LETHBRIDGE – The concept of applying composted manure to potatoes was introduced in Alberta in the late 1990s. The value perceived was adding organic matter and nutrients to potato fields.
“There are additional benefits, as well,” said Ross May, potato agronomist for McCain’s at Coaldale, Alta. “I’ve had some producers say the fields they added compost to, there was less dirt returned to the storages, which means less dirt they had to take back out to the field.”
At the recent Manure Management Update meeting held here, May said there are about 50,000 acres of potatoes grown in Alberta.
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He said that potatoes have a high nutrient requirement. Producers could apply up to 300 pounds per acre of nitrogen, 100 lb. of phosphorus and up to 350 lb. of potassium, to produce a 20 tonne per acre potato crop.
May said potato crops require 500 to 600 millimetres of water per year, so irrigation is required on crops in southern Alberta.
May said compost increases the nutrient supply throughout the season.
“We like to see a spread out of the nutrients, especially nitrogen, through the growing season,” he said.
“We monitor that by taking petiole samples, beginning in late June, to determine where they are in maturity levels and to control that by spoon feeding nitrogen. We’ve seen in research and in the field the addition of compost gave a reduced incidence of petiole crashes. A petiole crash can be difficult to get the numbers back up to where we’ll end up with a healthy crop.”
He said adding compost increases the raw product quality.
“We’re going to have more high quality potatoes, with less culled out. There’s a reduction in tonnage of composted material taken across the field (compared to raw manure). It’s a good source of organic fertility.”
May added that most compost sources are high in phosphorus and potassium, with moderate levels of nitrogen, so it fits with the nutrient requirements of potatoes and improves soil tilth.
Compared to adding raw manure to a field, weed levels will be reduced. Most often, the composting process will kill many weeds associated with raw manure. And there’s little or no odour during application.
However, uniform application of the composted manure was difficult in the early days. Specialized equipment has been developed – a variation on a spin spreader box-type fertilizer applicator – with GPS and rate controllers, to improve application.
Other disadvantages include the cost.
“Cost varies depending on location, how far the distance is from the compost to the field. Four tonnes per acre can easily run from $90 to $110 per acre. Most of the expense involved is transportation and application cost of that compost,” he said.
“It can be difficult to get timely application. Four tonnes per acre over 130 acres – you’re looking at 520 tonnes of compost. With 30 tonne truckloads, it takes a significant amount of time to get an accurate and enough amount of compost on. If you bump that up to six tonnes per acre, it increases rapidly.
“There can be equipment limitations to application and delivery. It can take up to one full day to apply 60 acres of compost at four tonnes per acre. For 130 acres, you’re looking at two days of perfect conditions, which means not windy. If you have a windy day, you’re going to affect your ability to accurately apply that compost.”
Other unknowns include the time delay to nutrient availability, the actual amount of organic matter increase and equipment availability.
“If there’s a high demand for trucks when you’re trying to get compost delivered to your field, it can become a headache quite quickly. There’s also the variation in analysis,” said May.
“If you look at commercial fertilizer, you have a guaranteed analysis. Urea will always have 46 percent nitrogen. There’s no guaranteed analysis in compost. That number can vary significantly over 1,000 tonnes, so it can be difficult to get good numbers to work with to plan your nutrient management package.
“Increased traffic over the field will increase compaction. If you’re applying four tonnes per acre and the box capacity of your applicator is only four tonnes, you’re doing one acre at a time. That’s a lot of trips across the field to get that product applied.”
May concluded in most cases the advantages of using compost on potatoes outweigh the disadvantages. But the cost of the transportation and application of the compost is critical to whether a potato producer is going to use compost as a soil amendment.
“The accuracy of application is getting better, but there’s places we can make improvements, in accuracy and reducing compaction in the fields.”
