Effect of sepiolite and palygorskite on plant available water in Arenosols of Namaqualand, South Africa
2019
Francis, Michele Louise
Sepiolite and palygorskite (attapulgite) are fibrous magnesium silicate clay minerals renowned industrially for their water-holding and sorptive capacities. They are only stable in arid region soils, which are regions where water is the limiting factor for agriculture and where ecosystems are the most dependent on soil as a medium for storage and timely release of water. The aim of this study is to determine how the presence of sepiolite and palygorskite affect the plant available water in sandy soils of the Namaqualand region in arid northwestern South Africa, an area where these clay minerals are present and where sepiolite is in some places present in high enough concentration to be detected by XRD in the bulk soil. The data for the water holding capacity of the soils, their mineralogy, organic carbon, CEC, soluble sodium, conductivity, resistivity and texture were extracted from the Land Type Survey records for each horizon and analyzed for statistically significant relationships. The results showed that the plant available water (defined as the difference between the water held at field capacity (−33 kPa matric potential) and water held at −500 kPa, the permanent wilting point) increases significantly with increasing CEC and clay content and decreases significantly with increasing medium sand. The presence of sepiolite in the Namaqualand soils is associated with both higher plant available water and higher amount of water held at wilting point for a given texture class (clay and medium sand) or CEC. This is significant (p > 0.001) for soils where the medium sand content is > 50%, suggesting that the effect of sepiolite on plant available water is positive for plant growth in sandy soils. In contrast, palygorskite-containing Namaqualand soils showed a lower plant available water than for a given texture or CEC, but this was not apparent for the water content at wilting point. Gypsum was not associated with a higher plant available water, even though gypsum-containing soils had significantly higher water at the permanent wilting point. The fact that sepiolite is associated with higher plant available water content in sandy soils signifies that it is a water storing mechanism available to plants in these areas, with a potential for impact on the biodiversity and land use.
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