Because nitrogen is closely related to chlorophyll levels, using a chlorophyll meter can help corn growers finetune their sidedresss nitrogen applications.
When it comes to corn production, nitrogen (N) is the most commonly applied nutrient and one of the most expensive inputs, averaging from 13 to 18% of the variable costs depending on the type of crop rotation. On the other hand, the nitrogen cost-to-benefit ratio usually exceeds that of other fertilizer inputs.
Nitrogen can be lost in several different ways such as through volatilization, immobilization, denitrification and leaching, so making sure to apply nitrogen when corn has its greatest need for this nutrient (in general, 30 to 45 days after emergence) to avoid nitrogen and money losses is something to always keep in mind. For more information on corn nitrogen guidelines, read some of Kurt Steinke’s great Michigan State University Extension articles: “Corn nitrogen guidelines and focusing on the right rate” or “Targeting corn nitrogen strategies for improved resilience.”
To minimize losses, corn producers can apply a smaller amount of nitrogen at or before planting and come later with a sidedress application. Because nitrogen levels can change drastically since the first nitrogen application, producers can rely on a variety of methods such as the PSNT (pre-sidedress nitrate test), leaf tissue testing and the chlorophyll meter test among others to help decide how much more nitrogen to apply to get a maximum return to nitrogen application. The goal is to make sure crop yield is not limited by lack of nitrogen throughout the crop life cycle.
This article will focus on how to use the chlorophyll meter test as a tool to minimize nitrogen losses and decrease costs. The use of the chlorophyll meter and other nitrogen sensing methods were demonstrated at the 2019 MSU Ag Innovation Day on July 26, 2019.
There are two methods for using the chlorophyll meter test to make in-season nitrogen recommendations for corn. The first method involves establishing a high-nitrogen reference area early in the season in each field to be tested (a small area where more-than-adequate nitrogen was supplied for the entire crop development). The second method does not require a high-nitrogen reference area, but only applies to fields with a recent history of manure or forage legumes.
For both methods, readings need to be taken from the fifth corn leaf (do not take readings on the leaf midrib or too close to the edge when corn is between V6-V8 leaf stage). At least 30 readings should be done; in cases where a field shows great variability, dividing readings into different zones may be necessary to determine if several different N rates would be appropriate for the field.
High-nitrogen reference value chlorophyll meter test method
Do not apply more than 15 pounds of nitrogen per acre in the starter in the fields you want to test. If you do not follow this rule, the results in the test might indicate that the crop has adequate nitrogen, but in reality it might not, resulting in underapplication of nitrogen that will most likely reduce yields. You should establish a high-nitrogen reference area where you applied 150 to 250 pounds of nitrogen per acre (at least two small hand-fertilized sections) in each field to be tested that has not been stressed. Take readings with the chlorophyll meter from the fifth leaf of 30 plants in the high-nitrogen reference area, record the average and clear all the data from the meter. Repeat, taking readings in the same way from the rest of the field.
Use the worksheet below to determine how much additional sidedress nitrogen you should apply (if needed).
- Relative chlorophyll meter reading = Average field readings / High N reference value
If the relative chlorophyll meter is ≥ 0.95, the N recommendation is zero. Otherwise, continue the calculations below.
- Yield factor = Expected yield (bu/ac) x 0.9
- Manure factor =17 x (0.75 if NO manure was applied since harvest or 3.5 if ANY manure was applied since harvest) x relative chlorophyll meter reading.
- Leaf stage factor = 19 x leaf stage of the corn crop x relative chlorophyll meter reading.
- Reference plot factor = 4 x High N reference value.
Final recommendation = 280 + Yield factor – manure factor – leaf stage factor – reference plot factor = lbs N/ac
No high-nitrogen reference area chlorophyll meter test method
Do not apply more than 15 pounds of nitrogen per acre in the starter in the fields you want to test. Only use this test on fields that have received manure since the previous crop was harvested or that are in the first year of corn following a forage legume. Take readings with the chlorophyll meter from the fifth leaf of 30 plants. There are different recommendations for readings taken at the six-leaf stage or seven to eight-leaf stages. Use the tables below to determine the recommendation based on the average reading.
|Table 1. Interpretation of chlorophyll meter readings taken at the six-leaf-stage in fields with a recent history of manure or a forage legume without a high-nitrogen reference plot.|
|Average meter reading for the field||Less than 42.0||42.0-45.9||≥ 46.0|
|N recommendation||Sidedress 80 lbs N/acre||Test again or sidedress 50 lbs N/acre||No sidedress N needed|
|Table 2. Interpretation of chlorophyll meter readings taken at the seven- to-eight-leaf stage in fields with a recent history of manure or a forage legume without a high-nitrogen reference plot.|
|Average meter reading for the field||Less than 43.0||≥ 43.0|
|N recommendation||Sidedress 50 lbs N/acre||No sidedress N needed|
Using a spadmeter is definitely easier than gathering PSNT samples, but there is still have a lag when the data have to be processed to a final nitrogen recommendation.
If you are interested in learning how to take the readings, watch this short video.