Manipulation of soil nitrogen to increase efficiency of mintuber seed production in Tasmania

The management and manipulation of soil nitrogen (`N`) was investigated as a means of influencing plant growth to increase tuber numbers produced from early generation seed potato crops. The project focussed on crops grown from minitubers, small potatoes produced under controlled environment conditions from tissue culture plantlets. The use of minitubers as planting material for first field generation seed potato crops reduces the risk of disease but is commercially challenging as the plants tend to produce low tuber numbers, limiting the seed multiplication rate. The first field generation is a high value crop grown on a small land area and there is therefore potential for intensive management of factors such as nutrient supply to increase the number of tubers and the subsequent rate of multiplication in the field. Nitrogen has been shown to influence tuber number and tuber growth in both hydroponics and field experiments, and was therefore the nutrient investigated in this study. Initial glasshouse and laboratory experiments demonstrated that manipulation of `N` availability could alter tuber development in potato plants. In contrast to evidence in the literature from hydroponics experiments, a high constant `N` supply did not delay or inhibit the onset of tuberization. High `N` supply increased tuber set and growth, demonstrating the importance of sufficient `N` supply during early plant growth up to tuberization. Rapidly reducing `N` supply to plants at the onset of tuberization increased tuber growth rate by 60% compared to control plants two weeks after treatment application. It was concluded that the treatment may reduce the rate of tuber resorption during the tuber bulking phase, and therefore increase tuber number at harvest. Under field conditions, application of a leaching treatment based on calculations from laboratory and glasshouse trials did not result in any significant change in tuber number. Three potato cultivars were used in the trial and 150 plants per treatment were assessed. Although `N` was applied to the crop as `NO_3^-` to increase the likelihood of leaching during treatment application, soil analysis indicated that `NO_3^-` concentrations were only reduced in the upper 20 cm of soil. Roots were distributed throughout the top 40 cm of the soil and therefore plants still had access to significant `NO_3^-` concentrations after treatment application. The use of a drip irrigation system, the volume of water applied and the presence of anion adsorption contributed to limited movement of `NO_3^-` through the soil profile. The field results demonstrated a need for better understanding of `N` movement in the Red Ferrosol soil so a more effective strategy for rapidly reducing `N` concentrations in the root zone could be developed. Accurate and detailed measurements of water and `NO_3^-` movement in the Red Ferrosol soil were obtained from laboratory experiments. The presence of anion adsorption was observed and therefore an adsorption isotherm for `NO_3^-` was developed. Using the data collected from the laboratory experiments, parameters for the Hydrus 2D/3D soil model were validated for the soil and this model was used to estimate the distribution of water and `NO_3^-` under different irrigation scenarios in the field. Simulations indicated an overhead irrigation system was a more effective method for rapidly reducing soil `NO_3^-` concentrations in the root zone than a dripper system due to the predominantly vertical displacement of `NO_3^-` however water applications of over 300 mm of water were required. Further simulations in analternative sandy soil indicated that the leaching volume required was less than half that of the Red Ferrosol due to the soils lower water holding capacity and absence of anion adsorption. It was therefore concluded that serious consideration of soil type should be made during the design of future experiments investigating strategic `NO_3^-` control in potato crops. This study provides preliminary evidence that careful management of `N` in potato crops has the potential to increase tuber growth rate and tuber number. However treatments that involve manipulation of `N` availability are difficult to apply in field environments where plant roots are widely distributed. Models such as Hydrus 2D/3D are useful tools for investigating water and nutrient movement under various flow scenarios however reliability of model predictions depends on the level of validation against measured data