Don’t guess, soil test. That’s John Heard’s message to farmers this fall, especially in fields that didn’t get seeded or were flooded this spring.
“In 2010, it looks like the average soil nitrogen level on fallow was about 60 pounds per acre, but that’s of no value to the individual farmer because he doesn’t know if he’ll be testing 20 or 120,” Heard, a soil fertility specialist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI), said during a webinar Aug. 30. “The real simple solution – farmers just need to soil test.”
Often nitrogen levels are higher on summerfallow, but if there are lots of weeds, nitrogen and other nutrients farmers applied last fall will have been taken up by the weeds. Those nutrients will become available sometime, but farmers will have to compensate with higher fertilizer rates to meet the needs of next year’s crop.
Excessive precipitation this spring and last fall might also have caused nitrogen to leach away.
Agronomists used to advise delaying sampling in fall until after soils cooled to below 10 C to get a more accurate nitrogen reading, but recent studies show earlier tests are reliable, especially following cereal crops, Heard said. Farmers might as well book their soil samples to be taken as soon as the combine leaves the field.
In some crops, such as canola or peas, nitrogen levels after harvest can rise by about a half-pound a day before cooler soils stop mineralization, Heard said.
“If a person wants a more accurate number they wait longer (to sample) but then it gives them less flexibility,” he said. “If they want to be perfect then they wait until spring, but they’ve limited their application options.”
The number of soil samples collected annually has more than tripled over the last 10 years, partly because more farmers are applying nutrients at variable rates, which requires more soil testing per field than single-rate applications, Heard said in an interview.
Soil testing has also become more credible with improved sampling equipment, including testing to two feet deep.
Many farmers and soil testers use GPS to record where samples were taken, making year-to-year comparisons more meaningful.
Heard says farmers should ride along with soil samplers, at least the first time a field is being sampled, to assure a representative sample is taken.
“It’s not so much where you sample as it is where not to sample,” he said.
Nobody knows a field better than the farmer. Where a single-rate application is planned, the farmer can point out “oddball situations” such as where an old barnyard might have been, or where scrub piles were burned. Sampling those areas could throw off the results for the whole field.
When a variable rate is going to be applied, those areas can be sampled separately and the application rate adjusted accordingly.
Fields that had crop in one area and not another should be soil sampled separately too, Heard said. The same applies where a part of a field is covered in silt left by flood waters. The silt should be worked into the soil and then sampled, he said.
“Let’s not overthink the obvious,” Heard said.
“If you have predominantly clay or a sandy soil it may improve the texture. You’ll move a clay soil closer to a loam; you’ll move a sandy soil closer to a loam.
“If it’s mixed in with your clay or sandy soils it may be somewhat beneficial so long as it’s not too thick.”
Next spring farmers will likely experience “fallow syndrome” – poorer nutrient uptake – on their unseeded land. It’s caused by less mycorrhizae fungi in the soil. Mycorrhizae form a mutual relationship with most plant roots. Mycorrhizae get carbohydrates from plants and in return mycorrhizae’s mycelium helps plants absorb more water and nutrients, especially phosphorus.
“There are still mycorrhizae in the soil, it’s just that they need to germinate some spore and infect the plant again,” Heard said.
“So we might not see phosphorus in dependent crops, which are flax, corn, soybeans and sunflowers. If you must follow with these crops on fallow soils just fertilize well with phosphorus.”
Many unseeded fields need to be prepared for planting next spring, Lionel Kaskiw, a Sourisbased MAFRI farm production adviser said during the webinar. Weeds should be controlled with glyphosate and then worked down so a seeder can get through it next spring.
Farmers should also consider using some of the new vertical tillage machines equipped with large, wavy discs that cut weed and crop residue, while causing minimal soil disturbance. [email protected]
“It’snotsomuch whereyousample asitiswherenot tosample.”
– JOHN HEARD