Schleiff Uwe: Research for Crop Salt Tolerance under Brackish Irrigation

RESEARCH FOR CROP SALT TOLERANCE UNDER BRACKISH IRRIGATION *)

Schleiff Uwe

Independant Expert for Irrigation&Salinity – Fertilizers&Crops – Soils&Environment

http://www.salinity.de; D-38289 Wolfenbuettel; P.O.Box 1934; schleiff@salinity.de

Abstract

In many countries of the dry areas irrigated agriculture and horticulture are exposed to increasing pressure to expand the use of brackish and even high-saline waters for crop production, as water resources of good quality are required for human consumption and industrial development. But experience shows that successful and sustainable application of lower quality waters such as brackish or saline waters requires special management practices, which need to be improved. The adaptation of irrigation management to saline growth conditions is often unsatisfactory, as our understanding on the process of water uptake by roots from saline soils and factors affecting crop salt tolerance is still far from being complete. The paper analysis the present concept for rating the salt tolerance of irrigated crops following the 4-layer-profile after FAO and USDA, where the vertical movement of solutes is considered. This concept does not reflect the effects of the lateral salt movement which dominate between water applications to cover the plant water supply. Field experiments with onions under irrigation with brackish waters of increasing salinity and under strongly arid climate (Saudi Arabia) have shown a continuous increase of soil salinization in the rhizospheric soil (soil adhering the roots) as compared to the bulk soil (soil far from roots). The presented model calculation demonstrates the effect of roothair length and rhizospheric soil volume on salt concentrations in the rhizospheric soil solution. The effects of root morphology in conjunction with soil properties are considered as an important link to complete our understanding on crop salt tolerance with respect to optimize irrigation technique and breeding of more tolerant crops.

KEY WORDS: saline agriculture, crop salt tolerance, soil salinity, brackish irrigation, rhizosphere, soil solution, root morphology, soil water potential, breeding

Zusammenfassung

Forschung für Salztoleranz von Kulturpflanzen bei Bewässerung mit Brackwasser

In vielen Ländern der Trockengebiete stehen Landwirtschaft und Gartenbau unter dem Druck, die Verwendung von Brackwasser und selbst salzreichem Wasser in der Pflanzenproduktion einzusetzen. Die Wasserressourcen guter Qualitaet werden fuer Haushalte und industrielle Entwicklung benoetigt. Die Erfahrung zeigt, dass die erfolgreiche und nachhaltige Anwendung von Wasser minderer Qualitaet wie Brack- und Salzwasser ein spezielles Management erfordern. Die Anpassung von Bewässerungsverfahren an Salzbedingungen ist oft unzureichend, weil das Verständnis vom Prozess der Wasseraufnahme durch Pflanzenwurzeln im Zusammenhang mit der Salztoleranz von Pflanzen noch sehr lueckenhaft ist. Die vorliegende Publikation analysiert das aktuelle Konzept zur Bewertung der Salztoleranz bewässerter Kulturpflanzen, dem nach FAO und USDA ein vertikaler Transport von Wasser und Salzen in einem 4-schichtigen Bodenprofil zugrunde liegt. Dieses Konzept berücksichtigt nicht die laterale Bewegung von Bodenwasser und –salzen, die zwischen zwei Wassergaben dominiert und von der Transpiration der Pflanze angetrieben wird. Feldversuche in Saudi Arabien mit Zwiebeln, die unter extrem ariden Bedingungen mit Brackwasser unterschiedlicher Salzgehalte bewässert wurden, hatten im wurzelnahen Boden deutlich höhere Salzgehalte als im wurzelfernen Boden. Die vorgestellte Modellrechnung macht auf die Bedeutung der Wurzelbehaarung für das Volumen des rhizosphären Bodens und die Salzkonzentrationen in der Wurzel umgebenden Bodenlösung. Die Bedeutung der morphologischen Eigenschaften von Wurzeln in Verbindung mit Bodeneigenschaften werden als ein wichtiges Glied angesehen, unser Verständnis von der Salztoleranz von Pflanzen im Hinblick auf die Optimierung von Bewässerungsverfahren und Züchtung salztoleranterer Pflanzen zu vertiefen.

