In the summer of 2014, UDC offered free soil testing to the community. I was happy to take advantage of this opportunity. My reported results were as follows: the soil's pH was neutral, the nitrogen level was reported as "sufficient," and all other major nutrient levels (phosphorus, calcium, magnesium, and potassium) were reported as "excessive."
What these results mean is that my soil is abundantly fertile; additional fertilizer is not needed. Excess fertilizer wastes money and resources, damages plants, and causes pollution of our watershed. I honestly cannot recall if I was using fertilizer 4 years ago, but I know that these results convinced me that I wouldn't need to. Today, I feed the soil, which in turn feeds the plants, by using only compost and mulch.
There are differences of opinion about how often one should have their soil tested. I've seen recommendations ranging from every year to every 3 years. Regardless, I'm pretty sure I'm overdue. For deciding how often to test your soil, think about your goals and what information you want from a soil test. My goal is to grow high quality vegetables on a frugal budget, spending as little money and requiring as little labor as possible. Yes, because I'm essentially cheap and lazy, I waited 4 years to retest, LOL! And the information I was looking for is whether or not I still don't need fertilizer.
At this time, UDC isn't offering soil testing. University of Maryland provides a list of regional laboratories. I decided to go with University of Delaware because (1) I wanted to support a public research institution and (2) their testing methodology is appropriate for mid-Atlantic soils. In addition to testing the soil's pH and major nutrient content, I liked that their $15 "home lawn and garden" package also tests for the soil's percentage of organic matter as well as the soil's cation exchange capacity (the ability to hold and release plant-available nutrients).
If this was my first soil test, I would have also sprung for University of Delaware's $25 heavy metal screening test. Luckily for me, UDC tested for heavy metals back in 2014 as well. Those results showed no evidence of contamination from lead, mercury, cadmium, selenium, nickel, or chromium. I don't see a need to retest as it's not like I've been burying industrial waste in my backyard or otherwise done anything that would contaminate the soil.
However, arsenic was reported at a level slightly above EPA guidance, 0.61 mg/kg (EPA guidance is 0.4 mg/kg). Naturally, this freaked me out at first, so I did some investigating. For comparison, soils in the central and eastern regions of Maryland have been reported to have naturally-occurring arsenic levels as high as 10 mg/kg, though the average is closer to 3 mg/kg. The DC Department of Energy and Environment considers 60 mg/kg to be a dangerously high level...that's almost 100 times the level found in my garden. Furthermore, research conducted by the University of Nevada suggests that vegetables don't accumulate arsenic from soil in dangerous amounts.
The way I see it, arsenic is like UV light; no amount is safe and yet it's a part of Nature, so there's no way to entirely escape it. Nonetheless, you can minimize your exposure. For UV light, we wear protective clothing and sunscreen. For arsenic, avoid touching, inhaling, or consuming soil: wear gloves while gardening, wear a respirator mask if conditions are dry and dusty, and thoroughly wash your harvest before eating. After all, we've all gotta eat and there are considerable health risks associated with not eating your vegetables.
Ok, I'm done talking about scary stuff. Let's discuss the results of my new soil test. Just as last time, my soil's pH was neutral and the levels of phosphorus, calcium, and magnesium were excessive. The potassium level was considered optimal. Nitrogen was not tested for; the explanation given by the lab was that current tests are unreliable. Besides, I'm confident that my use of compost provides all the nitrogen that my garden needs, as every compost recipe instructs you to use green materials like fresh plant clippings for nitrogen (along with brown materials like dried fallen leaves for carbon).
Using compost has resulted in high organic matter content of my garden's soil. Cornell University reports that mid-Atlantic soils naturally contain about 4% organic matter on average. My new soil test reported that my garden's soil exceeds 12% organic matter. Soils high in organic matter also have high cation exchange capacities; this was also confirmed by the soil test result. Thus the continued abundant fertility of my garden as well as its increased capacity for water absorption.
Maybe next time, I will wait only 3 years to retest. ;-)