The Soil Sampling Project

Our project was to take soil samples from the SAGE Garden, and analyze them in the lab for various nutrient content, carbon/nitrogen ratio, and pH. Our goal was to supply usable date to the SAGE garden to help them solve some growing problems and improve their garden management.

Aerial View of SAGE Garden. The Soil Type is Willamette Silt Loam

Welcome to the Garden

Compost Bins

The Starker Arts Garden for Education (SAGE) is an organic garden situated on a one acre plot located in the southwest of Corvallis. The SAGE garden is broken down into six plots, with additional space for composting, equipment storage, community meals and an area for chickens. The compost area contains large piles of yard debris with traditional composting, as well as vermiculture which is conducted in a worm box, where worms are fed food waste, coffee grounds and other organic matter to create a nutrient rich soil. The garden is tended by volunteers and funded by donations. It produces over 40 types of vegetables and other edibles, which are donated to the South Corvallis Food Bank and soup kitchens. For more information visit http://corvallisenvironmental.wordpress.com/edible-corvallis-initiative/sage/

Hey, let's face it folks, it doesn't get any more down to earth (soil?!?) than this. The process of gardening – or farming – is a direct link between soil and humans. Growers use plants to collect nutrients from the soil and air, putting them into a form that we as humans can use: fruits and vegetables. Tasty! Performing the process organically allows us to reap many of the benefits of conventional methods without the harmful side effects that chemical fertilizers and pesticides can cause. We went to the SAGE garden with a few predefined goals: First, we were there to collect soil samples for analysis; second, to learn how the SAGE garden functions; and third, to lend a hand to a worthy cause. And if the weather was any indicator, we were also trying to get ourselves sick, but that wasn't really an intended goal....

            The collection and analysis will be explained in more detail below, but the basic mission was to collect soil samples from across the garden and analyze them to give the SAGE garden staff an idea of what each section's nutrients and pH are now, so they know what they need to do to improve it. The analysis was carried out at Oregon State's Central Analytical laboratory (CAL).

Worm Bin

            Our jobs for the time we were at the garden consisted of weeding the plots, turning and building compost, and feeding the worms. Oh, feeding the worms isn't a euphemism, nobody died! Rather, it consisted of removing the covers for them, hand-digging small holes around the box and placing an assortment of the materials mentioned above in those holes and re-covering the box. Simple get-your-hands-dirty goodness.

Worms!

The major points of what we learned about the SAGE garden are above, but it bears repeating that this is a highly community-focused, VOLUNTEER-DRIVEN organization (hint, hint) that is carrying out an organic, hopefully sustainable project. In other words, these are good people, trying to do good things – both for other people and the Earth. So if you've got some time, head over to the garden. I'm sure Jenna and the crew would love to see you there!

SAGE Garden Soil Sampling

Garden Plots at the SAGE Garden

The Starker Arts Garden for Education (SAGE) stands on a gentle south-facing slope on re-purposed grazing land which, prior to that, was a wetland area.  The nearby green space, as well the prevalence of ducks and geese offer testament to the reclaimed nature of the land.  The SAGE garden area is divided into six plots along the slope with an additional area that was used for chickens. 

Soil Sample

Our group took six samples of the soils; one from each of the lower and upper plots, a single sample from the two middle plots and another from the chicken coop area.   To eliminate sampling bias, each sample consisted of soil collected at three sites within the sample area; these were then collected into a single container (Ziploc bag) and homogenized. On one of the plots we took samples just from the middle, and not from anywhere near the edges. During the previous harvest, the SAGE gardeners said that there was a problem with the vegetables coming from the outside area of that plot. Therefore, they wanted us to test the middle section to help determine what the variable was.  Aside from the general benefit of having soil information, the north-eastern plot (upslope and rightmost, as one looks upslope) has had stunting issues in the past, and the SAGE gardeners hope to figure out why from the information gleaned.  Plots two and four (bottom right and middle right, looking upslope) have dense leaf cover, while plots one, three, five and six do not.

Really hard soil!

Homogenized soil sample

When the samples were taken from the leafy area, we made sure to push aside the organic matter so that we only were testing the soil. In the chicken coop area, we had a problem with taking samples. It was much harder to take the samples because the soil was a lot more compact, especially where the chicken coop once stood.

