Soil Nitrogen Testing

Nitrogen in our production systems is slippery, and of course, it’s invisible to us. Makes it hard to manage without the right tools.

Thankfully, we do have the tools to “see the invisible!”

N bank versus available nitrate:
As plant growers, we care about both the forms of nitrogen that plants take up (mostly nitrate, NO3) and the amount of N “in the soil bank,” which will be available to plants over the growing season through mineralization of soil organic matter.

The “soil bank of Nitrogen” is everything inside the body of a plant or microbe. There’s vastly more of this “banked N” than what’s available to the plants on any given day. As they eat and get eaten, they excrete and take up the single-molecule forms of N such as ammonia, nitrite, nitrate, and nitrogen gas.

So how do we know how much is in that invisible soil N bank? And how do we see what’s available to the plants today?

First, the Quick N test tells us how much nitrate is in the root zone TODAY.

A Quick N test uses a pinch of soil from a well mixed field sample shaken with an extraction solution and a test strip that changes colour to estimate soil nitrate (NO3).

This snapshot of soil nitrate is not to be confused with the soil N bank, also called the “potentially mineralizable nitrogen (PMN). PMN is an estimate of what will be release from the soil organic matter over the growing season.

The old way the lab reports Potentially Mineralizable Nitrogen was with an “aerobically mineralizable nitrogen” (AMN) test. The new, better method is the “hot water extractable nitrogen” (HWEN) test. For either method, the number reported by the lab is translated into kg N/ha, and is an optimistic view of what would be available at ideal temperature and moistures over the growing season. Adjusting for the imperfect real world, we bump the estimate down to 50-70% of the lab number, and that’s what we expect the soil to deliver to the plants.

An analogy can be used to explain Potentially Mineralizable N versus a Soil Nitrate snapshot: PMN is like what’s in the pantry. Soil Nitrate is like what’s in the lunch box.

How much Nitrogen do blackcurrants need, anyway??

A three-year-old Ben Ard planting fully loaded with a 10 tonne crop contains about 80 kg N/ha in the plant body plus fruit. When the fruit is harvested, about 23 kgN/ha leave the orchard. These aren’t huge amounts of N, by any means.

I’ve been collecting some soil nitrate numbers to get experience using the test in Blackcurrants. A few results are below.

We’re just starting to get experience in what Quick N test results look like in blackcurrant fields. But for frame of reference, work in high yielding strawberries in California showed in-season soil nitrate levels of 10 kg/ha (quick N test), or even less, supported high (70+ tonne) strawberry yields. So I suspect that 10 ppm is enough to meet the needs of a blackcurrant bush as well.

The soil nitrate quick test is useful when we’re trying to decide if we need to put that spring fertilizer on the blackcurrant blocks. It was a wet winter–did the rain leach all the soil nitrate away? It has been warm this spring–are the soil microbes eating organic matter and pooping out nitrate in enough quantity to feed the plants?

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