Headline : From Symptoms to Solutions: Addressing the Underlying Causes of Water Quality Degradation – Part 2

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In part one of this series, I briefly discussed the importance of education for understanding how soil function and ecosystem processes play a critical role for water quality.  Now let’s turn our attention to determining the “right rate” for nitrogen. Nutrient management plans focus heavily on nitrogen because it is critical for plant growth and health. But how do know how much you need to apply? While there is no single “right rate,” it’s almost certainly much less than most farmers are currently applying.

The first question to ask is how much nitrogen can your soil supply to the crop without adding any N fertilizer? Nitrogen rate trials with zero N check strips can help answer this question. These trials help determine the nitrogen use efficiency (NUE) of your crop. This is commonly measured as pounds of nitrogen applied per bushel of grain harvested. Let’s use corn as an example. A producer recently shared some yield data from a replicated, on-farm trial they conducted (Table 1). How would you decide which is the best application strategy if this was data from your farm?

In this example, N use is clearly more efficient as application rate declines. In this situation it’s probably due to the spring application being more efficient than fall (less N loss), and/or the soil is inherently productive and just doesn’t need that much N. To make a fair comparison, we need to account for the cost of N and the yield differences. Column 4 shows the cost of N per bushel (not including application) using a price of $0.60/lb N for urea and $0.52/lb N for anhydrous ammonia. Revenue per acre after paying for the N is shown in column 5.

From an economic standpoint, spring urea at 120 lb comes out on top in this simple analysis. But there is more to it than that. What is the cost of the additional N that was applied for very little yield gain? Let’s look at NUE another way – how much additional N was applied per bushel of yield gain, using the lowest yielding practice as a baseline. Think of this as your marginal N use efficiency. Now things look a little less rosy for the higher N rates. At the 195 lb application rate it took between 12.8 and 20.2 pounds of additional N per bushel to achieve that last bit of yield gain. Now even the 120 lb N rate doesn’t look so great. Is it really worth spending almost $4/bu just for the nitrogen, with a corn price of $4.50?

Read more from the original article on here at understandingag.com.

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