Effect of Lime Source, Fineness and Granulation on Soil Permeation with Contrasting Textures under Simulated Mediterranean Climate Rainfall Conditions

Acid soils are considered a serious constraint to crop production, associated with root stunting due to metal phytotoxicities and nutrient deficiencies (Fageria and Nascente Citation2014). Soil acidity is a global problem, particularly in sub-Saharan Africa, where more than a third of soils are considered highly acidic (pH < 5.5) leading to low crop yields (Agegnehu et al. Citation2021). Application of limestone is the most efficient practice available to alleviate problems associated with soil acidity (Agegnehu et al. Citation2021; Li et al. Citation2019). However, it is particularly difficult to ameliorate subsoil acidity as movement of lime is very limited due to its low solubility (Azam and Gazey Citation2020). Incorporation of lime into the subsoil by mechanical procedures is an effective way to alleviate subsoil acidity (Agegnehu et al. Citation2021). This amelioration enhances root extension into the subsoil, enabling plants to use soil moisture and nutrients stored deeper in the soil profile (Azam and Gazey Citation2020; Tang et al. Citation2003). This is especially important in semi-arid cropping regions with low and variable rainfall (Tshuma et al. Citation2024). Unfortunately, incorporation of lime into subsoils by physical means is not always possible and often undesirable because of (i) wide adoption of no-tillage in crop production, (ii) the damage to perennial crop roots, (iii) the cost of deep ploughing, (iv) risk of erosion on steep slopes, and (v) exposing of infertile subsoil (Chan et al. Citation2004; Liu and Hue Citation2001; Pellissier et al. Citation2024). No-tillage systems form part of conservation agriculture, which is of the most effective farming strategies supporting sustainability goals (MacLaren et al. Citation2022; Tshuma et al. Citation2024). However, in no-tillage systems, lime can only be routinely applied on the soil surface (Rheinheimer et al. Citation2018). Grain and oilseed producers that practise conservation agriculture in the Mediterranean climatic regions of Western Cape Province of South Africa are increasingly reporting issues with subsoil acidity (Liebenberg et al. Citation2020).

  Pesticides Are Killing the Organisms That Keep Our Soils Healthy

Various field and greenhouse studies evaluating the movement of surface applied agricultural lime (aglime) in no-tillage systems, with a wide range of application rates, annual precipitation, soil textures, and evaluation time, have found liming effects restricted to a maximum depth of 10 cm in soil the profile (Barth et al. Citation2018; Bortolanza and Klein Citation2016; Ernani, Ribeiro, and Bayer Citation2004; Nunes et al. Citation2019; Rheinheimer et al. Citation2018; Van der Nest et al. Citation2022). However, liming effects to a depth of 20 cm or more take at least 10-years to achieve and is limited to coarse-textured soils with high annual rainfall (Azam and Gazey Citation2020; Costa et al. Citation2016; Dos Santos et al. Citation2018; Tang et al. Citation2003).

Thus, the vertical movement of lime also depends on the amount of rainfall or irrigation, and soil hydraulic conductivity, which is largely controlled by soil physical properties such as texture (Caires et al. Citation2015; Caires, Barth, and Garbuio Citation2006). Micro-fine limes might be able to permeate the soil more quickly and effectively than agricultural limestone sources, due to a higher rate of dissolution (Jones and Mallarino Citation2018). Hydrated and burnt limes have been shown to react far more quickly in terms of soil pH correction than calcitic limes due to their higher solubility (Du Toit, Swanepoel, and Hardie Citation2022; Mayfield et al. Citation2004), and therefore might be able to permeate the soil more effectively. Micro-fine limes are difficult to apply in the field, thus they are being commercially sold in granulated form, with binding agents such as clay, lignosulfates or molasses (Du Toit, Swanepoel, and Hardie Citation2022; Higgins, Morrison, and Watson Citation2012; Jones and Mallarino Citation2018). In South Africa, granulated micro-fine lime is being marketed as being more effective and rapid reacting than Class A agricultural limes at ameliorating soil acidity with depth. However, this is not based on published research as evidence. In a study by Du Toit, Swanepoel, and Hardie (Citation2022), it was found that molasses granulated lime neutralized soil acidity more slowly and to a lesser extent than class A lime, suggesting that the claims regarding granulated lime’s efficacy are overstated. Currently, no controlled studies have compared the movement of molasses-granulated micro-fine or hydrated limes compared to conventional agricultural limes under Mediterranean climate soil and rainfall conditions. We hypothesize that, due to the higher rate of acid soil neutralization of micro-fine calcitic and hydrated liming materials compared to conventional agricultural limes, they will affect soil pH, exchangeable cations and available P to greater depths than Class A agricultural limes. Therefore, a leaching column study was conducted to compare the soil permeation of surface applied granulated and powdered micro-fine calcitic limes in comparison to agricultural and laboratory grade calcitic and hydrated limes, in two soils with contrasting textures, under simulated Mediterranean climate rainfall conditions.

Materials and methods

This study demonstrated that hydrated lime is able to permeate sand soil more quickly and effectively than calcitic limes, but also resulted in the greatest extent of over-liming in the soil surface. Micro-fine calcitic limes were not able to permeate soils more effectively than agricultural calcitic limes, due to smaller particle size or higher rate of reaction as has been hypothesized. Importantly, granulated micro-fine lime was shown to react even more slowly than the agricultural limes, and is therefore not a quick-fix for subsoil acidity. The benefits of molasses-granulated micro-fine lime appear to be limited to its ease of handling and K content, as it costs substantially more than agricultural limes due to its processing. Therefore, the surface application of granulated micro-fine lime is not recommended for acid subsoil pH neutralization under Mediterranean climate soil and rainfall conditions, whereas surface application of hydrated lime showed promise for ameliorating subsoil acidity in sand soil within one season of rainfall. Future studies with more seasons of rainfall under field conditions would be recommended to compare the effectivity of surface applied micro-fine and hydrated limes on soil chemical properties and crop performance.