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  • Near-IR light measures nutrient content in manure

BioPhotonics
Oct 2006
Raquel Harper

Manure contributes to the fertility of soil by adding nutrients such as nitrogen, phosphorus and ammonia. However, not only is it possible not to get enough nutrients, but also crops can become saturated with nutrients if too much manure is applied. The excess nutrients can then be washed away by rainwater, polluting streams and other bodies of water. Ammonia also can be lost by vaporization into the atmosphere.

Plants need a balance of nitrogen and phosphorus to grow. Unfortunately, the right balance is not commonly found in manure (usually there is more phosphorus than nitrogen), which means that farmers must put down a layer of nitrogen-rich fertilizer to get the needed mix. Or they try to compensate by applying too much manure, which causes an excess of phosphorus to get the amount of nitrogen needed, and the excess eventually ends up in waterways and causes pollution problems.

To try and get the right balance of nutrients, many farmers send a manure sample to a lab for analysis of the nitrogen and phosphorus so that they can determine how much fertilizer to use — but this is costly and time-consuming and doesn’t give an accurate analysis of their entire manure pit.

James Reeves and colleagues from the Agricultural Research Service Environmental Management and Byproduct Utilization Laboratory in Beltsville, Md., created a manure analyzer for farmers to test samples at their own farms. Knowing that near-IR light can measure organic nitrogen content, they scanned hundreds from 107 dairy manures using a monochromator to determine the wavelengths of light they would need.


Using a prototype near-IR filter spectrometer, researcher James Reeves analyzes the amount of nitrogen in manure.


The scientists constructed a portable spectrometer instrument with a rotating wheel that holds about 20 filters. The filters allow the user to select the appropriate bands of near-IR radiation to quantify the compositional parameters that are of interest.

The analyzer shines light from a 20-W bulb through one of the selected filters onto about two tablespoons of manure placed in a small cup. The amount of light reflected allows the analyzer to measure the nitrogen and moisture content.

Reeves said he also is trying to figure out a way to measure the phosphorus in a sample, but finding a quick test for this is proving much more difficult.

The researchers are working on reducing the size of their prototype. It is currently a 15-in. cube that weighs about 20 pounds. Part of the reason for its large size is the rotating wheel. They initially designed space for 20 filters to allow flexibility in testing various types of manures. Reeves now believes that the analyzer needs only nine or 10 filters because there is redundant information in the near-IR spectrum and he doesn’t think it necessary to cover every wavelength to get the appropriate analyses. He added that the prototype instrument contains empty space, which he hopes to eliminate to create a shoebox-size machine that can be carried more easily.


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