The objective of this project was to investigate whether the concentrations of boron that could be present in alternative water sources will be an issue for four plant groups identified (pittosporum, elm, magnolia and citrus) as being sensitive to boron. The current limit for boron concentrations in irrigation water (0.3 mg/L) implies that there would be a negative effect on the growth of boron sensitive plants if this concentration is exceeded. However, there is a very narrow margin between the amount of boron required for plant health and the amount that results in toxicity, which can be identified by chlorotic leaves, i.e. the leaves becoming pale, yellow or yellow-white. This study aimed to verify the theoretical toxicity limit identified in literature.
A pot trial was undertaken to address the challenge, and the key activities and processes included the following:
- Applying five different concentrations of boron ranging from 0.03mg/L (tap water) to 1.5mg/L to four species of boron sensitive plants in two soil types (clay and sand), over a 12 week period
- Weekly visual and photographic observations of plant growth
- Recording of visual changes with a focus on identifying adverse effects resulting from boron toxicity
- Laboratory analysis of leaves showing adverse effects
- Laboratory analysis of all plants (leaf and root tissue) and soil on completion of trial
- Analysis of variance modelling to elucidate the statistical significance of both the plant growth and laboratory data
This pot trial sought to determine the following:
- Whether boron at concentrations between 0.03 (drinking water) and 1.5 mg/L (toxicity level from literature) affected sensitive plants; and
- Whether soil type enhanced or ameliorated the effect of boron on plants.
The results from the pot trial demonstrated that as the treatment boron concentration increased, the resulting boron levels in the soil and tissue samples (root and leaf) also increased. However, the observations of plant height, visual plant health observation, and analysis of tissue and soil samples showed that boron concentrations between 0.03 and 1.5mg/L had no negative effect on citrus, elm, magnolia or pittosporum.
It was also found that boron concentrations in the sandy soils were significantly higher than in the clay soil, particularly at the 1.5 mg/L concentration. The plant available boron was also higher in the sandy soil at a concentration of 1.5 mg/L. This however did not result in adverse affects on the sensitive plants.
The observations from this pot trial indicate that there was no effect on plants at concentrations of up to 1.5 mg/L.
It was concluded from this study that the current and future levels of boron in potable water are unlikely to be a threat to household gardens.