Water holding characteristics of pumice fragments in New Zealand Pumice Soils : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Nutrient budget models are used throughout New Zealand to monitor the movement of water and nutrients in farm systems. Currently there is limited knowledge pertaining to the water release and water holding characteristics of a variety of coarse fragments within soils, and of the influence that some clasts, such as pumice, have on nutrient and water movement through the soil. The role of pumice clasts in soil has been largely unstudied in New Zealand, with studies focusing on the physical and chemical characteristics of the soil profile. International studies on the physical properties of pumice clasts have found that the characteristics of pumice in different deposits vary. Due to the variability in the results of international studies and lack of data on the physical properties of pumice fragments in New Zealand soils, the properties of New Zealand pumice clasts cannot be derived or assumed. To fill this knowledge gap, the water holding capacity of pumice clasts produced in a mid-Quaternary New Zealand volcanic eruption has been quantified through the development of a water release curve. The curve, from a matric potential of -3 kPa to -1500 kPa has been produced from data obtained in a trial using suction plates under vacuum and a WP4C dewpoint hygrometer.

The study found that: The assumption that the behaviour of soil fines as a matrix is consistent as the ratio of pumice clasts to soil fines changes is false. At any given matric potential, the water content of the soil fines in relation to the proportion of the pumice clasts was variable. The method developed to determine the water content of the pumice clasts between saturation and -80 kPa was most effective when matrix variability was minimised by adding glass fragments to keep the total clast content of the core constant.