Soil Conservation

The Franklin District is home to some of the most productive and durable soils for vegetable growing in New Zealand but is prone to erosion and consequent soil and nutrient loss. Local growers initiated the formulation of a set of guidelines to sustainability in the late 90’s, as part of the Franklin Sustainability Project. One of the main focuses was erosion and sediment control. Building upon this, agricultural consultancy business Agrilink (along with growers and erosion control specialists) prepared the Erosion & Sediment Control Guidelines for Vegetable Production in 2014. Now Agrilink is verifying some parts of the guidelines by testing silt trap performance and other erosion and sediment control measures. 

In 2015, a three-year MPI Sustainable Farming Fund Project called “Don’t Muddy the Water” was begun. Managing Director of Agrilink, Andrew Barber, has been conducting trials testing the performance of sediment retention ponds to understand what can be done to effectively mitigate erosion in land under intensive cultivation. 

Much of the Franklin District has robust soils that can cope with continual vegetable growing. Indeed, some areas of the Pukekohe Hill have been cropped commercially for the better part of 100 years. However, such intensive arable land use can have significant negative impacts such as soil erosion, soil degradation and reduced quality of both surface and ground waters. 

Loss of soil when it moves within or off a paddock results in losses of productivity and profit. When it moves off the property altogether there is not only a loss to the grower but also to the community that has to bear the costs when sediment ends up on roads, in drains, streams, rivers and lakes. 

In May 1996 a deluge caused a flood of soil-laden water to wash through homes, businesses and a school at the foot of the Pukekohe Hill. As a result the Pukekohe Vegetable Growers Association (PVGA) initiated the Franklin Sustainability Project aimed at trialling and promoting good practice amongst growers on:

• surface water runoff management using contour drains, grassed waterways, silt traps, and raised access ways to reduce erosion, sediment loss and nitrate leaching
• the use of cover crops and mulching to reduce erosion and nitrogen leaching, increase organic matter and improve soil structure
• wheel track ripping to increase infiltration and reduce run off and erosion
• reducing cultivation to improve soil structure, boost organic matter levels, and reduce erosion
• the impacts of subsoiling - tilling the soil without turning it upside down - on infiltration and erosion
• irrigation management to improve efficiency, reduce run off and nitrate leaching
• fertiliser management, optimising fertiliser rates and timing to reduce nitrate leaching while maintaining production
• integrated pest management to reduce the use of pesticides
• soil quality monitoring so growers can monitor and manage changes in soil quality.

The resulting guidelines proved to be very successful because the project was community-based and had considerable grower support. 

More recently, Horticulture NZ and the PVGA commissioned Andrew Barber of Agrilink to prepare an updated version on erosion and sediment control, which is critical to sustainability (Erosion & Sediment Control – Guidelines for Vegetable Production, June 2014). 

Andrew says that reducing erosion and sediment loss is crucial to retaining soil health and avoiding downstream problems. 

“In runoff water you have really heavy material that drops out quite fast as soon as the water slows. Then there is suspended sediment that is finer and it needs more time to settle,” he says. 

“You need to deal with both. Reducing suspended sediment is important for maintaining downstream river water quality and supporting aquatic life. Erosion control starts essentially with minimising surface water runoff and thus any movement of soil within the paddock.” 

The first step, he says, is to do a paddock assessment – look at its size, history and slope, and identify where water runs onto the paddock and off it. Then take measures to stop or control water entering the paddock using interception drains, correctly sized culverts, benched headlands, bunds and/or grassed swales. 

The third step is to use management practices that will reduce the impact of machinery, rain and wind on the soil surface and help keep the soil in the paddock. These include the use of cover crops, windbreak crops, short row lengths, contour drains, wheel track ripping and a range of cultivation, harvest and post-harvest measures. 

“Wheel track ripping is important because when wheeled machinery goes up and down the rows it compacts the soil and when it rains the water flows down the compacted tracks frittering the sides of the raised beds. If you use a wheel track ripper the water goes into the soil and runoff is reduced,” says Andrew. 

“Research a few years ago showed a reduction in erosion from 20 tonnes per hectare down to 1 tonne after ripping. There are some soil types that don't suit it, which is why we advocate for being able to select from a toolbox of measures, but it certainly suits Pukekohe Hill.” 

Prevention is better than cure and so the above measures to reduce runoff are vital, says Andrew, because once silt exits the paddock it is degraded with poor structure and lower organic and nutrient levels. 

However, there will always be some silt laden water escaping from cultivated areas, especially after sustained heavy downpours. That water needs to be contained or slowed so that solids will settle. Measures to achieve this can include: 

• Planted buffers and riparian margins, hedges
• Super silt fences
• Stabilised discharge points and drains
• Decanting earth bunds and silt traps 

Bunds are essentially earth ridges constructed to slow or trap runoff and allow silt to settle. 

The erosion control guidelines were based on the best available knowledge and seemed to be effective but quantitative proof was needed. For example, the recommended size of silt traps was 50 m³ per hectare whereas the size for construction sites was 200 m³/ha. There are good reasons why the size for cultivated land should be smaller, as Andrew points out. 

“If you are building a house or a road with compacted clay surfaces essentially all the rain that hits them runs straight off whereas if you are on cultivated land less than half of that water runs off. It’s hard clay versus topsoil, so they are quite different situations and you can get away with smaller traps,” he says. 

“We are just about to finish year two of an MPI/Sustainable Farming Fund three-year project, called “Don’t Muddy the Water (quantifying the effectiveness of sediment control on cultivated land). So far the data indicate that the silt traps are performing as well as or better than what is being achieved on construction sites. And after all that rainfall in March we have got some big numbers and quite dramatic visuals on what would happen if you didn't catch all that soil. A sediment trap that was reasonably empty in February and then after the March downpours it was absolutely chock-a-block full of soil.” 

“The bottom line is that most growers have made good progress in controlling erosion and improving the sustainability of their businesses and it is certainly in everyone’s best interests that they do so.”