Just 5.5% of New Zealand soils have enough natural fertility for food production. Yet here in NZ, with our trademark ‘no.8 wire can-do’ attitude, you’ll find we manage to produce a very large amount of food. When you think about it though, that’s a hefty amount of no. 8 wire to drag a large proportion of our soils up to adequate fertility to be able to achieve this. So how is it possible?

With the intensification of farming in recent decades, New Zealand has become increasingly reliant on the use of inorganic fertilizers, like superphosphate, to make this happen. The alternative involves land uses that demand less nutrients from the soil, such as forestry. Unfortunately, this has resulted in many negative environmental impacts and a lot of controversy and division among New Zealanders. But there is another option with great potential for improving soil fertility that tends to receive much less attention, though is perhaps no less controversial.

Around 27 kg (dry matter) of biosolids are produced per person per year from sewage treatment plants. Yes, that’s the stuff left over after all the moisture has been removed during wastewater treatment. Biosolids typically contain a range of nutrients and organic matter. Application of biosolids is common due to their potential as fertilizers; around 40% of biosolids produced in New Zealand are in fact re-used in this way. Other key benefits from the re-use of biosolids are improvements in soil properties such as the water holding capacity and potential for carbon storage.

Despite this, it appears to be a common phenomenon that people will turn their noses up just hearing the word sewage due to rather ‘dirty’ connotations. So, I ask that before you poo poo the idea let’s take a quick moment to consider its merits.

Previously, pine plantations in low fertility soils have received biosolids to accelerate growth and improve the nutritional status of the soil and trees without negatively impacting microbial communities or increasing trace elements to unacceptable levels. Currently however, due to falling timber prices there is very little incentive to replant these soils in pine forests. But perhaps herein lies an opportunity to increase the biosolids we reuse in the establishment of a valuable native crop instead.

Mānuka (Leptospermum scoparium) is a New Zealand native species common throughout the country. It is also known as the ‘tea tree’, having been named by Captain Cook and the early settlers. In the early days of settlement, a war was waged to clear vast areas of mānuka stands for the establishment of pastoral farming. Yet today mānuka is recognised as an important species in re-vegetation efforts from stemming the flow of erosion, to providing a nursery crop for the restoration of other native vegetation. As if these useful traits aren’t enough, mānuka is also a species of high economic potential with a couple of valuable products to its name.

Mānuka honey is gaining much attention and becoming recognised not only for its delicious taste, but also because it has shown antibiotic properties. Another valuable product is the essential oils produced by mānuka (tea tree oil), which some claim to be just about anti-everything; anti-bacterial, anti-fungal, anti-inflammatory, anti-histaminic, even anti-allergenic! While this is a growing area of science, the increase of drug-resistant bacteria in recent years has seen further progress towards understanding the compounds responsible for these properties.

Mānuka is mostly found in low fertility or poorly-drained environments, so it would seem unlikely that it would be successful in nutrient rich soils, such as those that have undergone biosolid application. Yet, improving soils with biosolids has shown to be very beneficial in a study by Flavia Reis and colleagues at Lincoln University published in the journal of Environmental and Experimental Botany. Following biosolid application to soil there was an increase in the dry weight production and nutrient status of the mānuka plants. This suggests that the growth of mānuka could be accelerated using this method; chaa-ching! Good for those making a buck out of that honey.

This does appear to be dependent on soil type with low fertility and high pH soil showing the more pronounced response; lucky we have more than a few soils in New Zealand that fit that bill. Mānuka roots have also been found to forage biosolids which allows it to adapt to different patterns of application. This will be economically beneficial to farmers as they will be able to alter how they apply the biosolids in different situations.

So far, this is all sounding pretty good. However,  in order to benefit from these potential commercial gains, we must also consider the environmental impacts of biosolid application to land. In particular, the effect of land application of biosolids on both surface and ground water bodies. There is potential for biosolid particles to enter water bodies during periods of heavy rain or erosion. While water quality may be compromised at these times due to sediment run off, there are practical steps that can be taken to reduce the risk such as buffering strips between waterways and application areas and restricted application to only flat land.

The leaching of nutrients, such as nitrogen, into groundwater is also an environmental concern. However, this can be minimized with careful management where the amount of nitrogen in the biosolids applied does not exceed what the plants are able to take up. Evidence suggests that with the right management practices and continued monitoring, the environmental impact of biosolid application to land can certainly be minimized and need not cause undue harm.

In my mind, the application of biosolids to low fertility soils to grow a valuable native crop is not only an economically sensible choice, but one that makes sense ecologically too. Current practise sees approximately 60% of biosolids not being reused; in other words, sent straight to the landfill. But is this not a lost opportunity? Disposal in this way is a cost to the community which gives no environmental or economic benefit and not to mention, is in contrast to the New Zealand government strategy which aims to re-use 95%. Here is an opportunity to restore and regenerate native vegetation that has the potential to earn a buck or two along the way. Currently, the biggest barrier to making this a reality is the opinion of the New Zealand public so perhaps it’s time to give biosolids a chance and kick up fuss for a method of reusing biosolids that truly doesn’t stink.

The author Kate Monteath is a postgraduate student in the Master of Science taught at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.