I spoke with an agronomist about a grower who started variable rating nitrogen (N) in corn. Logically, given that high yielding corn takes up more N than low yielding corn, the grower was going to variable rate N by yield across the field – more in high yielding areas, less in low yielding areas.
The agronomist was cautious on this strategy however… while they agreed high yielding areas take up more N than low yielding areas, their concern was the strategy ignores variability in N supply across the field. For instance, some areas may yield high because of deeper topsoil and more organic matter, conditions which likely already mineralize more N. And some areas may yield low because of little topsoil or organic matter, conditions which likely already mineralize less N.
Observing N Response Down the Length of a Field
I looked at past trials pairing zero-N and N-rich strips (often called “delta yield” strips) down the length of a field. Strips are harvested with yield monitors so we can graph yields down the length of the field. Pairing these strips tells a nitrogen story, at least for that part of the field for that growing season:
- Zero-N strips indicate yields where N is only provided from soil mineralized N, and should partly reflect differences in the ability of soil to provide N
- The N-rich strip (fertilizer N applied at rate not expected to limit yields) indicates yield potential if N was not limiting
- The difference (“delta yield”) between the two indicates yield that can be captured with N fertilizer… large differences indicate higher responses/needs for fertilizer N while small differences indicate lower responses/needs for fertilizer N (Figure 1)
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