“You have two options: you can buy potash, or you can buy alfalfa seed every three years.”
– JOHN HEARD
Many farmers think hauling in potash at $900 per tonne onto their fields is something like bringing very expensive coals to Newcastle.
There’s some truth to that, because thanks to the feldspar and mica content in Manitoba’s clay soils, most fields are naturally rich in potassium (K), thanks to their glacial origin.
“It seems that we have an inexhaustible supply of potassium in our soils. Because our soils are young, we are still getting weathering, and that tends to recharge the soil, and recharge the exchange sites,” said John Heard, a soil fertility expert with Manitoba Agriculture, Food and Rural Initiatives, in a presentation at a recent soil fertility workshop here.
But on sandy or peat soils, and in places under continuous cropping, deficiencies may occur over time, especially if the straw is baled and hauled away.
Alfalfa and forages are particularly susceptible, simply because so much of the nutrient is taken up by plants and very little is returned to soil under hay production. This often appears as low potassium content in the feed test analysis and results in high rates of winterkill in alfalfa, with stands quickly petering out in as little as three years.
Fertilizing with K, even at less than the removal rate, has been shown to extend the life of an alfalfa stand to well over 10 years, with increased protein levels.
“You have two options: you can buy potash, or you can buy alfalfa seed every three years,” said Heard.
On low-testing soils where phosphorus and sulphur were adequate near Brandon, a study found that alfalfa yields tripled, from 1.47 tons per acre at eight per cent protein with no added potassium, to 4.73 tons per acre at 20 per cent protein with 100 pounds of added K per acre.
“So, a better nutritional status for the crop makes it better able to fix its own nitrogen with rhizobia, resulting in adequate protein,” he said.
Deficiency in corn shows up as “burning” or yellowing around the leaf edges, and in alfalfa, white spots on the leaves indicate K deficiency. In other crops, it shows up as weak stalks, lodging and poor seed development.
For beef cattle, however, too much potassium in the forage given to beef or dairy cattle in the third trimester can result in milk fever, and magnesium injections might be needed. One solution is to keep a field where fertilizer or manure is never used for feeding just prior to calving.
Studies have shown fertilizing with K in the first five years of grass forage production saw little yield response. However, after that, as the soil became more depleted, a 30 per cent increase occurred when potassium fertilizer was used.
“I always get these calls asking why yields tend to peter out after a few years,” said Heard. “People figure that they can farm for free as far as potassium goes.”
Under timothy hay production for export to Asia, for example, with no manure being returned to the field, harvesting three tonnes per acre was found to remove roughly 130 pounds of K per acre per year.
“Multiply that by five years, and that’s 650 pounds of potassium that you’ve removed without putting a lick of it back,” said Heard.
Sheep manure is the king of K, at 11 kg per tonne, followed by horse manure at seven kg, swine five kg and beef 3.5 kg. But because most of the K content in manure is in the liquids, especially urine, spreading dry manure helps, but could still end up shortchanging the system over time.
Using nitrogen to boost forage yields speeds up the K depletion rate, mainly because more biomass, and hence more nutrients, is taken up by the growing plants. When hay is grown without nitrogen fertilizer, the yield, and therefore the depletion rate, is much lower, he said.
Growing barley for grain uses up very little potash, but making silage removes a lot. That’s because 23 per cent is in the grain and 77 per cent is in the straw. Leaving the straw on the field to get rained on washes out a good portion of the K content back into the soil, and in depleted fields, the “bleeding out” effect can be seen in better growth in spots where last year’s swaths lay.
“In hemp, we measured about 200 pounds of potassium in a full stand in August. Then when the grain was harvested and the straw was dropped, by the time we got around to baling up the straw, most of the potassium had already washed out,” he said.
“By then, from that massive amount, it was down to under 25 pounds per acre actually being removed in the stalks.” [email protected]