Soil Testing

Many of our beloved garden plants originated somewhere else.  Brassicas and carrots are from Europe. Avocados, corn, and peppers came from Mesoamerica. Potatoes originated in the highlands of South America. Check out this amazing interactive map.

Each of these areas have their own climates with patterns of sun, rain, wind, which affects the composition of the soils in which they evolved. For example, the wild progenitors of blueberries grew near swamps on the conifer forest floors of western Canada. The soil would be exceptionally acidic from the water dissolving and carrying away the minerals in the soil and the accumulation of fallen conifer needles. If we want to grow blueberries here in the Willamette Valley, we need to make them feel at home. The better we can do this, the happier and more productive they will be.

It helps to understand our biome’s natural tendencies.  Here are some averages for our part of the Willamette Valley:

USDA Hardiness Zone: 8b
Low Temp: 18 F
Average Rainfall: 38 inches
Elevation: 200 feet
Growing season: April 30 – October 14
Length: 167 days
Last Frost Date: March 25
First Frost: November 5

Natural soil pH: between 5.5 – 5.8 (slightly acidic)

Preferred pH Ranges

Blueberries: 4.5 – 5.5
Most Garden Veggies: 6 – 7.5

Brassicas (broccoli, cabbages, and other plants that make me burp) as well as asparagus tend to like it a bit on the high side (more alkaline 6.5 – 7) whereas strawberries and leaf lettuces don’t seem to mind it a little lower. (5.5-6.5) 

Fruit Trees: 6-7.5 pH

Why Does pH Matter?

The soil pH affects the plants ability to uptake nutrients.  

When blueberries show signs of iron deficiency, which can look like yellow leaves (chlorosis) and dark green veins, check the pH to ensure it’s in the proper range. When the soil is in that very acidic range of 4.5 – 5.5., the iron already in the soil becomes available to the plant. 

Brassicas tend to be much more susceptible to a from a fungal disease called clubroot when the soil is  our natural pH level of about 5.5 (slightly acidic).  It could be because the plant is not as vigorous, or that the pathogen itself prefers our native pH to thrive. But either way, providing a pH nearing 7 is an excellent preventive measure for this plant family. 

When Should I Test for pH?

Fall & Winter: The dormant season is the perfect time to test and amend soil for pH.  When everything is sleeping they are less likely to suffer any deficiencies. The pH amendments also need time to break down and infiltrate down into the soil.  You may be able to squeeze in a couple rounds of testing and amending before planting season. 

How do I Test for pH?

(See my: Video Tutorial)

You have so many options here. You can send a soil sample to a lab like A & L Laboratories, or use one of a dozen different home tests. 

My preferred home test product is Rapitest.  The process is simple:

  1. Sweep aside mulch.
  2. Dig down a to the root zone. (Blueberries about 3”, garden veggies about 6”-8”)
  3. Take a scoop of soil from several locations in your bed.
  4. Mix
  5. Fill the pH test chamber to the line.
  6. Add the correct capsule of reagent.
  7. Fill with distilled water to the water line.
  8. Shake.
  9. Wait.
  10. Read the result.

A laboratory test will give you more concise results and they will offer recommendations on how to correct the pH.  They can also test NPK levels as well as mineral and micronutrient content.  I have been relying on the Rapitest for years and always feel a little uncertain about my interpretations, so this year—ta-dum!—I’m doing both.  (A & L Lab Results)

I read the Rapitest results to be nearly neutral, around 6.5-7.

The A & L Lab test showed my soil was nearly 7. (I did my usual application of lime in November. )

Amending for pH

With your results on hand you can now begin doing the math on how to amend your soil. 

For Vegetables:

If you wish to raise the pH for our vegetable friends, then we need to add calcium carbonate (lime), or dolomite lime which is calcium carbonate + magnesium.  A good practice is to raise the pH half a point at a time.  So if your pH is 6 then shoot for 6.5. Then amend again in a couple months to get to the desired pH.

For Blueberries:

To care for our conifer-forest-swamp-berries we will likely need to lower the pH (more acidic) using Ammonium Sulfate. Shooting for .5 reduction at a time is a nice way to ease your plants into this happier arrangement.  Keep testing every three months or so, then once a year after you get it balanced.

OK, so the pH is Good. What About NPK and Other Stuff?

