Spectrometer Testing for Nutrients in the Lab

Optical Emission Spectrometer

The SAGE garden is an agricultural producer.  The food grown there is donated almost exclusively to the South Corvallis Food Bank.  The soil management for that reason consists entirely of organic practices designed to maximize food production.  The ability of a plant to grow, and produce a nutritious yield, largely depends on two factors.  The first is the ability of the soil to hold nutritive ions, the positively charged ions called cations; this is largely a factor of the type of clay in the soil.  A highly weathered soil with kaolinite would have less cations in the soil, since there are less sites on which to bind those cations.  Smectite and vermiculite, with many more negatively charged receptors would bind more cations to the soil, making them available to the plants as they grow.  However, no plant can absorb what is not present in the soil and a soil manager needs to know the amount of cations present in the soil in order to fertilize properly.  An abundance of a particular cation, on the other hand, may mean that a completely different regimen is required. 

Soil Being Filtered

Soil in Agitator

Filtered Liquid Ready to Test

To test for cations in the soil, a sample of the soil has be submitted to intense temperatures so that the cations are first released and then incinerated.  Each chemical emits photons when incinerated, and these are emitted at specific energy levels.  We call them colors, but they are specific to each chemical and can be electronically read so that a researcher can then determine what chemicals are present in a given sample.  In this case the sample is SAGE garden soil.  The method for extracting cations from the soil sample was to inundate them with Mehlic 3, an acid solution containing Ammonium acetate, ammonium fluoride and other acids.  These were agitated

mechanically for five minutes then poured through filter paper into vials.  The resulting clear liquid was then numbered and placed in the Optical Emission Spectrometer, a fancy name for a fancy machine.  Calibrating itself to two chemically known samples, the machine then took samples from each vial to determine the cation (nutrient) content of each. 

 

Soil Data for SAGE Garden Soil

 Below is soil data from testing of similar soil. Not all elements were tested for, but where they were, they are quite similar to our results. Note that the samples go to about the same depth as our mixed sample, but are treated separately.
 

Soil Data for Willamette Series, California Soil Resource Lab

Depth cm	P ppm	K ppm	Ca ppm	Mg ppm	Mn ppm	Cu ppm	B ppm	Zn ppm	Fe ppm	C%	N%	pH
0-23	-	156	2200	276	-	-	-	-	420	2.45	.184	5.8
23-51	-	136	1360	324	-	-	-	-	420	1.02	.098	5.7
51-76	-	97	1740	360	-	-	-	-	420	.42	.055	5.5