with Robin Boom
Agronomic Advisory Services
Recently, I was asked to take some soil tests on a property that had been leased for a few years where the leasee was continually harvesting hay/silage crops and taking it away and spraying on seaweed to return nutrients to the soil.
The owner was concerned the pastures appeared to be getting poorer and more and more weed-dominant over time. I informed the owner that spraying on liquid seaweed at 20 litres/ha was only providing a tiny fraction of the nutrients being harvested, and that the fertility of the property was effectively being mined.
Seaweed naturally has around three per cent potassium, but not much else in terms of nutrients – although companies will often fortify liquid products with things like ammonium nitrate, mono ammonium phosphate and potassium nitrate and additional trace elements.
But even fortified products – which may be something like a 8-5-5 NPK rating when applied at 20 litres/ha – only provides 1.6 kg/ha N, and 1 kg/ha of both P and K. Seaweed, like most plant-based products contains some natural plant hormones like cytokinins, auxins and gibberellins – but not in sufficient quantities to make much difference unless again it has been fortified with extra hormones. It may however have a tonic value as it is reported to reduce the effect of some fungal toxins, and some research has indicated feeding certain seaweeds direct to livestock can reduce greenhouse gas losses. But as a source for replacing nutrients in the soil, it falls well short.
A three tonne/ha silage crop will remove approximately 60kg/ha potassium, 11kg/ha phosphorus, 9kg/ha sulphur and 6kg/ha magnesium. And a four tonne/ha hay crop will take out 20-30 per cent more than this. The biggest nutrient lost from hay/silage crops is potassium. Many farmers apply replacement fertilisers after being harvested but I’m more in favour of paying it forward and applying it as paddocks are locked up with some nitrogen to increase the yield. Plants will luxury feed on potassium, which is essential for tensile strength and stem development and also for seed and flower production. Any excess potassium applied pre-harvest will still be there after-harvest for pasture regrowth. As a leachable element, applying it in the autumn, post-harvest, can result in some of it being lost from the root zone and never utilised. For a silage crop, applying something like 400kg/ha Pasture Mag 15K when shutting up will provide a nitrogen boost as well as the nutrients being taken out. A hay paddock should have 500kg/ha of the same fertiliser applied to it. If phosphorus levels are good, then applying 150kg/ha of Muriate of Potash and 60kg/ha of Urea is an option.
Cowshed effluent is also an ideal replacement nutrient source, and I try to encourage dairy farmers to try and use effluent paddocks for hay/silage because the extra fertility on these paddocks can then be redistributed around the farm when fed out. This also saves money in not having to apply extra fertiliser to these before or post-harvest. Generally, effluent paddocks are close to the cowshed so many farmers prefer grazing these as they’re convenient for the milking herd and instead shut up paddocks at the back of the farm to harvest, which is really the wrong way around. It’s much better for the soil and bank balance to harvest effluent paddocks than non-effluent paddocks.
Many sheep and beef farms don’t need much potassium on their hill country pastures and focus on phosphate for their general fertiliser needs, but silage paddocks should have additional potassium applied to them, and again prior to harvest is best. Any harvestable country can have potassium and nitrogen applied to it by a truck or tractor, so applying these when shutting paddocks up will ensure a greater crop for contractors to harvest and give better utilisation of these nutrients.
Robin Boom, CPAg, member of the Institute of Professional Soil Scientists. Phone: 0274448764.