Pichu Rengasamy

Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: an overview

More than 60% of the 20 million ha of cropping soils in Australia are sodic and farming practices on these soils are mainly performed under dryland conditions. More than 80% of sodic soils in Australia have dense clay subsoils with high sodicity and alkaline pH (>8.5). The actual yield of grains in sodic soils is often less than half of the potential yield expected on the basis of climate, because of subsoil limitations such as salinity, sodicity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonate and aluminate. Sodic subsoils also have very low organic matter and biological activity. Poor water transmission properties of sodic subsoils, low rainfall in dryland areas, transpiration by vegetation and high evaporation during summer have caused accumulation of salts in the root zone layers. This transient salinity, not influenced by groundwater, is extensive in many sodic soil landscapes in Australia where the watertable is deep. ‘Dryland salinity’ is currently given wide attention in the public debate and in government policies, but only focusing on salinity induced by shallow watertables. While 16% of the dryland cropping area is likely to be affected by watertable-induced salinity, 67% of the area has a potential for transient salinity not associated with groundwater and other subsoil constraints and costing the Australian farming economy in the vicinity of A

Soil processes affecting crop production in salt-affected soils.

1330 million per year. A different strategy for different types of dryland salinity is essential for the sustainable management and improved productivity of dryland farming. This paper discusses the sodic subsoil constraints, different types of salinity in the dryland regions, the issues related to the management of sodic subsoils and the future priorities needed in addressing these problems. It also emphasises that transient salinity in the root zone of dryland agricultural soils is an important issue with potential for worse problems than watertable-induced seepage salinity.

Additive effects of Na+ and Cl– ions on barley growth under salinity stress

Salts can be deposited in the soil from wind and rain, as well as through the weathering of rocks. These processes, combined with the influence of climatic and landscape features and the effects of human activities, determine where salt accumulates in the landscape. When the accumulated salt in soil layers is above a level that adversely affects crop production, choosing salt-tolerant crops and managing soil salinity are important strategies to boost agricultural economy. Worldwide, more than 800 million hectares of soils are salt-affected, with a range of soils defined as saline, acidic–saline, alkaline–saline, acidic saline–sodic, saline–sodic, alkaline saline–sodic, sodic, acidic–sodic and alkaline–sodic. The types of salinity based on soil and groundwater processes are groundwater-associated salinity (dryland salinity), transient salinity (dry saline land) and irrigation salinity. This short review deals with the soil processes in the field that determine the interactions between root-zone environments and plant responses to increased osmotic pressure or specific ion concentrations. Soil water dynamics, soil structural stability, solubility of compounds in relation to pH and pE and nutrient and water movement all play vital roles in the selection and development of plants tolerant to salinity.

High concentrations of Na+ and Cl– ions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress

Soil salinity affects large areas of the worlds cultivated land, causing significant reductions in crop yield. Despite the fact that most plants accumulate both sodium (Na+) and chloride (Cl–) ions in high concentrations in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na+ accumulation. It has previously been suggested that Cl– toxicity may also be an important cause of growth reduction in barley plants. Here, the extent to which specific ion toxicities of Na+ and Cl– reduce the growth of barley grown in saline soils is shown under varying salinity treatments using four barley genotypes differing in their salt tolerance in solution and soil-based systems. High Na+, Cl–, and NaCl separately reduced the growth of barley, however, the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na+ and Cl– stress. The results demonstrated that Na+ and Cl– exclusion among barley genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. High concentrations of Na+ reduced K+ and Ca2+ uptake and reduced photosynthesis mainly by reducing stomatal conductance. By comparison, high Cl– concentration reduced photosynthetic capacity due to non-stomatal effects: there was chlorophyll degradation, and a reduction in the actual quantum yield of PSII electron transport which was associated with both photochemical quenching and the efficiency of excitation energy capture. The results also showed that there are fundamental differences in salinity responses between soil and solution culture, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.

