Considerations for Growing Vegetables in Urban Soils

By Cyndy Crist

soil profile at HeathGlen

soil profile at HeathGlen

It’s easy to take soil for granted.  I mean, it’s just there, right?  And dirt is dirt, isn’t it?  Many of us dug in it as kids, delighting in making mud pies and otherwise mucking about to our mothers’ distress (for my generation, it was always our mothers who were responsible for washing us up and keeping our clothes clean).

But when we start gardening, if we’re wise, we start paying more attention to soil, because it’s not all the same and the differences really matter.  The composition of our soil; it’s capacity to hold enough water to support plant life while draining enough water to keep them from rotting; the nutrients it holds or lacks; and its acidity are all important if we want to successfully grow what we choose to plant.

Many urban gardeners may have additional challenges.  It’s more likely that our soil has been compromised by things like pealing lead paint and automobile exhaust; there may have been multiple construction projects over time on or near the spaces in which we grow that compacted and contaminated the soil; and our boulevards and front yards may be more affected by salts and chemicals thrown up by snow plows and construction vehicles.  In addition, we may have neighbors who employ lawn services that leave those warning flags behind, indicating the application of chemicals that may leach into our soil or drift over onto our plants, lawns, and trees on otherwise friendly breezes.

Although a deep  understanding of soil requires more scientific and mathematical skills and knowledge than many of us have, there are some pretty simple things we can do to get, if you’ll pardon the pun, the dirt on our soil.  Here are a few things to get you started.

Soil Composition.  Good soil should be roughly composed of 50% solids, 25% air, and 25% water. Ideally, the solids will primarily be silt or loam containing good organic matter, but soil can be heavy in sand or clay.  Sand poses the challenge of holding few nutrients and little water.  Clay, by contrast, is particularly challenging for drainage and compaction, both of which reduce water drainage by closing the pockets of air needed by plants to grow and thrive.

One simple assessment of your soil’s composition is a quick visual test.  Dig down 3-4 inches and take a good look at the color of the soil (and do this quickly before the sun and air dry it out).  If it has a rich, dark color, it’s likely a good loamy or silty soil, though it may also contain clay.  If it’s light in color, it’s more likely to contain significant amounts of sand and be nutrient-poor.

moistness level of soil

moistness level of soil

Next, you can tell something about the soil’s composition and it’s ability to hold or drain water by performing a simple “ball” test.  Dig up some soil from your garden space and wet it down(not enough to make a mud pie, however – you don’t want it to be soupy, just moistened).  Grab a handful and press it firmly into a ball, then release your fingers.  If it immediately and completely falls apart, it’s likely to be heavy with sand.  If it remains firmly compacted into a ball, it’s probably heavily clay-based.  But if it forms a loose ball that is easily broken up, you’ve probably got good loam in which to plant your garden.

Soil Nutrients.  To know more about the composition of your soil and particularly what nutrients it may be lacking, there’s no substitute for a soil test.  A likely source to turn to for a soil test is your local extension service.  In Minnesota, the University of Minnesota Extension Service will complete soil tests for $15 with results usually sent within 2-3 weeks.  The report completed will include recommendations about how much NPK (nitrogen, phosphorous, and potassium, the three key nutrients needed by most plants) your soil needs, what it’s pH is (a measure of how acidic your soil is), and general information about its composition (a measure of organic matter and an estimate of the afore-mentioned sand, clay, and silt make-up).

Where to send soil samples

Where to send soil samples

It’s easy to prepare for a soil test.  Simply dig up 3-5 trowels-full of soil from different parts of your garden (representative of the total space in which you’re planting to plant) and mix them together in a ziplock bag or other bag that can be sealed easily and mailed safely.  If you’re concerned about lead in the soil, you may need to have a separate test conducted with a separate sample of soil.  Otherwise, a single test is all you’ll need.

Soil Contaminants.  Lead is a particular concern in neighborhoods with houses or other buildings that were painted at a time when paint routinely included this substance now known to cause major health problems if ingested and/or those located where traffic is consistently at high levels.

But there’s some good news here for gardeners.  First, research has shown that the concentration of lead needs to be pretty high in order for it to be taken up into edible plants to an extent that can compromise health.  Lead is commonly found in soil at levels of approximately 10 parts per million (ppm), and research has shown that edibles grown in soil are likely safe if the levels of lead fall below 300 ppm.

wash radishes & root veggies well

wash radishes & root veggies well

Second, since plants don’t usually absorb or accumulate lead, its presence in soil will primarily be a concern only in leafy greens and root vegetables, and with the latter, any lead will likely be on the surface only and therefore easily washed away. If you’re growing things like tomatoes, peppers, and strawberries, lead is likely not something about which you need to be concerned.

If you are concerned about levels of lead in your soil, there are a few things you can do to achieve improvements.  One is to raise the pH to at least 6.5; one easy way to do that is to add lime.  In addition, adding organic matter can reduce lead levels in your soil.  If you think your soil may contain more than “normal” levels of lead, by all means have a soil test done.  But rest assured that except in extreme circumstances, you likely have little to worry about which to be concerned.

Soil Amendments. Once you have all of the information garnered from the tests and procedures above, what do you do?  Again, it’s pretty simple.  If your soil is sandy, you’ll need to work in compost, well-rotted manure, leaf mold, or composted bark, all of which will improve the soil’s capacity to retain water and nutrients.  Appropriate amendments for clay soil include coarse sand (but be careful here, as fine sand can end up creating something more like concrete, not what you need to improve drainage) or lime.  Perlite is another option for either sandy or clay soil. Of course, you can also replace the top 4-6 inches or so of your garden soil with purchased top soil; this is a quick, but expensive, solution.

compost and fertilizers

compost and fertilizers

As for nutrients, a soil test will tell you what you’re lacking and give you figures about how much you’ll need to work in.  Commercial fertilizers will always provide a NPK ratio that you can use to calculate amounts.  Your extension service website will provide the ratios and other information you’ll need to determine appropriate fertilizers and applications.  This isn’t rocket science but does require a little basic calculation so that you’re not applying too much of a nutrient to the detriment of plant growth, wasting your money on unnecessary fertilizer, or contributing to water pollution by applying nutrients that can run off and cause environmental problems (for example, in Minnesota, there are laws restricting the application of nitrogen because of the problems it can cause when it enters the water supply).

* note on fertilizers and soil testing:  Phosphorous is not available to the plants until the soil warms up so be careful not to overload your soil with phosphorous if it doesn’t express itself early on.  It is not something we need to have appearing in our streams and water supply from the runoff.

You’ll want to work your amendments into the top 4-6 inches of soil before planting.  Later, you may be able to simply “top dress” your soil with compost if you have worked the soil well and kept it from becoming compacted. With this kind of friable soil (think crumbly like good chocolate cake), the new layers you add will work their way in over time with rain and ongoing cultivation to remove weeds.

Good Resources.  This is a big subject, and I’ve just scratched the surface here, so I encourage you to learn a little more about soil as you proceed with your garden. There are lots of sources you can turn to for more information about soil and its composition, preparation, cultivation, and amendments.  As mentioned previously, extension services in your state and across the country are terrific sources of free, research-based information.  Agriculture and natural resources departments, state and federal, are also great sources of information for the gardening public.  A growing array of websites and computer applications also provide all kinds of readily accessible information at little or no cost.  One example of a site with good overall gardening information can be found at www.gardenguides.com

Do a little judicious digging, literally and figuratively, and you’ll know what to do to improve your soil so that it will provide a good home in which to grow your garden.  Improving your soil isn’t something that will be complete overnight, but you’ll find it’s well worth the time and effort. So, get digging!

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