Using Greenseeker NDVI Technology to Monitor Nitrogen in Cover Crops

A video overview of using a handheld NDVI sensor to monitor cover crop biomass.
Using Greenseeker NDVI Technology to Monitor Nitrogen in Cover Crops - Videos

Description

This video provides an introduction to NDVI technology. In particular, it highlights how a hand held NDVI sensor can be used to monitor nitrogen in cover crop biomass in Pennsylvania.

Instructors

Agroecology Alternative Crops Conservation Cropping Systems Soil Organic Matter, Health, and Fertility Organic Agronomy

More by Kristy Borrelli 

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- [Kristy] Precision technology's growing in popularity for monitoring nutrient content in crops and it's being applied to improve nutrient use efficiency.

This video provides an overview of an NDVI sensor called a GreenSeeker that can be useful for monitoring nitrogen in growing crops.

This video will provide an introduction of NDVI.

We'll discuss NDVI's importance to nitrogen.

And we'll describe how to use a handheld NDVI sensor.

NDVI stands for Normalized Difference Vegetation Index.

More simply, NDVI sensors are monitors that are used to detect light emissions from vegetation.

Essentially, they measure the amount of light that is absorbed by a plant and compare it to the amount that is reflected.

Here's how NDVI works.

Plants use chlorophyll to absorb light and create energy during the process of photosynthesis.

As light hits the chlorophyll molecules the plant absorbs red and blue wavelengths of light and it reflects green light as well as near infrared light.

Although our eyes cannot see the near infrared light, green light is visible and its reflection is the reason why plants appear green when we look at them.

NDVI sensors are designed to detect near infrared wavelengths and using them is becoming a standardized way to measure healthy vegetation because healthier, greener plants will emit more green and near infrared wavelengths of light than sick or dying leaves that have less chlorophyll and are brown in color.

One practical use of NDVI sensors is to monitor nitrogen content in growing crops.

Nitrogen is a main nutrient needed for vegetative growth and chlorophyll production.

We are most familiar with it as a building block of amino acids and protein.

However, nitrogen is also a main structural component in chlorophyll.

The center of each chlorophyll molecule has four nitrogen atoms surrounding a magnesium atom at the center.

Because of this structure, nitrogen is an essential nutrient to help plant convert, absorb sunlight into energy during photosynthesis.

Because of this, NDVI sensors are useful for estimating the amount of nitrogen that spring cover crops will return to the soil at termination.

NDVI sensors can be mounted on the field equipment or attached to Unmanned Aerial Vehicles which are also called UAVs or drones.

However, a small handheld NDVI sensor is the most practical for small plot monitoring or standard field checks.

The GreenSeeker handheld sensor is a portable battery-operated NDVI sensor that is useful for monitoring nitrogen in cover crop biomass.

It is equipped with a light-detecting sensor and a finger trigger.

Using the GreenSeeker is easy.

Simply hold out the device at about waist level, walk it over the crop stand you are monitoring.

The sensor will detect near infrared light from the biomass and will present an average reading when you release your finger from the trigger.

Here's a video demonstrating how that works.

NDVI values range from negative one to positive one, where negative one would indicate pure water.

The GreenSeeker is not sensitive enough to detect negative values, so it begins at zero.

Zero would indicate no vegetation as shown by the reading on the black tabletop and also on bare land.

And anything approaching positive one indicates green vegetation.

High NDVI values indicate green healthy vegetation, where lower NDVI values indicate little to no vegetation.

Because it is detecting greenness the GreenSeeker will detect bare soil and flowers as low NDVI values.

In the previous slide we saw that it read zero with no vegetation, but here it is giving a low reading of 0.31 when soil cover is patchy with lots of bare spots.

It is also influenced when cover crops like canola are flowering since it does not detect the yellow flowers, giving a similar reading at 0.33.

Unhealthy crops or those deficient in nutrients or those that are starting to lignify and (indistinct) would have lower readings similar to these.

Once an average NDVI number is obtained for a field it can be compared to the nitrogen prediction curves to give an estimate of nitrogen contributions in the form of pounds of N per acre.

These calibration curves are specific to cover crops in Pennsylvania and therefore can only be used on crops in this specific area.

They were developed as part of a long-term cover crop research trial.

You'll also notice that they differ depending on the different types of covers that are grown.

The types here include legumes and mixtures, rye, and triticale and wheat.

In this example, the NDVI reading is at 0.84 on a legume cover crop mixture.

So first, locate 0.84 on the X axis and go up the solid line curve for legumes and mixtures.

Then follow it across to the Y axis and you will see that this cover crop has approximately 125 pounds of N per acre.

However, 125 pounds of N per acre is not what's available to the following crop since that cover crop biomass must undergo mineralization to become plant-available.

Mineralization is very difficult to estimate accurately since it is strongly influenced by a combination of variable factors such as the carbon to nitrogen ratio of the crop, soil type, (indistinct) days, and soil moisture and temperature.

Despite these challenges, knowing the nitrogen estimate of the cover crop before termination is very helpful.

This information can help determine when to kill the cover crop and it can be used in decision tools that help determine nitrogen mineralization.

More videos are coming that will help examine these details further to better help estimate nitrogen availability and mineralization.

So in summary NDVI is used to monitor greenness in plants based on its chlorophyll content.

These handheld sensors are therefore useful for monitoring nitrogen in cover crops which is also related to chlorophyll content.

Cover crop calibration curves exist but they are specific for Pennsylvania and should only be used in this area.

Although it provides a good estimate of the amount of nitrogen in a cover crop more information is needed to accurately determine how much nitrogen is being supplied to the following crop.

However, this information is still very valuable to know and can help improve our understanding about nitrogen dynamics and its relationship in agroecosystems.

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