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GroEdibles Blog

Oct 17

mar’sel/Terranea Edible Gardening Lesson #1: Starting from the Ground Up – Testing the Soil

Posted by GeriMiller on 17 Oct 2009. Filed under  Community Outreach Sustainability Gardens View Comments

It was a beautiful day to be playing in the dirt! A beautiful surrounding landscape, gorgeous views, great mild weather….gardening perfection at Terranea! Oh, remember those flowers I was afraid they’d plant? Well, guess what? Oh well, marigolds have a good companion quality so we’ll use them! I was there to do some necessary and important foundation work before we install our garden:
•    Testing the irrigation system and
•    Testing the soil

I know….B-O-R-I-N-G, right?  I can hear the collective sigh from all of you now….”What? Why can’t we just get to the fun part and put some veges in the ground?”. I know, I know…it’s not my favorite part either, but experience will tell you that unless you go through these steps now, you risk the failure of your garden later.

With the help of my new Terranea friends, Ruben (landscape foreman) and Corey (irrigation specialist), I acquainted myself with the core of  the Terranea landscaping team and…most importantly…how the irrigation system works! The drip irrigation system tested out well except for a lack of good coverage in 2 of the 4 beds. To solve this problem I’ll add a few sections of micro-tubing with emitters in those areas and we should be good to go!  Most veges need about an inch (some more, some less) of water per week making them among the thirstier items in your landscape.  Knowing your soil is crucial in determining exactly how often you have to water.  Now on to the next chore…

Testing your soil is an important step in preparing a site for an edible garden especially if you’re not familiar with your locale’s history if you’re an urban gardener or if there’s been recent construction on the site.  In our case, I was not concerned about heavy metals at Terranea.  My concern was primarily with fertility and pH since the property was “new construction”. Complicating things, we weren’t sure if compost had been added. It really didn’t look/feel like it to me.  I opted to test for pH, fertility (macro and micro nutrients*) and a heavy metal screen just to have a complete baseline for this new garden. While I was doing the soil sampling, I was THRILLED to find wonderful worms working their way through the soil….A GREAT SIGN!

For more information on soil testing and preparation, please see the Fall/Winter Gardening section of the HGEL website under Favorite HGEL Resources.

Step 1: Finding a Soil Testing Laboratory
The best way to find a local lab is to contact your county’s Cooperative Extension Office. A link to an extension office listing for California is on the Fall/Winter Gardening page of the HGEL website.  There are also simple inexpensive home kits that you can use but keep in mind that they are limited basically to pH and N-P-K analysis.

Step 2:
What to Test For
Heard the saying “I don’t know what I don’t know”?  This comes into play a bit when trying to decide how extensively to test your soil.  To a large extent, this depends on where you live and what you’re concerned about.   If you live in an inner city area where there may be a history of industry that involves heavy metals such as oil pumping, auto repair shops, metal industries, etc. a soil test that includes heavy metal screening may be a good choice.  I have posted a link to a great article from UC Davis about Trace Elements and Urban Gardens  on the HGEL Fall/Winter Gardening page of the website. And to add to your leisure reading, here’s a link to a great guide from UC Davis on what you can and should test for when doing soil testing. Though this guide is designed for family farms, it is still helpful in understanding  guidelines for soil testing.

Step 3:
How to Take a Soil Sample
Your lab will give you full instructions with your kit but here are the basics:

•    You will need to remove any vegetation from the soil surface, such as grass, plants or mulch.

•    Next, use a spade, trowel, or soil auger to remove a plug of soil that is approximately six inches deep.  Place the soil sample in a clean bucket or plastic bag.

  • Repeat this process ten to fifteen times, depending on the size of the area you’re sampling.  This helps to ensure that the sample is as representative as possible of the area that’s being tested.
  • Do not obtain samples from areas that have already been chemically treated, because they will not accurately reflect the composition of your soil.

Soil tests usually come back within 10 days and most companies will email the results to you with suggestions for corrective action.  When Terranea’s results are received, I’ll blog about our corrective actions, if any.
Again…please check the HGEL Fall/Winter Gardening page on the website for tips on raising/lowering pH and boosting N-P-K.
Macronutrients can be broken into two more groups:
primary and secondary nutrients.
The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking from the soil first because plants use large amounts for their growth and survival.
The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil so fertilization is not always needed. Also, large amounts of Calcium and Magnesium are added when lime is applied to acidic soils. Sulfur is usually found in sufficient amounts from the slow decomposition of soil organic matter, an important reason for not throwing out grass clippings and leaves.
Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities . These elements are sometimes called minor elements or trace elements, but use of the term micronutrient is encouraged by the American Society of Agronomy and the Soil Science Society of America. The micronutrients are boron (, copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micronutrients (as well as macronutrients) to growing plants.

WATCH FOR THE NEXT BLOG: Mar’sel’s Planting List

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