|dc.description.abstract||Commercial forestry nurseries use large quantities of water to irrigate the planting stock to meet the annual forestry industry planting demands. However, South Africa is a water scarce country and there is high competition for this limited resource with other sectors. Thus, sustainable water management strategies should be put in place to preserve this precious resource. In commercial forestry nurseries, sustainable water use may be achieved by carefully managing irrigation schedules, such as accurately measuring growing media water content and then replenishing the depleted soil water. Improved nursery irrigation management may not only save water, but also has the potential to reduce the prevalence of pests and diseases, excessive leaching of nutrients, irrigation costs and may produce robust planting stock that is better suited to adapt to field conditions.
Growing media water content can be directly measured by the gravimetric method. However, this method is laborious, time consuming, costly and does not allow for near real-time monitoring and control. Several indirect methods for estimating soil water content have been developed and are in use today. Some common methods are: frequent domain reflectometry, time domain reflectometry, time domain transmission and the dual-needle heat pulse method. However, each method has advantages and disadvantages. Considerations for choosing the most appropriate method are the ability to automate, accuracy and precision, skills required to use and the costs to purchase.
The main objective of this study was to calibrate the low cost commercially available Decagon EC-5 soil water content sensors using nursery growing media. The calibration equation was then used to measure and control irrigation for Eucalyptus grandis x Eucalyptus urophylla and Eucalyptus dunnii planting stock grown in seedling trays in a greenhouse. A web-based data and information system was utilised to share measurements and display greenhouse environmental conditions in near real-time. The data could be viewed or downloaded using the internet1.
Decagon EC-5 soil water content sensors were laboratory-calibrated, using nursery growing media, against the standard gravimetric method. Four nursery growing media were used for calibrations: coir/perlite mix (CP), coir/pine bark/vermiculite mix (CPBV), pine bark (PB) and sandy soil. The calibration relationships between gravimetric water content and sensor output for each growing media were established, and the manufacturer supplied calibration equation was evaluated against laboratory calibration equations. The appropriate laboratory calibration equation was programmed in the datalogger to measure and control irrigation. Greenhouse microclimate measurements of air temperature, relative humidity and solar irradiance were conducted. Hourly grass reference evaporation (ETo) was calculated by the datalogger. The greenhouse microclimate measurements were compared against an open area automatic weather station at the University of KwaZulu-Natal Agrometeorology Instrumentation Mast (UKZN AIM) system measurements.
The EC-5 soil water content sensors were used to schedule irrigation for E. grandis x E. urophylla hybrid clones (GxU) (Experiment 1) and E. dunnii seedlings (Experiment 2) grown in seedling trays inside a fully air temperature controlled greenhouse. Irrigation was controlled in three treatments: low, medium and high watering. All treatments were treated the same except for differing irrigation application. Total daily irrigation and drainage were measured per treatment. Irrigation scheduling for GxU was programmed at a set point and E. dunnii seedlings at lower and drained upper limits. Seedling measurements conducted were root collar diameter (RCD), heights, stomatal conductance (gs), root-to-shoot ratio and total leaf area. Total drainage and its electrical conductivity (EC) was also measured. The calibration relationship showed a linear relationship between gravimetric water content and sensor output for all four growing media with an R2 greater than 0.92. The manufacturer supplied calibration equation poorly estimated growing media water content compared to the laboratory calibration. Poor estimation exceeded the 5% error specified by the manufacturer.
Air temperature was consistently less than 25°C inside the greenhouse. The external air temperature, as measured by the UKZN AIM system, fluctuated and reached a maximum of 36.6°C during the study period. Solar irradiance inside the greenhouse was 60% lower than that measured by the UKZN AIM system. The relative humidity was consistently higher inside the greenhouse compared to that measured by the UKZN AIM system. Greenhouse grass reference evaporation was consistently lower than the UKZN AIM system due to low air temperature and high RH inside the greenhouse. For Experiment 1, the GxU clones were irrigated too frequently for short periods. This led to over- and under- irrigation in high and low watering treatments, respectively. However, these challenges were addressed in Experiment 2 using E. dunnii seedlings irrigated at lower and drained upper irrigation limits. In Experiment 2, variability in sensor measurements within each treatment were observed at drained upper limit and decreased at lower limit. This was likely caused by a change in the pore space volume from dry to wet growing media. The web-based system was successfully used as an early warning system to monitor soil water content measurements and greenhouse microclimate, averting experimental failure due to lack of irrigation on one occasion. Seedlings in the high watering treatment had the highest RCD, heights and gs followed by the medium and low watering treatments. Although the low watering treatment had the lowest growth rates and gs these seedlings were more robust, hardy and resistant to water stress. The root-to-shoot ratio showed no statistically significant differences between treatments. However, seedlings in the high watering treatment had slightly greater root volume. This was probably due to the increased total seedling leaf area for this treatment which facilitated increased photosynthetic activity and carbohydrates production, enabling increased root growth. The highest EC measurements were recorded in the low watering treatment. This was likely due to low irrigation and therefore nutrients were not washed off the growing media. Medium watering treatment EC was 30% lower than the low watering treatment whilst high watering EC was almost equivalent to the irrigation water. The analysis of economics showed that implementing the fully automated system could be costly. However, there are many potential benefits that may be offered by this system such as reduction in water use, pumping costs and management time. The early warning offered by this system could potentially help avoid the loss of planting stock if there is a problem with the irrigation system.
The study showed that irrigation may be automatically scheduled for nursery seedlings trays using low cost Decagon EC-5 soil water content sensors with reasonable accuracy. However, medium-specific calibration is important to improve the soil water content measurement accuracy. The study also showed that reducing irrigation may result in reduced growth rates of seedlings. However, other benefits such as seedling resistance to water stress, robust seedlings, irrigation water savings and a reduction in washing off nutrients may be achieved.||en