Crops in Silt
A science initiative promoting crop resilience on silt-affected land.
In February 2023, the tropical Cyclone Gabrielle tore across the north and eastern regions of Aotearoa New Zealand causing flooding, loss of life, and severe damage to property. Hawke’s Bay was one of the worst affected regions, with several million cubic meters of silt (also known as alluvium) dumped on orchards, vineyards, farms and roads, as well as buildings and homes.
Very soon after this catastrophic event, Plant & Food Research began investigating a variety of ways to help with the tasks of recovery. Over the course of 12 months, the Plant & Food Research cyclone response team began several initiatives involving over 50 research and support staff across 13 science teams. One of these projects, Promoting Crop Resilience in Silt-Affected Landscapes, began in October 2023. It has been funded by Plant & Food Research and led by crop scientist, Dr Eduardo Dias de Oliveira.
The project is designed to address challenges to cultivation brought about by severe weather events, and to assist recovery of production following these events, which, as Dr Dias de Oliveira acknowledges, are only going to become more severe, and more frequent.
For millennia, humans have been successfully growing crops in alluvial soils. Deposits from flooding rivers have generally been regular in nature and have supported soil fertility. However, recent alluvial soils generally lack in structural development compared to original topsoil. Cyclone Gabrielle, with its extensive damage from the millions of cubic meters of alluvial soil (commonly called silt) left on cropping land and the associated financial losses of that, has forced growers to retire land while they assess the viability of returning to cropping in these areas.
Eduardo explains that very little is known about the movements of water and nutrients in those soils and there are few international studies on recovering from events like this to draw on. While potentially fertile, these alluvial deposits may initially lack readily available nutrients compared to topsoil. Also, its fine texture can easily compact, reducing drainage and potentially leading to waterlogged conditions. This lack of drainage restricts oxygen availability for plant roots, which may affect growth. Adding another layer of complexity, silt depositions vary in depth and in composition, depending on the source catchment and the flood debris it carried. This variability makes it hard to predict how suitable the land covered by these deposits will be for agriculture. “That’s why we are doing this work, to understand how suitable this silt can be for growing crops that are usually grown in the region.”
The first stage of the crop resilience project is expected to be completed by June 2024. As well as looking at the physiological impact of silt on plant establishment, growth and yield, the project hopes to answer a few questions around land management practices. Should silt be stripped off immediately and how much can be left behind? Can it be incorporated into soil, and if so, what is the optimal depth for that? What is the timeline for full soil recovery for different crops and management practices? Dr Dias de Oliveira says these, and other questions will inform the direction of research, as well as focusing on what will help to enhance the overall performance and adaptability of the crops.
Some of the trials from the project are being run in greenhouses at Plant & Food Research in Havelock North. Here they have been evaluating growth and establishment and testing a variety of soil and alluvium combinations and their impacts on root growth and plant performance in the different crops being evaluated.
NH Packing is a commercial grower and packhouse operator, with a focus on production of squash and pumpkin for export and domestic markets. The company has been operating in Hawke’s Bay for about 20 years. It has made a portion of cyclone-affected land, and their knowledge and expertise, available to the project, so field trials can be incorporated into the research.
Across a one-hectare site, varying silt depths have been planted with four vegetable crops of small, medium and large seed size (carrots, peas, and maize) and transplanted broccoli seedlings. Eduardo says the selections were made based on diversity of seed size and plant root structure, as well as their economic significance to the region.
Initial findings suggest that the smaller the seed, the more difficult it is to establish a healthy plant and root system. Eduardo suspects the lack of early vigorous growth may affect the plant’s ability to push through the silt to reach the more fertile soil beneath, or to access what nutrients may be available in the alluvium itself.
While the outcome of the current research is important, the goal is to provide industry-specific resources that help with making decisions and speed recovery after Cyclone Gabrielle, as well as contributing to future recovery efforts following similar occurrences. Some future strategies could involve a rethinking of landscape design, targeted selection of more resilient crops and developing management practices that enhance soil health.
Eduardo adds, “the past 12 months have been exceptionally challenging for affected growers, but they have demonstrated incredible resilience and we’ve witnessed the industry coming together in response to the cyclone, and this project is just a small contribution to supporting the ongoing recovery efforts.”
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