By Allan Dawson
A new spread sheet-based tool to help organic farmers assess and manage crop nutrients via a bioassay of green manure crops has been developed at the University of Manitoba.
“We are hoping we can take some of that guesswork out of when farmers actually need to think about applying phosphorus or potassium or sulphur sources back to their soil because up until now it has been basically that you wait until there are visual symptoms of deficiency and then apply something,” Joanne Thiessen-Martens, a University of Manitoba natural systems agriculture researcher said in a recent interview. “This just gives us a more precise measure.”
The green manure bioassay involves collecting samples of a green manure and testing its nutrient concentration. The spread sheet is a convenient way for organic farmers and agronomists to record data and field observations. The spread sheet will process some of data, which then can be used to assess nutrients that should be available to the following year’s crop.
The key is to sample from a known sample area so we can accurately calculate that biomass production. – Joanne Thiessen-Martens
Managing soil fertility and crop nutrition is key to any farm, but there are differences between conventional and organic, Thiessen-Martens explained to participants in an agronomy program co-ordinated by the Prairie Organic Grain Initiative during a webinar in June.
On conventional farms, soil tests are used to measure nutrient levels and synthetic fertilizer is applied to make up deficiencies.
On organic farms, nutrients come from plant- and animal-based materials, such as legume green manures and animal manure. And those nutrients, which are released slowly, don’t show up on standard soil tests. Soil tests, while still important, do not provide a complete picture of nutrients on organic farms.
“The soil test basically asks the soil how much do you have to offer?” Thiessen-Martens said. “The plant test comes from the other side and asks the plant how much were you actually able to get from that soil? We might have some crops that you could say are more persuasive than others and can actually extract more nutrients from the soil than would show up on the soil test.
“We have found instances (where) soil tests are showing phosphorus is getting low and yet they still seem to be able to grow very good crops with no signs of phosphorus deficiency. That is what got us asking the question, is there another source of information to help us interpret soil tests or perhaps even replace soil tests on organic farms?”
“What we are finding is in organic systems sometimes we are providing those nutrients in plant and animal-based materials… rather than synthetic fertilizers. These nutrients are generally in an insoluble form initially. They become available through biological and chemical processes, dissolving and being cycled through the biological systems. It just means nutrients act a little differently in organic systems than they do in conventional systems.”
A green manure crop should be at least 50 per cent legumes by weight to ensure enough nitrogen to feed all the other plants in mix, Thiessen-Martens said in an interview. Also, soil nitrogen had to be relatively low before the green manure was planted, otherwise the legumes will use the soil nitrogen instead of producing their own.
The green manure bioassay spread sheet isn’t available for general release because Thiessen-Martens and her colleagues are still tweaking it.
“We don’t know a whole lot about exactly how all those nutrients are released from the green manure to be made available to the next crop,” she said. “Potassium is fairly straightforward. It is released quite readily, but phosphorus and sulphur there are a few more unknowns about exactly how that is released. Sometimes it can be immobilized in the soil and other times it is actually made available. We are not saying too much specifically about what will be available next crop, but rather we are using that green manure as an indicator. If the green manure was able to get X amount of phosphorus out of the soil then we can expect the following crop can likely get a similar amount.
“We are hoping to learn more about this, especially the phosphorus immobilization and mineralization processes in the soil, but we are not quite ready to make a nice neat recommendation for phosphorus based on what we know just yet.”
It’s important that the green manure crop be fixing adequate amounts of nitrogen. If nitrogen is deficient it will limit the amount of other nutrients the crop takes up, making test results less valid.
When doing a bioassay of a green manure, sampling should be done as close to crop termination as possible, Thiessen-Martens said. That’s usually at flowering — the time when nitrogen fixation has peaked.
“The key is to sample from a known sample area so we can accurately calculate that biomass production,” she said.
Soil tests — zero to six inches and six to 24 inches — should be done at the same time.
University of Manitoba researchers have set guideline values for nutrients found in the green manure, but field observations need to be also taken into account, Thiessen-Martens said.
If soil test phosphorus is low, but plant (green manure) phosphorus is normal, a farmer will likely be able to grow good crops, especially highly mycorrhizal crops such as flax, corn, grain legumes or oats, the university’s document says. The farmer should plan to add phosphorus by applying compost or manure in the future and in the meantime watch for signs of phosphorus deficiency.
If both soil and plant phosphorus are low it may be difficult to grow good crops without adding phosphorus immediately.
If soil and plant phosphorus are normal and crop performance is good, but phosphorus deficits are common, a farmer should plan to add phosphorus
“The value of this bioassay test is in cases where soil test phosphorus is low but the crop is still actually able to get enough phosphorus out of the soil, or in other cases isn’t able to get enough out of the soil,” Thiessen-Martens said.