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Subsoil salts affect root function, shoot growth and ionic balance of wheat plants

Harsharn Singh Grewal1, Shane Norrish1 and Peter Cornish1

1University of Western Sydney, Hawkesbury Campus, NSW 2753, Australia www.uws.edu.au, Email: h.grewal@uws.edu.au
s.norrish@uws.edu.au
p.Cornish@uws.edu.au

Abstract

GRDC-funded research focusing on subsoil constraints to crop production in the northern grain zone of Australia commenced in July 2002. The aims of the research are to evaluate the incidence, severity and causes of subsoil constraints, and to develop appropriate management strategies for overcoming these constraints to crop production. Results of analyses of Vertosol soils from paddocks in north-western NSW indicate that electrical conductivity, exchangable sodium percentage (ESP) and chloride concentrations commonly increase markedly as soil depth increases, suggesting major constraints in the subsoil. Chloride concentrations in some of the paddocks increased from 20 mg/kg in topsoils to > 1210 mg/kg in subsoil. Similarly, ESP increased with depth, and was more than 30 in some subsoils. The results of a glasshouse experiment involving different subsoil salinity levels (0, 50, 100, 150, 200, 300, 400, 500, 1000 and 1500 mg of added NaCl/kg soil) in a Vertosol soil (EC 1:5 soil water: 0.36 dS/m, Cl 30 mg/kg and ESP 1) reveal that increased subsoil salts had detrimental effects on root length, root: shoot ratio, water use efficiency and ionic balance (Ca:Na and K:Na ratios) of wheat plants. The above-ground dry matter of young wheat plants (Z 41) declined significantly when 300 mg/kg of NaCl was added to subsoil. At this concentration of NaCl, the leaf tissue concentration of Na and Cl was 433 mg/kg and 10300 mg/kg DM, respectively.

Media summary

Increased subsoil salts had detrimental effects on root and shoot growth, water use efficiency and ionic balance of wheat plants.

Key words

Subsoil constraints, salinity, sodicity, ionic balance, wheat, water use efficiency

Introduction

The majority of researchers have traditionally concentrated on the best management of topsoils (0-15cm) to enhance crop productivity. However, subsoil characteristics can limit development, penetration and function of roots in lower layers of soil (Rengasamy et al., 2003). Subsoil properties may restrict water and nutrient extraction from subsoil, which affects crop yield and water/nutrient use efficiency. Due to highly variable and typically low in-crop rainfall, dryland crops in the northern grain zone of Australia generally rely on soil water and nutrients stored in subsoil to achieve economic yields. Subsoil constraints may have a considerable impact on productivity, stability and sustainability of crop production.

Impediments to subsoil root growth and function may be associated with single or multiple factors such as subsoil salinity, sodicity and acidity In addition, nutrient deficiencies, and/or toxicities due to high concentrations of boron, sodium, aluminium, chloride or carbonate in subsoils can reduce crop growth and yield. Some subsoils also have very high bulk density, low organic matter and biological activities (Rengasamy 2002).

The Grain Research and Development Corporation (GRDC) initiated a project in 2002 to investigate the quantitative and interactive effects of subsoil constraints on crop yields and to develop appropriate strategies to manage crops with subsoil constraints in Australia. Current research includes soil analytical studies and experimentation in the field and glasshouse. In field work we have an emphasis on participatory approaches in collaboration with growers and advisors. Field experiments aim to improve management strategies for crops growing in sites with subsoil properties that are hostile to root growth and function. Experiments have investigated the tolerance of crop species and varieties to subsoil properties, nutrient uptake, and the use of soil ameliorants such as gypsum. The work reported here briefly describes soil properties and wheat crops found in commercial paddocks in north-western NSW, typical of sites with high subsoil Na and Cl concentrations. Results are also reported from a glasshouse experiment investigating the tolerance of wheat to NaCl in the subsoil. In combination with other research, the project will contribute to identifying the primary mechanisms through which crop growth is restricted on sites with subsoil constraints and to developing practical and cost-effective solutions for managing these landscapes to optimise economic and environmental sustainability.

Methods

Commercial crop monitoring: After characterising soil profiles (EC, pH, ESP, Cl and extractable nutrient concentrations, clay content, -1500 kPa soil water content), growers’ crops are monitored for seasonal dry matter, rooting depth, soil water use and grain yield.

