Soil testing my raised vegetable beds was long overdue, it'd been seven years since my previous test. In that time period I renewed the soil every so often with added bagged compost, fertilizers, plus I grew cover crops to improve soil structure, fertility and keep the soil in the beds protected during the winter months. For the most part my vegetable plants produced fine since 2012. I had a couple of tough seasons keeping them well watered, the soil is super fast draining (then and now) and the weather was pretty darn hot the past two summers. A couple of seasons my tomatoes suffered from Early Blight and Tomato Spotted Wilt Virus. One season my direct seeded basil came down with downy mildew, no more raised bed planting for my basil, now I seed exclusively in Smart Pot containers.
Below are the results from 2012 and 2019. Normal ranges are in parentheses under the 2012 column. *See notes at end of the results for more information. Testing was done both times at the Colorado State University Soil, Water and Plant Testing Laboratory in Ft. Collins, CO.
2012 2019
pH* 7.3 High (6 to 7.2) 7.6 High
Electrical Conductivity or Salts 4.3 very high (less than 2.0 is fine) 0.3 Low
Lime less than 1% (over 2% is high) less than 1%
Texture Estimate Sandy Loam Sandy Loam
Sodium Absorption Rate, not requested in '12 0.7 Low (good)
Organic Material 3.1 Low (up to 5%) 6.4 High (not good)
Nitrate* 139 ppm Very High (not good) (10 to 50 ppm) 10.0 ppm Low (not good)
Phosphorus* 100 ppm High (not good) (10-45 ppm) 94.8 High (not good)
Potassium 1759 ppm High (not good) (<200) 533.5 ppm High (not good)
Zinc 9.9 ppm Adequate (<100 ppm) 19.1 ppm Adequate
Iron 52.2 ppm Adequate (<10 may be deficient) 54.9 ppm Adequate
Manganese 6.8 ppm Adequate (<40 -50 ppm) 7.7 ppm Adequate
Copper 3.7 ppm Adequate (<20-30 ppm) 4.3 ppm Adequate
Boron* 1.04 High (1 ppm or less is adequate) 0.50 ppm High
*pH: is the acidity or alkalinity measurement of the soil. On the pH scale, 7.0 is neutral, below 7 is acidic, above 7 is base or alkaline. The levels have to do the properties of long time established soil and the climate. Many parts of the country with good moisture have acidic soils, so naturally here in CO, we run on the alkaline side. 6.0 to 7.2 is the preferred range for good plant growth. Our plants can fairly easily tolerate 7.2 to 7.9 pH levels if the correct supplemental nutrients are added. Soil pH influences the quality of plant growth and nutrient availability to plants.
Lowering the pH isn't easy and takes time and work. One hurdle to changing pH is the free lime content, if high (over 2%), then any added soil acidifiers such as sulfur are neutralized so won't change the pH. Click here and scroll to page 222-2 for more information.
*Nitrate: 20 ppm to 50 ppm nitrate-N is needed for good growth of trees, shrubs and turf. For vegetables and flowers, the amount of N required depends on the soil Organic Matter or O.M. content. If the soil O.M. is 5% or greater, then 10 ppm N is required for good plant growth. If the O.M. is less than 1%, then 50 ppm N is required for good growth.
*Boron: any reading above 4 ppm is toxic to plants. If the level is too low than the soil lab is very specific when recommending how much to add because even a small amount can be too much and cause plant harm.
In 2012 our beds were filled with a vegetable specific "planters mix" of soil from a reputable sand and gravel company here in Denver. I took soil samples for testing this new soil the day it arrived for filling the empty (newly built) raised beds.
The soil test result notes advised me to leach the beds with 6-12 inches of water to push excessive salts deeper into the soil to dilute the salt effect. Effective leaching depends on using quality water and having good drainage. Despite the high Nitrate in '12, plants produced fruit and leafy crops did fine. Often too much Nitrate will result in bushy plants with small or fewer fruit. The leaching in my case helped dilute the high salt level.
Why the other changes in my soil from 2012 to now? That's easy, I added stuff in the form of composts (store bought) and incorporated cover crops which added more O.M. and fertility. I also added dry fertilizers, mostly slow release organic type. My O.M is high, no need to add any more bagged soil or turned over cover crops for a few years.
The fertilizer I used contained both *phosphorous and potassium, clearly not needed based on the results. Colorado alkaline soils contain plenty of both of these nutrients. However, they can be low if it's a new planting area where O.M is low or if the pH is high - 7.8 - 8.3. For more reading on phosphorous and potassium, click here. Scroll to page 232-5.
What jumps out is the low Nitrate level in 2019. Clearly my vegetables used up what nitrogen was available since 2012 and what I added since that time wasn't enough. The comments on the soil test suggested adding 0.3 lb N/100 square feet. They listed urea, ammonium sulfate, bloodmeal, corn gluten meal or alfalfa meal pellets as Nitrate options. I'll use bloodmeal since I have it on hand. Be careful if adding corn gluten meal because not only is it an organic nitrogen source, it will prevent seeds from germinating for a few weeks - any seeds like lettuce or other direct seeded ornamentals, vegetables and some weeds.
Additional Resources:
A Little Soil Sleuthing can Save You
Do You Know Dirt About Soil? Here's a Three-step Primer
Soil Testing: It's a Good Thing
How Your Soil Test Results are like a Margarita
Interpreting Soil Tests: Unlock the Secrets to Your Soil from New Mexico State University Extension, helpful, they have similar soil conditions to CO
Soils, Fertilizer and Soil Amendments - Several Topics to Choose
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