How do you determine N, P, & K needs for your farm?

 This may seem like a very basic question, and you are probably thinking I've been doing this for years, I think I have this part of the equation figured out (or perhaps the more you've learned the less certain you are on this part). In either case, I encourage you to keep reading, because while I agree this topic is fundamental it is anything but basic.

When I think about this topic there are two concepts that immediately spring (season joke for those of you counting at home) to mind: mass balance & dose-response. You might have different names for them, but since I’m trained as an engineer I've had them ingrained in me. However, the important point isn't what we call them, but what they tell us and what they mean.

So what is the “mass balance” approach? In the mass balance approach we often use and equation like:

In – Out = Change in Storage

There are a few ways to conceptualize this, but to me the easiest analogy was always the haymow (if I grew up on a swine farm it might have been a gain bin, or if I was a small child growing up on the farm today perhaps I’d think of a commodity shed). For those of you not versed in hay making the haymow refers to that place where you store hay, particularly the upper part of the barn (alternatively called hay loft). So how does this analogy work? Well if “ins” are greater than “outs” we’d be accumulating more hay in the barn. This would happen every summer, we’d make hay and stack it in the barn. We’d still feed some hay every night, but thankfully we were making it at a faster rate than we were using it. Come winter time the opposite would occur, we’d be making zero hay (since the world was frozen and snow covered), but we’d still be feeding some to the cows every day. Clearly the goal was to build up a big enough surplus in the summer so that our supply would last us until the next in-flow event (harvest).

So, what’s the analogy to this in nutrient management? I’m going to talk about it from the perspective of phosphorus, but it would work for any other nutrient as well. So in our hay example I used the “mow” as the place where we might have a change in storage, in the our agricultural system this will be our field, specifically from just above the soil surface to the depth our crop roots (I tend to picture about 4 feet deep, but it depends on the crop – Christmas trees root vastly differently than corn, just like alfalfa might root drastically different than soybean). We also have ins and outs, i.e., phosphorus is added to and removed from our field. Additions of phosphorus could include adding manure an putting on commercial phosphorus fertilizers (MAP, DAP, triple phosphate). Outs are things like the removal of phosphorus in the grain and crop residues we harvest, the loss of soil particles and attached phosphorus with erosion, or any dissolved phosphorus that flows from the field in surface runoff (this could also include phosphorus that moves vertically below the root zone of our crop as water leaches through the soil, but usually this is a really small amount in Iowa. A common management strategy would then be to try to balance the inflows and outflows of phosphorus from our field so that our soil stays like it was. That is we keep our inputs and outputs in balance so that we are neither gaining or losing phosphorus and hopefully our soil productivity remains the same.

An alternative way to view this is through the concept of dose-response. Dose-response refers to the relationship between the amount of a substance that an organism receives (the dose) and the effects on the organism (the response). In this approach the idea is to figure out how much response you might get from applying some dose. To get an idea of this one let’s think about the “do your chores” analogy. A parent might offer some economic incentive (an allowance) to try to achieve a desired response (for example unloading clean dishes from the dishwasher). In this case the parent would try to pick the lowest amount that would motivate their child to perform the task. The parent could of course pay more than this amount, but they don't receive any additional benefit from it.

In an agricultural system this would be akin to developing a curve that would predict the relative increase in yield that would be achieved by adding a certain amount of some nutrient. Again it will work on a variety of nutrients, but the one I’m going to pick as an example is nitrogen (because the work has all been done for me – see the maximum return to nitrogen tool @ http://extension.agron.iastate.edu/soilfertility/nrate.aspx). I’ve given an example of what this curve looks like for Iowa (at least on average) for corn following soybean. Just like the parenting analogy, our goal here would be the same – to provide just enough of what we are dosing (nitrogen addition) to maximize our response (I’m showing yield in the graph below, but we are doing it to maximize profit from our acre of crop), so supplying just enough nitrogen so that the next bushel of corn the extra nitrogen we add to our yield, will just pay for the costs of putting that little bit more fertilizer on.

So where does that leave us? Well, I’ve introduced two concepts for you to think about here: The mass balance approach and the dose-response approach. In the next couple posts I'll try to give more detail about both for the cases of nitrogen, phosphorus, and maybe even potassium. I won’t promise any answers about which one is right for your farm, but hopefully it will get you thinking about it in a new way and I’ll give you some insight into my thoughts.

PS - If you are already itching for more information on this topic I actually introduced a comparison between the mass balance approach and the dose-response in the blog post Iowa manure management plans: Yield goal versus maximum return to nitrogen (which you can find here: http://themanurescoop.blogspot.com/2015/01/iowa-manure-management-plans-yield-goal.html.