How to build a soil-health bank account

Joanne Thiessen Martens. Photo: Jennifer Blair

This organic production technique offers a big-picture view of nutrient management — and a ‘tough love’ approach to soil fertility

By Jennifer Blair
OrganicBiz staff

Soil is a little like a bank account for nutrients — in order to manage them properly, you need to balance the budget.

“No matter how big the bank account is, if you only take stuff out, eventually it runs out,” said University of Manitoba research technician Joanne Thiessen Martens. “We need to look at replacing those nutrients in some way if they’re going to remain at good levels for crop production.”

But the trick is knowing “not just how to add those nutrients, but when,” Thiessen Martens said at the recent Organic Alberta conference.

“When we’re growing crops, we need to make sure they get all those nuts and bolts from the soil in the right amount and at the right time. That supports healthy growth, good yields, and high-quality products,” she said. “In conventional, it’s pretty straightforward — you just spray the fall before, and the nutrients are there for that crop. Organic requires longer-term planning.”

Organic producers can use manure, forage rotations, legumes, green manure, and other methods to top up the bank account. But understanding when to make those deposits is key to knowing “where to invest our time and resources — and money — in managing it.”

“You just need to be aware that you are exporting nutrients and that you will need to, at some point, replace those nutrients in order to be sustainable.”

Where we promote healthy soil biology, we give the plants a very good start with adequate — but not excessive — nutrients. – Joanne Thiessen Martens

One of the ways producers can do that is through soil testing, which she likened to talking to a teller at a bank.

“A soil test basically asks the soil how much nutrients are available right now for a plant,” said Thiessen Martens. “But we want to know a little more than that.”

Asking the plant directly is a similar approach.

“We can do that through different types of plant tests where we take plant samples, analyze them for their nutrient concentration, and compare them to critical values that say either, yes that plant was getting enough, or no it wasn’t.”

But you get a much more comprehensive picture by talking to the bookkeeper.

“You can measure how much of that nutrient is actually moving on and off of the field by quantifying all of the products that are coming onto a field or farm and leaving that field or farm, and seeing what the balance is. Are you taking out a lot more than you’re putting back in?” she said.

“There’s a lot we can actually put a number to, and that will help guide our nutrient management decisions. And that’s called nutrient budgeting.”

A nutrient budget basically add everything that comes onto the field (seed, nitrogen fixation, manure, forages, and other inputs) and then compare it to the outputs (grain, straw, hay, animal gain, and other measures).

“We get a balance at the end of what came in versus what went out, and we can see whether there’s a surplus or a deficit,” said Thiessen Martens. “What that does is let us identify and quantify some trends over time. Is this field on a constant downward trend of only nutrient removal and no inputs? Or is one particular nutrient actually accumulating on this field?”

There are a couple of different ways to create a nutrient budget.

The first is to look at the whole farm in a one-year period. The second is to focus on one field over time, which she described as a “more useful planning tool.”

Once the nutrient budget is built, producers need to decide whether they want to replace all of the nutrients or let the soil biology take care of some of it. Don’t be “on either extreme,” she said.

“We ought to find a balance between providing everything to the plants and providing nothing to them and just relying on the soil biology to make nutrients out of nothing. That doesn’t happen,” said Thiessen Martens.

“The middle path is what I like to call ‘tough love for crops,’ where we promote healthy soil biology, we give the plants a very good start with adequate — but not excessive — nutrients, we do good management, but we also make the crops fend for themselves to a certain degree.

“At the end of that, I believe we can have healthy crops.”