Aggregation of soil by fungal hyphae

Despite the fact that most plants accumulate both sodium (Na+) and chloride (Cl–) ions to high concentration in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na+ accumulation. There have also been some recent concerns about the ability of hydroponic systems to predict the responses of plants to salinity in soil. To address these two issues, an experiment was conducted to compare the responses to Na+ and to Cl– separately in comparison with the response to NaCl in a soil-based system using two varieties of faba bean (Vicia faba), that differed in salinity tolerance. The variety Nura is a salt-sensitive variety that accumulates Na+ and Cl– to high concentrations while the line 1487/7 is salt tolerant which accumulates lower concentrations of Na+ and Cl–. Soils were prepared which were treated with Na+ or Cl– by using a combination of different Na+ salts and Cl– salts, respectively, or with NaCl. While this method produced Na+-dominant and Cl–-dominant soils, it unavoidably led to changes in the availability of other anions and cations, but tissue analysis of the plants did not indicate any nutritional deficiencies or toxicities other than those targeted by the salt treatments. The growth, water use, ionic composition, photosynthesis, and chlorophyll fluorescence were measured. Both high Na+ and high Cl– reduced growth of faba bean but plants were more sensitive to Cl– than to Na+. The reductions in growth and photosynthesis were greater under NaCl stress and the effect was mainly additive. An important difference to previous hydroponic studies was that increasing the concentrations of NaCl in the soil increased the concentration of Cl– more than the concentration of Na+. The data showed that salinity caused by high concentrations of NaCl can reduce growth by the accumulation of high concentrations of both Na+ and Cl– simultaneously, but the effects of the two ions may differ. High Cl– concentration reduces the photosynthetic capacity and quantum yield due to chlorophyll degradation which may result from a structural impact of high Cl– concentration on PSII. High Na+ interferes with K+ and Ca2+ nutrition and disturbs efficient stomatal regulation which results in a depression of photosynthesis and growth. These results suggest that the importance of Cl– toxicity as a cause of reductions in growth and yield under salinity stress may have been underestimated.

Root-zone constraints and plant-based solutions for dryland salinity

Several authors have proposed that soils are made up of aggregates of various sizes, stabilised by different organic and inorganic materials. Fungal hyphae have been shown to bind microaggregates ( 250 µm diameter). This paper examines the aggregation of soil clay by saprophytic (Rhizoctonia solani and Hyalodendron sp.) and mycorrhizal (Hymenoscyphus ericae and Hebeloma sp.) fungi. The results support the hypothesis that fungal hyphae bring mineral particles and organic materials together to form stable microaggregates at least 50 µm diameter.

The response of barley to salinity stress differs between hydroponic and soil systems.

Limitations to agricultural productivity imposed by the root-zone constraints in Australian dryland soils are severe and need redemption to improve the yields of grain crops and thereby meet world demand. Physical, chemical and biological constraints in soil horizons impose a stress on the plant and restrict plant growth and development. Hardsetting, crusting, compaction, salinity, sodicity, acidity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonates and aluminium are the major factors that cause these constraints. Further, subsoils in agricultural regions in Australia have very low organic matter and biological activity. Dryland salinity is currently given wide attention in the public debate and government policies in Australia, but they only focus on salinity induced by shallow groundwater. However, the occurrence of transient salinity in root-zone layers in the regions where water tables are deep is an important issue with potential for larger economic loss than water table-induced seepage salinity. Root-zone constraints pose a challenge for salinity mitigation in recharge as well as discharge zones. In recharge zones, reduced water movement in sodic horizons results in salt accumulation in the root zone resulting in chemical and physical constraints that reduce transpiration that, in turn, upsets salt balance and plant growth. High salinity in soil and groundwater restricts the ability of plants to reduce water table in discharge zones. Thus plant-based strategies must address different kinds of limitations in soil profiles, both in recharge and discharge zones. In this paper we give an overview of plant response to root-zone constraints but with an emphasis on the processes of salt accumulation in the root-zone of soils. We also examine physical and chemical methods to overcome subsoil limitations, the ability of plants to adapt to and ameliorate these constraints, soil modification by management of agricultural and forestry ecosystems, the use of biological activity, and plant breeding for resistance to the soil constraints. We emphasise that soil scientists in cooperation with agronomists and plant breeders should design site-specific strategies to overcome multiple soil constraints, with vertical and lateral variations, and to develop plant-based solutions for dryland salinity.

