How to deal with silt deposits post-flood

Posted on behalf of Prof Derrick Moot (Dryland Pastures Research Group)

The Dryland Pastures Research Group are thinking of their east coast NI farmers – and we are feeling a long way from being able to help. So, we have put together some practical considerations for when the focus goes on pasture renewal on silt covered land.

If the silt is more than about 8 cm the resident pasture under it is likely to have died. What you do with the silt depends on whether the paddock can be worked with machinery or not – but there are some things that remain the same.  

Step 1: Soil test first

Get a soil test so you know what level of pH and nutrient you are dealing with. The lightest silt will have floated to the top – that is dusty fines that can blow around – but the deeper silt will be coarser material and may contain clay particles. In all cases it is likely that soil is nitrogen (N) and sulphur (S) deficient because those nutrients will have washed away as nitrate and sulphate. The soil is also likely to be low in organic matter because as that will have washed away. So rebuilding the organic matter and fertility is important. This can happen within a season or two.

Step 2: Determine if you are wanting permanent pasture or winter feed

For permanent pasture

If the soil test returns an Olsen P level of 15 or above: Identify which legume to sow. In summer dry (normally) regions the winter annual subterranean clover can be sown now (autumn). It is fast to establish and will fix its own N. It is unlikely to need rhizobia added but sulphur fertiliser may be necessary – so extra sulphur super or similar is advised based on soil test result. White clover and red clover can also be oversown in regions that are usually wetter in summer. In low pH situation (e.g. pH<5.5) the perennial Russell lupin may be an option. In these cases where lime may not be economic or easily able to be applied ensure the trace element Molybdenum (Mo) is included in any applied superphosphate.

Including a grass (e.g. perennial ryegrass or cocksfoot) in the mix at this time of year is an option. The lack of N in the silt deposit will slow the grasses and should allow the clovers to establish with less competition than usual. The bare ground may also provide an opportunity to establish specialist finishing pastures with no grass but using chicory or plantain with the most suitable legumes (herb/legume mix). When we see road side slips often gorse/ broom and Lotus are the first colonisers. This is because they are legumes so can fix their own N. We want to control which legumes become our first colonisers as we rebuild soil fertility.

If soil tests show low soil fertility then addressing that immediately is crucial. Legumes need phosphorus (P) for photosynthesis and nitrogen fixation so address any nutrient deficiencies in low fertility (P <10) soil.

For winter feed

Italian ryegrass will establish quickly but may need a nitrogen/sulphur boost to grow through winter. Grazing this in situ will return some nutrients to the paddock and start rebuilding soil organic matter.

Step 3: Manage the new pasture

Grazing with light stock is preferred for establishing pastures once they pass the finger and thumb pull test. The mid-summer nature of this event plus ongoing rain mean the pastures should establish in March/April because there is soil moisture, which is often not the case at this time of year. These months are warm enough to establish pastures and get that light grazing to encourage tillering and branching before winter temperature reduce growth rates. Of course, this will depend on your ability to control stock, which will depend on the immediate need to fence and provide stock water but … getting on and establishing pastures now is important. It is critcal to take advantage of the warm temperatures to get pastures/feed re-established – and can fence/deal with infrastructure as they are growing. Waiting until fences are fixed to start on pastures will add to winter/spring feed deficit problems.

In areas that cannot be drilled

Oversowing that includes a low rate (3-5 kg/ha) of Italian ryegrass in any oversown pasture can provide quick germination and offer some protection to establishing legumes and herbs.  

We are happy to answer any questions and would hope other farmers who have dealt with such events recently can also offer advice if questions come in.