Amendment incorporation to increase soil water retention : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

This thesis comprises two small core studies and two lysimeter studies, which are aimed at understanding how incorporation of soil amendments at cultivation can increase soil water retention in New Zealand’s shallow stony soils. The overall objective was to determine the potential of this management practice to increase water retention of these vulnerable soils and to develop an understanding of the benefits and limitations of this practice in a spray irrigated system. Experiment 1 (Chapter 3) - A lab based study was initially conducted to determine the potential of a range of amendments to increase soil water retention. This experiment involved incorporating organic (dairy shed manure (DSM), municipal compost (MC), sphagnum moss (SM) and biochar (BC)) and synthetic (polyacrylamide-based hydrogel (PM), silicate gel (SI) and starch gel (ST)) amendments in to a Templeton silt loam soil that had been under long term intensive cropping management. The amendment application rate was 1% and 0.1% on a mass by mass basis for the organic and synthetic amendments, respectively. The original hypothesis was that incorporation of these amendments would increase the water-retaining mesoporosity of a cultivated silt loam and that water repellence and carbon mineralisation would not be increased relative to the control. This experiment confirmed the original hypothesis for some amendments. Mesoporosity and total mesopore volume increased relative to the control in the order of PM>SM>DSM>BC, and the remaining amendments failed to produce a significant increase. Contrary to the original hypothesis, both water repellence and carbon mineralisation were significantly increased relative to the control by the incorporation of DSM, although no other amendment had a significant effect on these properties. From analysis of changes in soil physical properties such as bulk density, total porosity and the volume change due to the strain of amendment addition it was concluded that these changes in mesoporosity were caused by two modes of action that were unable to be separated: (1) inter-particle pore spaces were created through soil and amendment particle interaction, (2) the introduction of amendments bought new intra-particle pore spaces within the new particles which held water within the mesopore range. The results and conclusions from this initial experiment allowed subsequent questions to be developed that contribute towards the original aim. Experiment 2 (Chapter 4) - If the concentration and volume of water-retaining mesopores is controlled by the introduction of new inter-particle pore and intra-particle pore spaces, what effect would 1) modifying the maximum particle size of the amendment have? and 2) modifying the application rate have? For this experiment a readily available amendment (municipal compost) was selected, screened to three different maximum particle sizes and incorporated at four rates to determine if reducing particle size would allow water retention to be increased at lower application rates than traditionally recommended. Increasing MC application rate increased both the concentration and net volume of mesopores, and maximum mesoporosity was achieved with the smallest MC particle size (<0.25 mm) at the maximum application rate (80% wt/wt). It was concluded that reducing particle size of MC was an effective method of increasing water retention at lower (and potentially more economical) application rates. Experiment 3 (Chapter 5) - The findings of Experiments 1 and 2 were expanded by answering the following question: Are the increases in water retention associated with sphagnum moss (SM) and polyacrylamide (PM) measured in Chapter 3 and municipal compost (MC) in Chapter 4 maintained under spray irrigation? What effect does spray irrigation application rate have on water retention and movement within the amended soil? This experiment identified that the incorporation of SM, MC and PM significantly increased soil mesopore volume and water retention compared to the control. The change in pore size distribution following amendment addition also resulted in an increase in the proportion of macrpores which increased water movment through the soil in lysimeters where MC and PM were incorporated. This experiment was unable to define any yield effect due to plants not becoming water stressed under the set irrigation schedule. Experiment 4 (Chapter 6) - The final experiment builds on the findings of Chapter 5 by asking if increasing water retention can prolong plant water use, and whether this translates to an increase in dry matter production? To answer this question, rye-grass in the weighing lysimeters used in Chapter 5 was allowed to grow in non-irrigated conditions until plants had fully senesced. This experiment found that incorporation of MC and SM both increased dry matter yield and prolonged water use compared to the control. The results from this experiment were applied to a simple irrigation calculator and the increase in water retention equated to a potential irrigation saving of 30 mm (SM) or 20 mm (MC) over an irrigation season in Lincoln, Canterbury.