Tensile strength of dry, remoulded soils as affected by properties of the clay fraction

Many studies on salinity stress assume that responses in hydroponics mimic those in soil. However, interactions between the soil solution and the soil matrix can affect responses to salinity stress. This study compared responses to salinity in hydroponics and soil, using two varieties of barley (Hordeum vulgare L.). The responses to salinity caused by high concentrations of Na+ and Cl– were compared to assess any consistent differences between hydroponics and soil associated with a cation and an anion that contribute to salinity stress. Concentrated nutrient solutions were also used to assess the effects of osmotic stress. The effects of salinity differed between the hydroponic and soil systems. Differences between barley cultivars in growth, tissue moisture content and ionic composition were not apparent in hydroponics, whereas significant differences occurred in soil. Growth reductions were greater under hydroponics than in soil at similar electrical conductivity values, and the uptake of Na+ and Cl– was also greater. The relative importance of ion exclusion and osmotic stress varied. In soil, ion exclusion tended to be more important at low to moderate levels of stress (EC at field capacity up to 10 dS m–1) but osmotic stress became more important at higher stress levels. High external concentrations of Cl– had similar adverse effects as high concentrations of Na+, suggesting that Cl– toxicity may reduce growth. Fundamental differences in salinity responses appeared between soil and solution culture, and the importance of the different mechanisms of damage varies according to the severity and duration of the salt stress.

Phytotoxicity of aluminium to wheat plants in high-pH solutions

Abstract Changes in tensile strength with clay type, clay particle size, and amounts of spontaneously and mechanically dispersible clays were assessed for dry, remoulded samples of eight Australian Vertisols and Alfisols varying in clay mineralogy. The effects of different clay content and particle size on tensile strength were also measured. The average particle sizes of whole clay, and of spontaneously and mechanically dispersible clays were also determined. Tensile strength of soils was influenced by the type and amount of clay present, clay particle size, and amount of dispersible clay. Soil strength increased with increasing clay content. When the clay content increased beyond 20%, there was a dramatic increase in soil strength indicating the possible role of particle arrangement. Correlations between different clay types, clay particle sizes, cation exchange capacity and tensile strength clearly illustrated that the sensitivity of tensile strength to these factors was greatest in soils dominated by smectite, followed by illitic and then kaolinitic soils. Tensile strength of soils was positively and significantly correlated with both spontaneously and mechanically dispersible clays. A significant negative correlation was obtained between clay particle size and tensile strength for illitic soils, whereas the same linear relationship was not significant for smectitic soils. However, a significant negative exponential regression was obtained between the cube of clay particle size and tensile strength when the results of all soil samples were pooled together. Transmission electron micrographs (TEM) showed a wider clay particle size distribution in smectites than in illites. Soils with the highest amounts of fine clay and the widest clay particle size distribution had higher tensile strengths. The size of spontaneously dispersed clay particles was smaller than that of mechanically dispersed clay. No difference in clay particle size was found between spontaneously and mechanically dispersible clays from mixtures of a fine clay with a basic sand and silt matrix. Regression, collinearity diagnostics and principal component analyses were used to analyse the data. A high correlation was found between the cation exchange capacity (CEC) of clays and soil strength. The principal component analysis indicated that spontaneously dispersible clay, percentage of randomly interstratified minerals (RIM) and CEC were important factors in predicting the strength of remoulded soils.

Effects of gypsum and stubble retention on the chemical and physical properties of a sodic grey Vertosol in western Victoria

Phytotoxicity of aluminium in acid soils is well known. At pH ≥6.3, aluminate [Al(OH)4–] is the principal hydroxo-aluminium species in soil solutions; however, its phytotoxicity has not received much attention. Sodic subsoils in Australia are generally alkaline and have pH above 9. During our survey of 8 subsoils in South Australia, we found aluminate ions at concentrations greater than 0.8 mg/L (29.7 μmol/L of aluminium) in soil solutions when pH was greater than 9, with corresponding high uptake of aluminium by wheat plants. We studied the phytotoxicity of aluminium to wheat plants in solution culture by maintaining the pH of alkaline solutions at 9.2. Relative root lengths of wheat plants, compared with those in reverse-osmosis deionised water, were significantly reduced in alkaline solutions and CO2-free air indicated toxicity of hydroxy, carbonate and bicarbonate ions. Further reduction of root lengths due to aluminate toxicity was also evident. Relative root lengths of wheat plants, when comparing between +aluminium and –aluminium treatments, were reduced up to 50% in alkaline solutions containing as low as 1 mg/L of aluminium. Aluminium accumulated mainly in the roots, thereby reducing their growth. In bicarbonate solutions, aluminium toxicity under alkaline pH was highly significant (P<0.001). However, at the same level of added aluminium in carbonate solutions, relative root length was not reduced. This study concludes that when aluminium species are present at a concentration of about 1 mg/L in soil solutions with pH greater than 9, the growth of wheat plants could be significantly affected.