SCHLÜSSELWORTE: Bodenversalzung, Salztoleranz von Kulturpflanzen, Bewässerung mit Brackwasser, Bodenloesung, Rhizosphaere, Wurzelmorphologie, Bodenwasserpotential, Pflanzenzuechtung,

1 Introduction and Definitions

In the dry areas of the world agriculture and horticulture are exposed to increasing pressure to develop brackish water resources for irrigation and to reduce the consumption of fresh water, which are required for human consumption and industrial development. In Oman the over-pumping of good quality groundwater along the Batinah-Coast has caused a continuous intrusion of seawater into the groundwater, resulting in severe soil salinity and expanding problems for crop growth, even for palm trees (Ahmed et al., 2004). The Government of Oman is now developing a strategy to limite agricultural production in coastal areas and to transfer the production of selected crops on suitable land inside the country, where brackish groundwater resources can be developed for crop irrigation (MRMEWR/OMAN et al, 2005). Similar strategies to expand the use of brackish and treated waste waters for irrigation are under discussion in many countries of arid regions (Dichtl N., 2002).


As there is no internationally accepted clear definition of the term ‘brackish water’, I will give my understanding applied for this paper. The term ‘brackish water’ was originally used for waters found at the mouth of rivers, where sweet river water is mixed with saline seawater. Under such conditions the salinity of brackish waters may vary in wide ranges following the tides. As the term is not clearly defined with respect to salinity for irrigation, we proposed for a regional study in Oman the rating of brackish groundwater salinity as given in Table 1:


Table 1: Brackish Water Classification for Irrigated Agriculture (proposed for brackish groundwater resources of Oman, 2005)


Unit

Fresh

Brackish

Saline

Mildly

Moderately

Strongly

dS/m

<2

2 – 5

5 – 10

10 – 15

>15

g/l

<1.5

1.5 – 3.5

3.5 – 7

7 - 10

>10


The term ‘crop salt tolerance’ evaluates the agronomic effect of soil salinity. The ‘crop salt tolerance’ appraises the relative yield loss of a crop as compared to the crop yield from a non-saline soil under comparable growing conditions. The relative crop yield (Y in %) at a given average root zone soil salinity (ECse = EC of soil saturation extract in dS/m) for a specific crop can be calculated from:

Y = 100 – b (ECse – a)


where: ‚b‘ = yield loss in % per unit (1 dS/m) increase of soil salinity;

(with ranges from <3%/(dS/m) for salt-tolerant to >30%/(dS/m) for sensitive crops)

‚a‘ = soil salinity threshold value: ECse, where yield loss starts;

(ranges from 1.5 dS/m for salt-sensitive to 10 dS/m for salt-tolerant crops)

‚a‘ and ‚b‘ are crop specific: published in Ayers&Westcot, FAO, 1985


The following Fig.1 presents this definition graphically for crops differing in their salt tolerance, but is completed for halophytes.

Fig.1: Rating of relative crop salt tolerance after FAO (Ayers&Westcot, 1985) completed for halophytes (after Schleiff, 2003)


The growth of halophytes is included as they are sometimes taken into consideration for excessive salinity levels, e.g. re-use of saline drainage waters. Their agronomic value is still under discussion (Lieth, 1999).

2 Analysis of FAO-Concept for Rating of Crop Salt Tolerance

The rating of crop salt tolerance after FAO considers the root zone as a 4-layered soil profile, the top layer of 0-25%, the middle layers of 25-50% and 50-75%, and the bottom layer 75-100% as presented in Fig.2. To cover the water demand of plants the water depletion follows the pattern 40/30/20/10% (0.4/0.3/0.2/0.1 ET = EvapoTranspiration), which reflects the decreasing rooting density: 40% of the ET demand is taken from the top 25% of the rooted soil layer, 30%, 20% and 10% from the 2nd, 3rd and bottom layer. Consequently the share of water for salt leaching (LF = leaching fraction) and controlling soil salinity decreases with soil depth. As a result the root zone salinity is not considered as homogeneous, but increases with soil depth. Rating of crop salt tolerance is based on this concept and all ECe-values given for relative yield losses of crops refer to the average root zone salinity calculated from the 4-layer soil salinity. For special cases this concept is adapted by modifying the water depletion pattern


However as pointed out on the right half of Fig.3 there is one important aspect of water and salt movement under brackish irrigation that is not taken into consideration. It is the lateral movement of saline soil solution to cover plant water requirement and which dominates between 2 water applications, when the soil water content drops below field capacity. Stimulated by transpiration, which is usually stronger under arid weather conditions, a flow of saline soil solution is directed to the water absorbing root surface. The soil water is taken up by roots, but in saline soils most salts remain in the soil solution surrounding the roots. As shown earlier the uptake of damaging salts by most crops can be considered as nearly negligible in this context (~<5% of soil solution salinity by most crops; Schleiff 1982a, b).

