The Sink and The Source

4-10-17 Relationships from Central Valley Ag on Vimeo.

by Mike Zwingman

by Mike Zwingman

I threw this idea out a few weeks ago and have kind of left it hanging ever since. But now that it’s had some time to simmer…

The goal with the sink and the source is to strike a perfect balance between them, an achievement that means your plants get all the N they need and not a scratch more. It is perfection: your plants experience zero want and you experience zero waste. The environment and groundwater chalk up wins as well.

But there’s absolutely no way to hit this hard target (and highly desirable goal) without a thorough understanding of both the sink and the source. In service of that fact, read on.

Let’s start with the more familiar of the two concepts: the source. The source is the sum of all the nitrogen you apply and all net nitrogen released in the N cycle through mineralization. While the first part is easily accounted for, the second bit there is not. How much N is released via mineralization depends on a variety of factors that include things like moisture, temperature, and soil type.

The sink is perhaps an unfamiliar concept made up of an ultimately familiar thing: your plants. The entirety of your plants: all their green tissue and grain. You know well then what a moving target the sink can be. Beneficial conditions increase the size of the sink; stresses decrease it.

Neither the sink nor the source is straight forward. Both fluctuate according to conditions. So how in the world do you work to make all things equal?

The best thing you can do is to build the biggest sink you can. This is also to avoid the worse case scenario, which is an overabundance of source, which is loss. The good thing is that this is what you try to do. Every. Single. Season.

The source, you can tinker with. You can add nitrogen to your field in subsequent passes as conditions and signs of mineralization warrant to ensure that the fuel tank of N is never empty. The sink however. That’s something that you set in motion and in many ways can’t change.

Insert here, friends, a reminder to plant with care.


We all talk pretty often about nitrogen efficiency. The undercurrent to this conversation always seems to assume that achieving efficiency means reining in overapplication. This isn’t always so though. Inefficiency with nitrogen can also mean that you aren’t building a big enough sink. And consider: while fixing overapplication will save you a few bucks, building a bigger sink will make you many more.

But Mike, you say, we try to “build a bigger sink” every year! I know. This year, let’s try it from this perspective though. Let’s manage the abiotic stresses on our plants that work to decrease the size of our sink and manage the source as our plants at their critical yield determining timeframes, planting/emergence, V5-V9, and during grain fill. Reducing stress or at very least mitigating it will help us maximize the yield determining factors of number of ears, kernels per ear, and kernel weight which for those of you who do hand yield checks have been doing the math on the sum of the sink for years.

We have the tools to do this. (Cough cough VR cough cough.) Over the course of the summer, we’ll roll out the goods, the tools that help us understand the complexities of the sink and the source. In anticipation, can I leave you with a little game? Besides N, there are two other nutrients critical to balance between the sink and source. I’ll bring a coffee to whoever names them both first…