We made sure to not put the samples in anything metal (like buckets) because metal buckets are galvanized which can interfere with testing results.  The homogenized samples (approx. a single baggie-full) were assigned sample numbers by the Central Analytic Lab, and taken there for testing. 

Soil Samples ready for lab

After our group took our samples, we then helped with other jobs around the SAGE garden. Two people turned compost, two pulled weeds, and two fed the worms. Over all, we were grateful to be able to visit and take samples from the SAGE garden and have such a wonderful learning experience.

Soil Testing in the Lab

The CAL Lab

After getting the six soil samples from Sage Garden, the next week we headed over to the Central Analytical Laboratory where soil testing is done to get a hands-on perspective of how soils are tested, what they are tested for, and why.

Laboratory soil tests are useful in helping develop and maintain more productive soil and increase crop production. The tests provide information on the available nutrient content of soils. Soil testing helps to select the correct kind and amount of fertilizer and liming material a soil needs.

The standard soil test that most laboratories perform includes, but is not limited to: the measurement of organic matter, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and soil pH (acidity). The pH test is the best way to determine how much lime is needed in a soil. Certain crops might have higher requirements for specific nutrients.

In our visit to the lab we did soil testing on the following measurements:

P, K, Ca, Mg, Mn, Cu, B, Zn, Fe, C%, N%, and pH.

pH Analysis Procedures

Adding Dye to Soil Sample

Testing soil pH will help ensure that you're getting the healthiest and biggest yield of crops from your garden. pH is a measurement of how acidic an entity is. Conducting a soil pH test is the best way to find out how acidic your soil is. Different types of plants have different requirements for their optimal pH level. By testing soil pH in each garden bed at Sage, they will have the opportunity to provide the garden it's best opportunity for having successful growing conditions. Placing plants in garden soil that does not match their pH needs can weaken the plants. Most plants will grow well in soils that test within a pH level in the range of 6.5 to 7.0.

pH testing was performed in the lab for all six samples that were taken from Sage Garden. The test was done by saturating a small sample of each soil with a selected dye, swirling lightly to mix, and then comparing the color to the colors on the pH scale.

Dyes for pH Testing

Color Matching - not so easy!

The soil dyes used were:
Bromcresol Green for pH 3.8-5.4
Chlorophenol Red for pH 5.2-6.8
Bromythol Blue for pH 6.0-7.4

The results were:
Soil Reference Western Oregon sample - 6.4
Sample 1 - 5.8
Sample 2 - 5.6
Sample 3 - 5.4
Sample 4 - 5.6
Sample 5 - 5.4
Sample 6 - 5.6

The reference soil, “Soil Reference Western Oregon “ or SRW, is known to have a pH of 6.2. Our reading was within the margin of error, so the other readings are probably reasonably accurate too. The Soil Survey data for Willamette Silt Loam indicates a range of pH 5.6-5.9, so the SAGE soil is generally within that range.

C/N Testing in the Lab

The Soil-baking Oven

Because soil is a complex habitat, it hosts a complex variety of soil organisms whose needs are equally diverse.  Depending on one's soil goals (agriculture, industrial, construction, etc.) the types of soil organisms may play an important role.  In agriculture especially, soil organisms contribute to soil fecundity and structure.  Structure in turn has an important role to play in water infiltration, soil aggregation, and pore size.  Because of the complexity of these characteristics, maintaining a habitat that is most conducive to a particular soil organism is vital.  The carbon to nitrogen ratio, then, is an important piece of information for a soil manager.  Most soil biota enjoy a low C:N ratio, since nitrogen is difficult to acquire. 

Soil Being Placed in "boat"

To test the carbon to nitrogen ratio in the SAGE garden samples, soil was placed in a ceramic container known as a “boat.”  Each sample was weighed, and the tare weight of the “boat” subtracted to ensure accuracy.  Then each sample was heated in an oven to 2300 degrees to release the carbon and nitrogen from the sample.  These were read by a computer and a print-out generated with results for each sample.  The SAGE garden has a fairly low C:N ratio of ~10:1, on average.

C/N Ratio Data, plus Sulfur. The "Oatmeal" data is for reference

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