The Benefit and Results of Soil Testing
By Paulette Mouchet
Originally published in "The Rose Garden" newsletter, September 2003.
I recently learned the very hard way that using tested and certified compost is a must in the organic garden.
In May 2003, Rob, my long-time friend and gardener, brought me 3 bulk loads of dark brown, triple-screened compost. Rob purchased it from a source he'd used for quite some time and considered reputable. Rob rolled this compost onto my front and back lawns. I used it to amend the soil where I planted 35 new Rainbow's End miniature roses. I spread this compost as mulch in a new rose bed where I'd recently planted Charisma, Incognito, Hot Cocoa, and Brandy. Finally, this compost was added to five of my raised vegetable beds and a corner of my herb garden.
Everything this compost came in contact with died. Everything.
Within 2-3 days of application, my lawns turned brown. Dark brown strips marked the edges of the roller. It look like I'd spray painted brown lines on the lawns. The back lawn had been newly over-seeded and all the new grass died.
Within a week, all the roses had crispy brown leaf margins. The poor Charisma roses, which I'd special ordered months in advance, were the first to drop their leaves and die. The Rainbow's End minis shortly followed. Then Incognito, Brandy, and Hot Cocoa. When the dust settled, I had one Incognito and six Rainbow's End left.
My summer vegetable garden was a disaster as well. My green and wax bean starts shriveled up and died within days. New seeds refused to sprout. The strawberries and the watermelons went the way of the beans. The corn and pepper starts simply wouldn't grow. The squash struggled along for awhile and produced a few deformed zucchini and crooknecks. I eventually yanked everything out.
In my herb garden the thyme, basil, and parsley all bit the dust.
I was in tears. I had a lot of time and energy and money invested in these plants and in making my soil healthy and hospitable for beneficial soil microorganisms. Every time I looked at my formerly beautiful yard and saw the death and destruction, my stomach twisted into knots. My plants were dead and my soil biology severely damaged. In short, I was going to have to start over.
As soon as the lawns turned brown, I called Rob and he immediately came out. Rob contacted the compost supplier and was assured the compost was regularly tested for nitrogen, phosphorous, and potassium (N-P-K) and six herbicides, and there was nothing wrong with it.
This wasn't good enough for me. Clearly, there WAS something wrong with it and I wanted proof of what that was and how to fix it. I wanted answers and apparently it would be on my dime. So, I collected four samples
- amended soil from one of my unaffected rose beds
- native soil taken from a depth of 2-4 feet
- tap water
That's right. I even had my water tested. I wanted to make sure that my soil and water were not contributing to the bad compost problem. I bundled everything up and sent it off to the Soil and Plant Lab, Inc. in Santa Clara, CA.
Within an hour of my samples' arrival in Santa Clara, I had a phone call (talk about great service!): the compost was loaded with potassium salts. The test for total soluble salts in the soil is called an electrical conductivity test (ECe). Healthy soil has an electrical conductivity of about 3 dS/m. Plants grown in soil with a higher ECe, show signs of salt burn: crispy brown leaf margins, leaf drop, death. Cotton is one of the very few plants that can tolerate a highly salty soil. It dies when the ECe is above 10 dS/m. The ECe of my compost was 10.8 dS/m!
If the supplier of this compost had been testing for N-P-K like he claimed he was, then he would have known the potassium level was sky high.
Rose Bed Soil
As for my rose bed soil, according to Soil and Plant Lab report, it's in pretty good shape:
- plenty of organic matter
- plenty of nitrogen in the preferable nitrate form
- ECe of 3.2, which is very slightly high
- pH 7.2, which is slightly alkaline (roses prefer pH of about 6.8)
- plenty of calcium
- lots of potassium and phosphorous
- adequate copper and zinc
- boron was slightly high
- iron was fair
Since the rose bed soil is quite fertile, the lab recommended that for the rest of the year, I fertilizer only with an organic, slow-release nitrogen fertilizer such as cottonseed or alfalfa meal. Cottonseed meal is a good choice because it is slightly acidic and will help lower the soil pH. The lab also recommended the addition of iron, so I'll be spreading some chelated iron along with the cottonseed meal. Finally, the lab recommended soil sulfur to help lower the soil pH. But, since I know sulfur can kill the beneficial soil critters, I'm going to skip this and depend on the cottonseed meal to lower my pH.
As for the ECe, which is very slightly high, the lab suggested extra summer water to leach out the excess salts.
Testing of my native soil produced some interesting results. I was very happy to learn it is sandy loam. This was particularly good news in terms of the salt-laden compost that had been tilled in. The potassium salts in it will eventually percolate down through my native soil and disappear. Application of lots of water will get the job done. If my native soil contained a lot of clay, it would be difficult if not impossible to get rid of the excess potassium salts.
My native soil also contains quite a bit of nitrogen, which is unusual, and a fair bit of potassium. The pH is 7.0-not as high as I expected considering I live in the desert where alkaline soil is typical.
About 40 percent of my water comes from wells and about 60 percent comes from the California Aqueduct, which comes from northern California, not the Colorado River.
My water is disinfected with sodium hypochlorite, a.k.a. chlorine bleach. When bleach is added to alkaline water, two of the end products are sodium and chloride. If too much bleach is added at the source by my water company, I could end up with water that is too salty. Fortunately, my tap water is not high in sodium or chloride. Also, the total soluble salts are low so irrigation with my water will not cause salt buildup in the soil or salt burn on the plants.
My water is slightly alkaline and coupled with my slightly alkaline garden soil, means I need to continuously add amendments to lower the pH. The lab recommends using soil sulfur or iron sulfate to lower my soil pH.
I felt a lot better after having everything tested. Now I know that the long-term effects of the salt-laden compost will be minimized by my native soil composition and my water quality. I can begin to rebuild beneficial populations of soil microorganisms and they will help repair any remaining damage.
As for the salt-laden compost. I had it removed from the property-as much as possible. Some had been tilled it and that couldn't be removed. The compost supplier is not interested in reading the test report and refuses to take phone calls or make himself available in person. He's lost my business as well as that of my gardener.
In the future I will only purchase compost that comes with a written test report. Yes, a tested and certified product costs more money but I lost over $700 in plants to the salty compost and I don't want a repeat. Never mind the lost summer vegetable harvest and the weeks of frustration I suffered.
When you buy bagged compost, it lists the ingredients and basic N-P-K test results. These tests and package labeling are regulated by your state. As long as you buy a name brand product from a reputable retailer, you can be pretty sure you're getting a quality product. When buying compost in bulk, demand to see a list of ingredients and test results for the batch of compost you are going to buy. Test results and ingredients can vary from batch to batch. Buy Certified Organic compost and mulches whenever you can. This will minimize any concerns about the quality of the product.
Better yet, make your own compost! Then you'll know exactly what went into it and how well it was composted.
Compost is an integral part of a healthy organic garden. With a little effort on your part you can insure you're getting the best possible product.