The research team were interested to see just what sort of levels were necessary in the sewage compost, before metal levels in the plant became unacceptable. But even here, the only metal that came anywhere near the recommended limit for human consumption, was cadmium. Interestingly, the growth stimulation in plants after treatment with composted sludge, seems to go well beyond what one might explain from nutrition alone. It has been suggested that this response might be some form of biological stimulation at work, involving organic compounds that are absorbed by the plant roots and which directly stimulate growth. A simpler explanation however, is that in improving the structure of the soil, the compost has significantly improved the availability of moisture and air to plant roots.
Water is very often the most limiting factor to crops in the soil.
Adding compost has made the soil more like a potting mix. We've opened it right up and in doing that, we've increased the proportion of useful soil pores. The water that's held in these large pores is much more readily available to the plant. It appears as though the total amount of water that's held by the soil is not significantly affected by the compost, but the water is more readily available to the plant. So moisture stress is lessened.