toxicity result in necrosis (death) of the terminal (bottom) end of the leaf with sodium toxicity showing striations that are perpendicular to the midrib. The best method to determine whether sodium or chloride is the toxic element is through a standard leaf analysis test. Care must be taken to not confuse leaf-tip burn caused by sodium or chloride toxicity and the effects of Almond Leaf Scorch (Xyella fastidiosa) which is a bacterium transmitted by some sap sucking insects (Figures 3 & 4). Trees infected with Almond Leaf Scorch usually show symptoms in early January and there is a golden yellow band between the green and necrotic parts of the leaf. Thankfully the prevalence of Almond Leaf Scorch is very low in Australian orchards.
The first step in managing the effects of salinity is leaf sampling. This should already be part of the routine management program and conducted in mid-January. Leaf samples however only give an indication of the current status of the trees (in particular the trees used for sampling). Yield and tree health may have already been compromised and therefore more attention should be placed on prevention of salt uptake. As with water stress, once the visual symptoms appear the damage is already done. The best way to keep salinity in check before visible symptoms occur is to monitor the levels in the soil and take remedial action before symptoms are observed or leaf tissue alarm bells start ringing. Taking annual soil samples from selected locations in the orchard should be part of every growers’ management program. Why??? The obvious signs of increasing salinity levels are not always evident (Figure 5). The high prevalence of drip irrigation in the Australian almond industry and the resultant smaller rootzone compared to full cover sprinkler irrigated orchards means the accumulation of salt can occur around the rootzone fringes in greater concentrations. The normal practice of irrigation scheduling is to apply enough water to keep salt leached out of the active rootzone. In sprinkler irrigated orchards the salt is pushed downward out of the rootzone however in drip irrigated orchards the salt can also be pushed sideways towards the midrow as well as downwards. Why is this important for tree health? Modern irrigation scheduling of daily irrigations based on consumptive water use and improved fertigation practices promote a rootzone with more fine root hairs compared to structural roots. The greater surface area of fine root hairs would suggest more efficient water and nutrition uptake. Figure 6 shows how salt can accumulate in a drip irrigated rootzone. If the main rootzone is depleted of soil moisture and roots on the margins are forced to draw in more water, salt uptake could be increased. Alternatively, if the main rootzone is low in soil moisture and rainfall occurs salt could be pushed inwards towards the main rootzone and salt uptake could be increased. What is a safe level of soil salinity? Table 1 shows the ranges of salinity levels and the corresponding potential yield decline that could occur.
|Potential yield decline||main threshold||90% loss||75% loss||50% loss|
|Soil Salinity ECse (dS/m)||1.5||2.1||2.9||4.3|
Table 1: Salt tolerance data for Almond (Prunus dulcis)
Drip irrigated orchards
Soil samples should be collected from three depths (two minimum) in the rootzone usually every 30cm down to one metre. Two locations should be sampled; one 20cm from the dripper and the other on the edge of the wetted area, usually 60cm from the dripper. A double handful of soil is collected from each depth/location and placed in a bag before sending to a laboratory for analysis.
Sprinkler irrigated orchards
The sampling pattern for sprinkler irrigated orchard is similar to drip irrigated orchards except three locations are samples instead of two. As most sprinkler systems don’t have a perfect distribution uniformity, samples should be taken with two metres of a sprinkler, one third distance between sprinklers on a diagonal and in the middle of the sprinkler pattern. The aim of this method is to identify if there are variations of salinity in the sprinkler pattern by location and depth. If the irrigation system has a designed and measured distribution uniformity of 80% or greater or salinity results from two or more years indicate there is no variation by location, then the number of locations could be decreased.
In both drip and sprinkler irrigated orchards the number of soil salinity monitoring sites will be guided by soil type and irrigation management units. Sites that representative of an irrigation management unit should be chosen for routine sampling. It’s important to return to the same site each year for continuity of results. If soil salinity levels are consistently low, monitoring could be conducted every alternate year or even as many as every three years.
- Continue to conduct routine leaf sampling programs for representative sites in the orchard, both for salinity and general nutrition status
- Monitor soil salinity levels by having soil samples tested. This helps establish a baseline for further management decisions.
- Specific leaching irrigations should be timed with rainfall (both sprinkler and drip) during the early winter months.
- Smaller, frequent leaching irrigations are better than one large irrigation especially if there are drainage concerns.
- Soil nutrition tests can also be done in the main rootzone whilst sampling for salinity to gauge soil health i.e. cation exchange capacity, calcium:magnesium ratio.