Glasshouse experiment: A glasshouse experiment involving addition of different NaCl concentrations (0, 50, 100, 150, 200, 300, 400, 500, 1000 and 1500 mg of NaCl/kg soil) in subsoil was conducted in a controlled environment (20C day and 10C night temperature) to investigate the impact of subsoil salinity on wheat growth (cv. Sunvale) during the vegetative phase. Heavy texture clay soil (vertosol) (EC 1:5 soil water: 0.36 dS/m2, Cl 30 mg/kg and ESP 1) collected from Moree in north western NSW was air dried and passed through a 2-mm sieve. Polythene-lined cylindrical PVC pots (40 cm long, 10.5 cm diameter pots) were used for growing plants. The 3.8 kg of soil per pot was divided into 0.8 kg for topsoil and 3.0 kg for subsoil. Salt treatments were applied to subsoil only and mixed in soil before potting. Soil for both zones (topsoil and subsoil) was watered to the drained upper limit (0.32 g/g) with double deionised water and allowed to equilibrate at a temperature of 25C in a glasshouse for four weeks before sowing wheat. Plants were harvested at boot stage (Z 41), dried at 70C and analysed for parameters of interest.

Results and Discussion

Field sites: Soils from growers’ paddocks indicate a marked variation in topsoil and subsoil properties and surface properties were generally a poor indicator of subsoil properties. Electrical conductivity (EC), exchangable sodium percentage (ESP) and chloride concentrations increased markedly as soil depth increased (Table 1). Relatively minor increases in profile pH and clay content contrasted to the large increases in subsoil EC, ESP and Cl concentrations, which were many times those found in surface layers. On these soils some crops accumulated elevated tissue Cl (ie; 1.6% at anthesis) and the Na concentrations had a marked effect on the ionic balance (Ca:Na, K:Na and Mg:Na ratios). Zn concentration of wheat shoots measured at anthesis at some of the sites was also low (ie; 6.4 mg/kg) suggesting Zn deficiency.

Table 1: Physico-chemical properties of a typical field crop site monitored near Walgett, NSW

Soil depth
(cm)

pH
(CaCl2)

EC 1:5

ESP

Cl
(mg/kg)

Clay
(%)

0-10

7.4

0.17

5

30

62

10-30

7.8

0.25

8

39

66

30-50

7.9

0.38

16

72

69

50-70

8.0

0.49

17

137

70

70-90

8.1

0.63

22

299

70

90-110

8.1

1.04

23

820

71

110-130

8.1

1.50

25

950

71

130-150

8.1

2.02

26

1210

71

Glasshouse experiment: The results showed that increased subsoil salinity reduced wheat growth (Table 2). At early boot stage (Z 41) there was a significant reduction in wheat DM when 300 mg/kg of NaCl was added to the subsoil. At this concentration of soil NaCl, the leaf tissue concentration of Na and Cl was 433 and 10300 mg/kg respectively. The salt-affected wheat plants also had reduced root and shoot dry matter, root: shoot ratio and water use efficiency. Increased subsoil salt concentrations markedly increased the Na and Cl concentrations in leaves and stems. The ionic balances (Ca:Na and K:Na ratios) of wheat plants were severely affected at high subsoil salinity (Table 3).

Subsoils with high Na and Cl concentrations are widespread in the northern grain zone, particularly in the lower rainfall areas. Although reductions in rooting depth and crop water extraction have been observed (not reported here), these results suggest that reductions in wheat growth may be due, at least partly, to several nutrient deficiencies and toxicities related to salinity, that may be important in these soils.

Table 2: Dry matter of shoots and roots, root/shoot ratio and water use efficiency of wheat in response to subsoil NaCl.

Soil NaCl
(mg/kg)

Shoot dry matter
(g/4 plants)

Root dry matter
(g/4 plants)

Root/shoot ratio

WUE1
(mg/ml)

0

4.27

1.60

0.38

1.91

50

4.19

1.63

0.39

1.91

100

4.15

1.62

0.39

1.87

150

4.14

1.58

0.38

1.88

200

4.10

1.53

0.37

1.84

300

3.92

1.32

0.34

1.80

400

3.89

1.30

0.34

1.78

500

3.56

1.21

0.33

1.72

1000

3.49

1.09

0.31

1.68

1500

3.38

1.04

0.30

1.67

         

LSD 0.5

0.25

0.09

0.02

0.06

1 Water use efficiency of shoot dry matter

Table 3: Ratios of Ca:Na and K:Na in leaves and stems of wheat grown under variable subsoil salt concentrations

Soil NaCl
(mg/kg)

Leaves

Stems

 

Ca:Na

K:Na

Ca:Na

K:Na

0

45

301

16

256

50

43

270

14

235

100

31

192

11

212

150

27

169

11

177

200

24

148

9

158

300

21

122

8

141

400

21

125

8

120

500

20

114

7

118

1000

14

81

6

95

1500

10

48

4

58

LSD 0.5

2

12

1

13

Acknowledgement

This work is supported by the Grains Research and Development Corporation. Technical assistance from Ms Linda Allanson and Mr Mark Emanuel is gratefully acknowledged.

References

Rengasamy P (2002). Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: an overview. Australian Journal of Experimental Agriculture 42, 351-361.

Rengasamy P, Chittleborough D and Helyar K (2003). Root-zone constraints and plant-based solutions for dryland salinity. Plant and Soil 257, 249-260.

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