Farmers dealing with silt damaged paddocks in the wake of Cyclone Gabrielle are encouraged to get a soil test done as soon as possible and consider their resowing options.
Lincoln University-based plant scientist Professor Derrick Moot says a soil test will give farmers an understanding of what they are dealing with.
He says it is possible that the nutrient status (phosphate levels) of the silt will be high if it came off pasture areas of hill country, but low if it came off forestry blocks.
“Soils are the key and a soil test is important.”
He says the decision about what to plant depends on feed requirements. An annual ryegrass would be an option for fast-growing winter feed crops, but additional seed and resowing areas of flood damaged winter crops should be fine provided the soil is still there and fertility is high.
“Some DAP down the spout could be useful if fertility is low.”
According to information in Beef + Lamb New Zealand’s flood recovery information, if silt has completely covered pasture it will not survive and will need to be re-grassed. Because silt is typically relatively infertile with no organic matter or nitrogen and poor soil structure, cultivation is recommended to combine topsoil and silt if possible.
Where sediment is between 10-25cm in depth, levelling will help achieve a consistent depth of silt across the paddock and once the silt has dried, heavy machinery such a swamp plough can be used to mix the silt and topsoil.
It can then be cultivated as normal.
If the resulting soil is predominantly silt, then short-term ryegrasses, forage oats or other deep-rooted short-term crops are recommended before returning the paddock to permanent pasture.
Sediment over 25cm
Where flood sediment is over 25cm, then the paddock can either be oversown with a helicopter or the silt cultivated and drilled. In either case, the paddock will probably need to be recultivated the following spring or autumn due to poor soil structure.
If cultivating deep silt, the silt should be left to dry sufficiently before light machinery is used to break up the surface before drilling either forage oats or a short-term ryegrass and harrowing.
After grazing the oats over winter, the oats can be mulched to help build organic matter or made into silage.
Oversowing with a short-term ryegrass is an option for clay/silt loams and can be done when the soil is still damp and sticky. Once the silt has caked and cracked it is too late for oversowing.
It is a riskier option than cultivation or direct drilling, so a higher-than-normal seeding rate will be required and it is important to only use coated seed.
If the silt is too wet, the seed will rot and if its too dry then the surface of the silt will cake and crack and seedings struggle to grow primary roots.
Stocking of newly sown areas should begin as soon as possible without pugging. If feasible, mulching before re-grassing is a good option to help build up organic matter.