3 Salt Dynamic around Irrigated Roots

The following results from a field investigation carried out under extremely arid conditions in Saudi Arabia with onions show that these considerations are not just theory, but can be even found also under the difficulties of field experiments. Onions were irrigated for around 4 months with brackish waters of different salinity levels ranging from about 2.3 dS/m to 8.0 dS/m, which corresponded to Cl-concentrations from 13 to 52 meq/l (see Fig.3). The plots were irrigated once (winter time) and twice (summer time) a week with an application rate of 50mm. Soil samples were taken from the rooted bulk soil (soil far from roots) and soil adhering the roots (rhizospheric soil) at two occasions:

As shown in Fig.3, the Cl-contents of the bulk soil followed the Cl-concentration of the irrigation waters and increased from about 0.5 meq/100g to nearly 2 meq/100g soil. However the Cl-contents of the rhizospheric soil exceeded the Cl-contents of the bulk soil even immediately after the water application with values in the range from about 3 to about 5 meq/100g soil. Thus the water application of 50mm did not level the Cl-contents of the bulk and rhizospheric soil. The following period of 4 days transpiration caused a Cl-accumulation in the rhizospheric soil, which was 2 to 3 times higher than immediately after the water application and ranged from 8-9 to 10-12 meq/100g soil (Schleiff 1981b).


The results from the presented field experiment have clearly shown that soil salinity is not only heterogeneous in the vertical dimension as supposed in the FAO-concept, but there is also a significant gradient in the lateral dimension. This gradient is not considered in the present concept for rating the crop salt tolerance, but doubtless is of particular importance for plant water supply and consequently crop salt tolerance (Schleiff 2005).

4 Modelling of Soil Solution Salinity as Affected by the Volume of the Roothair Zone

The conclusion from the field measurements is that salinity stress for plant roots under irrigation is rapidly varying within short periods and can be much stronger than expected from calculations or measurements of the average soil solution salinity. The level of salt concen-trations in the rhizospheric soil solutions that can be achieved through plant transpiration depends from several factors. One important factor is the water potential of the leaves as al-ready reported for maize, barley, wheat and sugar beets (Schleiff 1981a, 1982b, 1983, 1984). Numerous experiments with many crops have clearly shown that it is the salt concentration or osmotic water potential of the root surrounding solution, which the roots are actually exposed to and that is decisive for the water uptake by roots. However the level of the osmotic soil water potential the roots were exposed to was usually taken from the average soil solution salinity, a gradient of soil solution salinity between bulk and rhizospheric soil was not considered.

Fig.3: Concept for research on crop salt tolerance under brackish irrigation

Fig.3: Chloride contents of bulk and rhizospheric soil under brackish irrigation (Schleiff, 1981b)


The results of the following model calculation presented in Fig.4, however, points out the considerable differences of the effective soil solution salinity around roots following a period of water depletion and depending from the morphology of roots. The simplified model calculation compares the effect of a water depletion of 10µl/10cm root length by roots, which are equipped with roothairs of 0.5, 1.0 and 2.0mm length and form a rhizocylinder volume of 78.5, 314 and 1256 mm³. The columns of Fig.4 show a large difference in the increase of the rhizospheric soil solution salinity. In case of short roothairs (0.5mm length) the water depletion effects an increase of soil solution salinity from 10 to 28dS/m, in case of medium roothair length (1.0mm) from 10 to 12dS/m only and in case of longer roothairs (2.0mm) the increase can be neglected. These ECe-values close to the roots correspond to osmotic soil water potentials of about -0.9, -0.4 and -0.35 MPa and may differ greatly with respect to their effect on water uptake by roots.


Even when the presented model calculation may suffer from some simplifications of minor effects (e.g. diffusion effects, water flow from bulk into rhizospheric soil, ion uptake), it clearly attracts our attention on the substantial contribution of root morphology for the salt concentrations in the soil solution near roots.


Fig. 4: Effect of water depletion by roots of different morphology on rhizospheric soil water salinity


5 Conclusions

As recently stressed by Hu and Schmidhalter (2004) further progress in research for crop salt tolerance is expected, when focus will be put on mechanisms of plant salt tolerance at the whole-plant level, organelle and molecular levels. From the results presented in this paper I conclude that with respects to practical irrigation management and breeding of more salt tolerant plants, interactions between plant roots and soils are an important and most promising field of research that has been overlooked in the past. However some important steps to develop an experimental set-up to understand the interactions between roots of different morphology and saline soils with respect to plant water supply were already presented (Schleiff 1987a, b, 2005).

6 References

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*) The paper was presented on the Scientific Conference SOIL AND DESERTIFICATION –Integrated Research for the Sustainable Management of Soils in Drylands; Biocentre Klein Flottbek/University of Hamburg, Germany, organized by DesertNet and AK BOGEO