Curt Swift, Ph.D.
CURT'S CORNER
Grand Junction Free Press Gardening Columnist

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October 11, 2012
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GARDENING: Soil testing is easy, inexpensive, and it can save your crop, garden or lawn

Fall is the time to have your soil tested in preparation for spring planting. Whether you intend to plant a new garden, orchard, lawn, lavender field or field crop, a soil test can help your planting be a success. Collecting a soil sample is easy. Dig a hole and take a slice of the soil of the edge. Interpreting the results and making recommendations based on those results is the technical part.

Over the last several year while working with Colorado State University, I interpreted and made recommendations specific to the plants being grown for over 700 soil test reports. In some cases the recommendations were unfortunately made after the crop had already suffered from assorted soil problems. A great example of this is the field of dying pinto beans Ron planted in Montrose. Shortly after I identified his plants as dying from Fusarium root rot, a disease of beans and many other plants, Ron collected and sent a soil sample off for analysis. He emailed me the report. Beans should have never been planted in this field; the nitrogen level was much too high.

Beans have a symbiotic relationship with Rhizobacteria that form nodules on their roots. These bacteria convert nitrogen gas in the soil atmosphere to plant-available nitrogen. When soil already contains a high level of nitrogen, these nodules do not form and the bean plant is therefore unable to obtain the nitrogen it requires. While this may not make sense, the health and yield of beans and other legumes suffer when soil nitrogen exceeds a specific level. 

This critical level differs for each crop whether it be beans, peas, honey locust trees, or other legumes. In addition to yield loss, these nitrogen-deficient plants are unable to produce the root protectants necessary to prevent invasion by soil-inhabiting root pathogens such as Fusarium. A soil test prior to planting would have provided the information necessary to develop recommendations on how to correct the high nitrogen problem and provided Ron with a list of crops that would benefit from the high nitrogen level.

Sometimes a high soil nitrogen problem is linked to a high soluble salt level. Sometimes both are linked to a high water table. The other day I was talking with Don Mundy of High County Lawns at his home and sod farm at 2426 H Road. We were reminiscing about the problems we have run across during our many years working in the Grand Valley. His recollections reminded him of a situation where hydroseeding and laying sod both failed. As soon as he entered the property he knew the reason for the failure. A test of the soil he collected and had tested for soluble salts confirmed his suspicions. The contractor responsible for the installation would have known the soil was unsuitable for grass if a soil sample had been submitted for analysis.

The salt level of this site was extremely high. In addition, the water table was at a level that attempting to wash the salts out of the soil would have been a failure unless a drainage system was installed to move the salty water off site. I'm not sure what the homeowner was charged to hydroseed one year and then attempt sod the following year, both of which attempts failed, but it was certainly much higher than what the cost of a soil test and related interpretation and recommendations would have been. With the proper recommendations and follow-through the initial hydroseeding could have been successful.

A soil analysis determines the nutrient level, pH, soluble salt status, and organic matter content. Each of these directly affect plant health, yield, and quality of the fruit, vegetables, and other crops you are growing or intend to grow. A soil test also indicates what you should avoid in the way of fertilizer elements even when applied in organic form. For example, if you already have an adequate phosphorus level and you continue to apply products containing phosphorus, whether organic or synthetic, you will kill off the mycorrhizal-forming fungi critical to the health of plants. The requirement for mycorrhizal-forming fungi on conifer roots is absolute; at last count 98 percent of dicots had also been shown to require mycorrhizae. Even onions require mycorrhizae. Applying mycorrhizal-forming fungi to the soil when phosphorus exceeds a certain level does not replenish the soil with these critical fungi.

High phosphorous levels also restrict the production of the phytochelates that solubilize plant-deficient micronutrients thereby permitting their uptake. Without having your soil analyzed you cannot know the phosphorus status of your soil. Even when you have the soil test results in front of you, someone needs to interpret those numbers and provide you recommendations specific to the plants you intend to grow or are growing. Soil test numbers without interpretation and recommendations are meaningless.

Let's take another example - nitrogen. Peas and beans are legumes and have their associated nodule-forming Rhizobacteria. Peas may need the soil nitrogen level increased to permit establishment until these bacteria can start producing nitrogen, while the growth and yield of beans will suffer from the same nitrogen application. Zinc is another example where interpretation and recommendations are critical. When zinc is deficient, some fruit tree should be sprayed with this nutrient in the fall while other types of fruit trees require their Zn application during the growing season. The soil test does not provide you this minutia.

A soil test provides you the raw numbers but does not provide you the details you need to produce quality plants. While I don't do the analysis, as a consultant I will interpret the results and provide you recommendations for the specific plants you are growing or intend to grow.

My blog item on the importance of soil testing and a list of my fees for this service goes into more detail. You can access that article at http://swiftsgardeningblog.blogspot.com. If you do not have access to the internet and want to receive a copy of this article, give me a call at 970-778-7866 and I'll mail it to you.

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Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curtis.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.


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The Post Independent Updated Oct 11, 2012 05:26PM Published Oct 11, 2012 04:10PM Copyright 2012 The Post Independent. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.