Looks like N deficient, P deficient, and K OK. The Lab test confirmed the N deficient, but showed a Phosphorus build up from the high level of organic matter (compost) in my garden boxes.

Spoiler alert. If you are following the recommendation to perform soil tests in the fall & winter then your likely nitrogen (N) depleted. Phosphorus and potassium are likely OK if you’re into adding lots of organic matter and compost as modern gardeners are wont to do. Sometimes we add too much and end up with a phosphorus build up. Calcium (lime) we have to add for both nutrient and pH balance. Other minerals and micronutrients (e.g. iron, copper, boron, etc. ) are important but often stay more stable in the soil. Rely on a laboratory test before adding any amendments for those.

When do I amend for those nutrients?


First ensure your pH amendments were sufficient to get your beds in the correct range. Then when soils are warming and you are a few weeks out from planting, it’s a good time to amend the soil for nitrogen and other deficiencies you may have.  

Why not During the Dormant Season?

Nitrogen is a mobile nutrient, meaning that heavy rains will wash it out of your soil where it will likely contribute to pollution of the watershed and aquifers.  Also, you don’t want to encourage vegetative growth in the winter because it will make overwintering plants more susceptible to frost, pests, and disease. It’s like drinking an espresso before bed. Not all nutrients are this mobile, but since most fertilizers are in a blend, it’s best to wait until spring. 

How Often do I Need to do Soil Tests?

I test my soil every winter. But if you dial in your system such that every year you get same results and performed the same amendments, then you could skip a year and follow your routine.

If my pH is way off, I will test in December and amend to raise or lower .5 on the pH scale. Then wait a month and amend again, lather-rinse-repeat until spring. Then I would test again the spring to see if the pH is close to what it needs to be.  (The amendments can take weeks or months to break down and affect the pH levels.)  While you are getting your soil pH balanced, it is good to retest every 3-6 months. Then once a year after that, because your soil will tend to return to that natural pH of 5.5, although all the organic matter in our wood-framed raised beds will likely retain a more stable neutral pH.

I will also do a home test ad-hoc if things aren’t going well.  Chlorosis (yellow leaves) is a common indication of nitrogen deficiency, but it could also be a symptom of other problems as well.  So if I’m getting a lot of yellow leaves in May, I might test to see if I need to bump the nitrogen or look deeper to see if there is another issue. 

Doing the Math 

My total garden bed area is about 350 square feet. (Thank you basemap!)

Let’s pretend the soil test recommended 150 pounds of 100 score lime for every 1000 square feet.

Super Sweet comes in at 90 score lime.  So the calculations would look like this.  (I rounded the numbers.)

(150lbs / .9) · (350sq ft / 1000sq ft) 

= 167lbs of Super Sweet · .35sq ft 

= 58 lbs of Super Sweet distributed evenly over 350 square feet of my garden space.  

If this recommendation was going to increase the pH from 5.5 to 6.5. Then I might decide to spread half now, then the rest sometime again before spring planting.

So I would do about 28 pounds now and in March do the remaining 27 pounds of Super Sweet. If my test showed low magnesium as well, I would do the last 27 pound application using dolomite lime. 

What? I Didn’t Get into Gardening to do Math. 

I feel you. Truly. 

A few years back I attended a workshop and when the subject of lime came up I raised my hand and asked, “How much do we add?” and she replied, “Let it snow.” I followed this advice successfully for many years prior to doing my own soil tests. Now, I would amend that to say, “Distribute an even sprinkling of lime (calcium carbonate) over your lawn and growing beds (not blueberries or lignon berries) every fall. Test in March to make sure it is somewhere within the neutral zone. If it is testing below 6.5 add a sprinkling of dolomite lime to further sweeten the soil and add magnesium.” This has worked for me so far.  My lab soil tests came back showing a pH of 7 and 6.9.  

So geek out with me if you want.  But of course, it’s OK to wing it too.  Gardening is a choose-your-own-adventure activity. Read the directions on the box or bag and do your best approximation and you’ll likely be fine. If you have new garden boxes with freshly purchased soil, it will likely be fine without any pH adjustments. 

Whew, I’m famished. Do you have time for tea?  I just baked some biscuits this morning. I’ll set some out. 

Did you catch my video tutorial? Warning: I’m all kinds of awkward on screen, but somethings are easier to see than read.

Want more information?

Good on you. Here are a couple articles from Oregon State University.

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