The Part-time Gardener #4: How to interpret your soil test results

I got my soil test results!

As I covered in The Part-time Gardener #3, a few weeks ago I submitted two soil samples from my backyard to the UMass Extension Soil and Plant Nutrient Testing Laboratory. The first sample primarily analyzed nutrients in the soil, while the second sample analyzed heavy metals. I recently learned how to do a quick “read” of the results, which I wanted to share here.

SOIL TEST REPORT

To begin, here is my actual report:

Soil Test Analysis 1

Page 1 of the Soil Test Report. Note all of my macronutrients are off the charts!

Soil Test Analysis 2

Page 2 of my Soil Test Report. Comments and resources specific to my results are given.

pH
My pH level is 7.2.  You can read loads more on this anywhere online but basically pH describes the relative acidity or alkalinity of a solution – with the pH scale ranging from 1 to 14. 1 refers to a highly acidic soil, 14 refers to a highly alkaline soil and at pH of 7, the soil is called neutral. Most crops do best when the pH of a soil is slightly acidic to neutral (5.5 to 7.5). In California, most soils range from pH 5.5 to 8.5, but a pH near neutral or slightly alkaline is most common (California Master Gardener Handbook Second Edition, Pg 49).  With my pH of 7.2, I’m pretty good to go.

Macronutrients
Next, let’s check out the macronutrients – Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S).  They’re ALL well above the optimum range listed. For example, P is at 163.5 parts per million (ppm) where the optimum range is 4-14 ppm.  What’s going on!? One clue could be that P is relatively immobile in soil because it binds strongly to organic matter. Even though I haven’t ever fertilized this area of the yard, it’s possible the previous owners or owners before that did. The Phosphorous could still be there from those fertilizers.   Calcium and Magnesium are similarly immobile.

Are these levels an issue, though? Turns out this test is geared towards New England soils, which are more acidic. Apparently gardeners out there frequently add lime to the soils, which explains why there’s an area for limestone reccomendations. If a local company were used (like Wallace Labs in El Segundo) my results could be different. The bottom line, though, is that I probably don’t need to add anything to my soil.

Micronutrients
Not much to comment on here.

Lead and Aluminum
Lead and aluminum are also recorded at acceptable levels. (Though read on to see what the Total Sorbed Metals Test had to say about lead…)

Cation Exchange Capacity (CEC)
CEC is a basically a measure of how well the soil can hold onto and release nutrients, specifically the positively charged nutrient ions called cations. According to the UMass Guide to Interpreting Your Soil Test Results:

CEC is reported in units of milli-equivalents per 100 grams of soil (meq/100 g) and can range from below 5 meq/100 g in sandy, low organic matter soils to over 15meq/100 g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to cation nutrient loss through leaching.

With a CEC of 23.5, I’m assuming my soil – which I previously tested to be a sandy loam  soil – is within the acceptable range.

TOTAL SORBED METALS TEST

I also went the extra mile by opting for a Total Sorbed Metals test:

Total Sorbed Metals

Total Sorbed Metals Test Results.

Most concerning here is the lead concentration found (332 mg/Kg), given that the USEPA Threshold is 400 mg/Kg. Here’s where I got really confused – the Soil Test Report showed my lead was low, but my Total Sorbed Metals Test shows I’ve got something to be concerned about. My guess is that the Total Sorbed Metals Test is more fine tuned to detect lead, meaning, it’s likely more accurate.

To read up on lead, I turned to a great publication by the University of California Agriculture and Natural Resources entitled “Soils in Urban Agriculture: Testing, Remediation and Best Management Practices.”   According to Table 2 of this paper, my lead concentration should be considered a “potential risk.” Some of their remediation recommendations include:

  • Relocating to a lower risk area of the garden
  • Increasing use of soil amendments
  • Barriers (mulch)
  • Raised beds and containers
  • Wear gloves and use tools to reduce soil contact and ingestion
  • Decrease planting of root vegetables
  • Increase planting of fruiting vegetables, vegetables that grow on vines, fruit trees

Unfortunately,  there is only one part of our yard suitable for edible gardening in terms of sunshine and space. So moving our vegetable operations isn’t much of an option. However when starting our new beds, we incorporated all of the other recommendations: I amended the ground soil with compost and dumped mulch on the ground around the beds. Going forward in the beds, I’ve also decided to give up root vegetables (like beets) and many vegetables that grow on the ground (like lettuce) and instead am opting for crops like tomatoes and cucumbers. I always wear boots and gloves when I’m working outside. Depending on what I’m doing, I also wear a face mask.

This could be being overly cautious, but it doesn’t hurt.


Overall, I’m glad I decided to get the tests done because now I know what I’m working with in my backyard. I probably don’t need to add anything extra to the soil, and my pH level should be fine. Most importantly, without the tests, there’s no way I would know about the lead in my soil. UMass was a cost-effective option and returned the results quite quickly (6 days for the first sample and 13 days for the second sample).

What’s your experience been with soil tests? Have you used a local soil lab instead? Leave a comment below!

With hands to soil,

Erin

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