Using granular polyhalite to improve structure stability of various soils types and their productivity – From mechanisms to application

Uri Nachshon and Meni Ben-Hur

Background

Soil structure is controlled by two main factors: (i) arrangement of soil particles in the soil space, and (ii) the strength of the forces that hold the soil structure stable against destruction actions. The pores between the soil’s particles, which characterized by their total volume, size distribution, tortuosity, and connectivity, affect the soil hydraulic properties, the root zone aeration, and the nutrients mobility toward the roots system (Hillel, 2004). The soil structure is a dynamic factor that could be altered by aggregate breakdown (e.g., Tisdall and Oades 1982; Le Bissonnais and Arrouays 1997; Chenu et al. 2000; Lado et al., 2004; Ben-Hur and Lado, 2008; Barto et al., 2010). FAO and ITPS (2015) indicated that at the last decades, the land use intensification in farmlands (e.g., intensive soil tillage and crops production, irrigation with low water quality, applications of sewage sludge and manures with high Na content, and inefficiency irrigation managements), has led to a significant decline in soil structure stability, which, in turn, decreases the soil fertility and productivity.

In most of the soils, particularly in arid and semiarid regions, the dominant factor that hold the particles together in the aggregates, and stabilize the soil structure, is the clay fraction (Shainberg and Letey, 1984; Singer, 2004; Bronick and Lal, 2005). In these soils, changes in the attraction or repulsion forces between the clay particles have significant effects on soil structure stability (van Olphen, 1977). For example, Ghezzehei and Or (2000) indicated that soil wetting decreases the adhesion forces inside the aggregates, which in turn, facilitates aggregate breakdown. Many other studies (e.g., Frenkel et al., 1978; Shainberg et al., 1981; Keren and Singer, 1989; Edelstien et al., 2010; Buelow et al., 2015; Tanner et al., 2018) indicated that soils with high exchangeable Na percentage (ESP) and containing expandable clay minerals, such as montmorillonite, are very sensitive to soil structure breakdown.

Ben-Hur et al. (2009) indicated that, during wetting and leaching of the soil, there are four main mechanisms that could breakdown the soil structure: (i) mechanical breakdown of the aggregates at the soil surface by the impact energy of water drops under rainfall or sprinkler irrigation, (ii) clay swelling, (iii) clay dispersion, and (iv) an aggregate slaking. Slaking occurs during wetting of dry soil, when the aggregates are not strong enough to withstand the pressure formed by entrapped air, the mechanical impact of running water, rapid heat release, and differential swelling of the aggregates (Collis-George and Green 1979; Kay and Angers 1999).

Soil structure breakdown could affect negatively on the hydraulic properties, fertility, and productivity of agricultural soils in different ways. Hydraulic conductivity is a key parameter, controlling the water and solutes movement through the soil profile (Hillel, 2004). The saturated hydraulic conductivity (K_s) of the soil is strongly governed by soil structure, and its breakdown can decrease sharply the K_s value. This decrease could lead to, waterlogging, low aeration in the root zoon, and limited water and nutrients movement toward roots system (Kadu et al., 2003; Ben-Hur, 2008; Shabtai et al., 2014; Tisdall and Oades, 1982; Le Bissonnais and Arrouays 1997; Chenu et al. 2000; Lado et al., 2004; Ben-Hur and Lado, 2008; Barto et al., 2010). For example, in a long-term experiment of irrigation with treated waste water (TWW) of orchard with calcareous, clayey soil in Yizre’el Valley, it was found: (i) The ESP of the soils in the TWW irrigated plots increased significantly, which led to soil structure destruction, K_s reduction, and to waterlogging. (ii) Depravedness and then death of the grapefruit trees, which were irrigated with TWW (Fig. 1).

Under rainfall or sprinkler irrigation, a thin and dense structural crust layer is usually developed at the soil surface. Agassi et al. (1981) indicated that the crust is formed by two complementary mechanisms: (i) a physical breakdown of the aggregates at the soil surface by the raindrop impact energy, and (ii) a physio-chemical dispersion of the clay, which caused by ESP increases and EC decreases. Consequently, the following negative process could be occurred by the crust formation:

1. Decreasing the soil infiltration rates (IR), which leading to an increases of the surface runoff (Ben-Hur and Agassi, 1996; Ben-Hur, 2004). In a field experiment with loess soil and 3% slope in the western Negev, which was irrigated with sprinkler, moving irrigation system, it was found that preventing the runoff flow along the slope increased the peanut yield by 37.5% in average, compared to the control treatment (runoff flowed freely).
2. A seeds germination injury in the field as a result of formation of hardness crust layer at the soil surface (Fig. 2).
3. Erosion increase of the fertile, top layer in the field (Fig. 3) with the surface runoff, which lead to reduction in soil productivity and quality.

Recently, ICL developed a new product, polyhalite (K₂Ca₂Mg(SO₄)₄•2H₂O; market name “Polysulphate”), which is sold as fertilizer. In a previous experiment of leaching of two soil types, loess and vertisol, in columns with a Polysulphate solution it was found: (i) The Polysulphate is a semi-soluble salt, and can be used as a source of Ca²⁺, Mg²⁺ and K⁺ ions, and high concentration of electrolytes. (ii) Leaching the soils with the Polysulphate solution decreased significantly their ESP. (iii) The positive effects of the Polysulphate on soil K_s were different in the two soils types, and caused by different mechanisms. It can be concluded from this study that the proper Polysulphate application should be determined based on the mechanisms of the Polysulphate that prevented soil structure breakdown. These mechanisms are controlled also by the chemical and physical properties and mineralogy of the soil.