Nutrient-rich wastes include:
They are rich in nutrients such as nitrogen, phosphorus, potassium and/or organic matter.
Nutrient-rich wastes can be used in different ways:
There are benefits to using nutrient-rich wastes.
Using municipal wastewater can decrease the amount of clean water needed. It also provides an enriched water supply for nurseries.
Adding slurries, sludges or solid waste to soil helps it hold more water. The wastes improve the soil’s structure and biological activity. They also provide nutrients. These benefits encourage productivity.
The use of nutrient-rich wastes in forestry and nurseries supports New Zealand’s move to a circular economy.
The potential benefits need to be balanced against the management of risks. Potential health, safety and environmental risks depend on the type of waste used. Most risks can be managed by:
There can be social and cultural issues, in particular with the use of human waste (biosolids). These issues can be dealt with through appropriate engagement processes.
Nutrient-rich wastewater is beneficial for nurseries. Water access is critical for nurseries. The wastewater decreases the amount of clean water used and provides an enriched water supply.
Using solid or slurry waste can reduce the need for fertilisers or substrate.
Being near to a wastewater plant is an important factor. This study found 6 nurseries servicing the commercial forestry sector within 5km of a waste processing plant.
| Waste | Use | Incentive for use |
|---|---|---|
| Wastewater: municipal, dairy factory, meat processing, food processing |
Irrigation | Beneficial in water-deficient areas. Avoids water stress. Decreases amount of clean water needed. Re-uses wastewater. |
| Sludges/solid materials | Supplements to seedling beds | Helps soil hold more water. Improves soil structure. Improves soil biological activity. Adds slow-release nutrients. |
| Composted/modified sludges/solid materials | Planting mix | Improves soil biological activity. Re-uses waste materials/offsets use of new materials. Reduces need for inorganic fertilisers. |
The main opportunity for using nutrient-rich wastes in forestry is to increase production. The wastes can cut rotation time (the number of years between planting and harvesting) with little loss to wood quality.
Applying wastes to commercial forestry sites has lots of potential. About 260,000 hectares of commercial forestry is on sandy soils. These sites are within 20km of a processing plant that produces nutrient-rich waste. Sandy soils have the potential to benefit most from the waste.
| Waste | Use | Incentive for use |
|---|---|---|
| Sludges/solid materials | Site preparation | Adds slow-release nutrients for growth promotion. Helps soil hold more water. Improves soil structure. |
| Wastewater: municipal, dairy factory |
Irrigation | Avoids water stress. Decreases the amount of clean water needed. Reduces discharge to aquatic systems. |
| Sludges | In-forest application |
Adding slower-release nutrients helps deficiencies and promotes growth. |
Using nutrient-rich wastes for native plantings can increase growth rates resulting in quicker canopy closure. This helps survival rates too.
Other wastes like woody debris are useful for managing weeds and excluding browsing animals.
Native plantings on poor quality soils will get the most benefits from nutrient rich wastes, especially when the aim is to bring back the native ecosystem. Poor quality soils have poor structure and little organic matter and nutrients. The natural humus layers and woody materials in the soil are gone.
New plantings need water to survive. Using tankers to apply nutrient-rich wastewater reduces the need for clean water.
| Waste | Use | Incentive for use |
|---|---|---|
| Woody debris, woody mulches |
Site preparation | Protects microsites. Prevents erosion. Saves water in dry/warm sites. Suppresses weeds. Stops browsing animals (deer, hares, rabbits). Enhances nutrition when mixed with compost. |
| Sludges and modified sludges | Site preparation | Improves degraded soils and builds soils. Helps soil hold more water. Improves soil structure. Improves organic matter. Adds slow-release nutrients. |
| Wastewater: municipal |
Irrigation after planting | Decreases amount of clean water needed. |
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This research was commissioned by the One Billion Trees programme.
Read about other One Billion Trees science projects on the Ministry for Primary Industries’ website.