Research Projects

Research Projects

Developing treatments for the remediation of an olive orchard grown on highly sodic soils

Arnon Dag, Guy Levy, Isaac Zipori, and Uri Yermiyahu

In semi- arid and arid regions, the shortage of fresh water necessitates the use of low-quality water (i.e. treated wastewater (TWW) and saline-sodic water) for irrigation. Those types of water may contain large quantities of soluble salts, predominantly, comprising chloride (Cl) and sodium (Na) ions. Such type water may be sodic, and thus cause deterioration of soil structure and its hydraulic properties (Levy et al., 2014). Presence of Na ions in soil enhances (i) soil clay sensitivity to swelling which leads to the narrowing of water conducting pores and (ii) can ultimately cause the dispersion of soil clay particles which brings about clogging of soil pores and the breakdown of soil aggregates (Levy, 2012). Initial awareness to soil sodicity led the US salinity laboratory (1954) to suggest that exchangeable sodium percentage (ESP, a term used to express sodicity of the soil solid phase) of 15 as a critical level above which soil structure is severely deteriorated. Later on, McIntyre (1979) proposed ESP 5 as more appropriate for separating between non-sodic to sodic soils. Recently, according to the Australian Soil Classification, a sodic soil is defined by an ESP >6% (Isbell and NCST, 2016).

Currently, sodic soils represent 581 million hectares globally and over half of these, 340 million hectares, are found in Australia where they affect about 60% of Australian cropping soils. A decadal scale study in Australia in a region where sodicity was initially low to moderate in the upper 0.5 m of the soil profile but high in deeper layers, showed that with time a trend of increasing soil sodicity which was greater at deeper layer than at the upper 0.5 m layer. Most of the statistically significant increases in ESP occurred in areas under irrigated horticulture, with this likely due to the continued addition of sodium to the soil system in the same location and no soil tilling (Filippi et al., 2018). In Israel, Assouline et al. (2015) arrived at a similar conclusion whereby the negative effects of soil sodification are expected to be particularly severe in orchards planted on clay soils irrigated in the same spatial configuration (e.g. drip irrigation) during many years.

In Israel, some attempts to remediate soils irrigated with saline-sodic water with a source of Ca, i.e., phosphogypsum (PG, a byproduct of the fertilizer industry),  were carried out in loess soils in the North-Western Negev in the 1970-1980’s, mostly in cotton and wheat fields (i.e., the soil was subjected to cultivation every year). The proposed management was to spread the PG (5 ton ha-1) on the soil surface prior to the rainy season in order to (i) prevent seal formation at the soil surface, (ii) maintain high infiltration rate, and (iii) decrease the sodicity level in the upper soil layer (Agassi et al., 1985). Farmers have abandoned the combined use of saline-sodic soils with PG after 10-15 years, because of a gradual decrease in seedling emergence, yields and the need for using greater amounts of water for irrigation (Guy Levy personal knowledge). An additional possible explanation is the accumulation of Na in soil subsurface layers that consequently increases soil sodicity hazard (Magaritz and Nadler, 1993).

The inner coastal plain in Israel is characterized by clay soils and the use of TWW for irrigation for a long period. This combination has led to continued increase in the SAR values in table grapes vineyards and olive orchards, towards values that are considered harmful to crop production (Netzer et al.,2014; Erel et al., 2018). Moreover, olive trees are moderately resistant to salinity (Ben-Gal et al., 2017), however, the combination of salinity with waterlogging (caused by soil sodicity) might lead to severe damage to trees, and even to their death (Aragues et al., 2004).

Amelioration of sodic soils involves the addition of a source of Ca ions that can replace the adsorbed Na ions and thus reduce soil ESP to acceptable levels. Although most of the soils of Israel contain CaCO3, it is of limited value mainly because of its low dissolution rate that fails to provide an adequate amount of Ca ions for exchanging the adsorbed Na (Gupta and Abrol, 1990). Hence, use of an external source of Ca ions needs to be considered (Mace and Amrhein ,2001). To date, no proven treatment that can be applied to mitigate this problem on a commercial scale has been developed for the Israeli conditions.