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Often readers write in with general questions about garden soil. Readers may comment that they have “wonderful” soil because it is “dark”,  but nothing will grow in it. Or they may complain that it is so dark it gets too hot for plants. Others will talk about “fixing” their earth for a specific planting by adding in a bag of potting soil to the spot where the new plantings were to go only to find out later that this technique didn’t quite work. Others won’t have much to say about their soil but will ask what amendments to add to their garden soil so their plants will do better. So, what’s the real dirt on garden soil and what can gardeners do to create healthy rooting zones for their plants?

Cabbages Planted in March into Tested, Amended Soil

Well, first off, you can’t tell what’s really going on with your soil simply by looking at it. Just because it appears dark doesn’t mean it is rich and nutritious. As well, just because it appears moist, it may not be. And, amending soil only in the place a plant will be placed will not make for a good long term solution.

The best way to know what’s going on with your soil is to have it tested. Yes, you can purchase a small home soil kit to determine your soil’s pH (how acidic or alkaline your soil is) and what your basic nutrient values are (NPK). However, if you take a sample and send it to a soil lab you will be able to determine much more information, and many labs will give you solutions for adjusting your soil based on what is currently in the soil as well as what you plan to grow in the space. If you do your test at home, you may determine that you have excessive Nitrogen (N) and sufficient Potassium (K), but did you know that Nitrogen in excess can suppress your Potassium? A lab test will look at your true levels and consider these aspects of soil science as well as just what’s in there.

Additionally, soil tests can provide you with details on material percentages in your soil. Not only can you determine how much mineral v. organic material is in the soil, but also which mineral material (sand, silt, clay) is in the soil and in what percentage. Knowing this detail will help you understand how nutrients are held or lost in the soil (and the same for water).

Some labs will also provide details on heavy metal soil levels. Remember, lead and arsenic can exist in the soil. Even if you aren’t growing food crops, which can take up these elements and put them in your zucchini, you will still be exposed to whatever levels exist in your garden — touching them each time you weed or play on the lawn with your kids and dog and tracking them into your home each time you stroll in from the garden path.

Same Cabbages Ready for Harvest in July

And, some labs will also provide details on your soil microbial levels. Healthy soil is inhabited by a balance of insects, worms, bacteria, fungi and more that live together in harmony. When their balance is off, so too is soil and plant health “off”. For instance, in the presence of high phosphorous (P) mycorrhizal fungi may be suppressed. These fungi are critical to plant root health and function.

So, to get the real dirt on your soil? Have it tested.

And, a brief note on those who would prepare soil by digging a hole, adding a bag of potting soil and then planting. Please don’t. Essentially, this method can result in creating a pot or container-like soil environment within the existing soil. Your potting soil may likely hold too much water, which has a distaste for passing between layers of dissimilar material. If this happens, your new plants roots may become water logged. Or, in other situations, the roots may hesitate to pass out of the nutrient-high potting soil environment into the surrounding garden soil. This can cause circling, kinked and girdling roots — none of which lead to a healthy plant. Better to test your overall soil, adjust as recommended and plant into a soil that is blended throughout.

And, just a reminder to test your soil by digging in before watering. Just because soil looks moist or dry doesn’t mean it really is. Watering by the calender or by visual clues isn’t enough. Dig in to test for moisture needs and avoid wasting water by over-watering or stressing plant roots by under-watering.

Now to find the right soil lab for your needs. Here are a few to consider (and note that each of these offers other types of testing such as crop nutrient detailing and tissue sampling):

• Umass Soil Testing: Amherst offers several different tests including a basic soil nutrient and composition test for about $9-$13 and a separate texture (% of sand, silt, clay) test for around $50.
• Woods End Research Lab: Woods End offers a soil test for microbial content levels.
• Midwest Labs: Offers a huge array of testing options from simple nutrient details to nematode levels to nitrate concentrations to much, much more.

Certainly, these aren’t the only labs out there. If you’re running a lab or working with one I haven’t mentioned here, please add it via the comments below to share with everyone. Remember folks, to know if your soil is hospitable for your plants takes more than just a quick look. Knowing whether to add an amendment takes more than just understanding your crop’s requirement or looking at a calender — it takes knowing what you have at the get-go.

Mulching, crop rotation, and watering can go a long way toward maintaining a healthy soil environment, but having a baseline to start can make all the difference. And, despite our best efforts to manage our plants and soil, sometimes even veteran gardeners find themselves filling up a bag of soil from a troublesome bed to send into a lab for on-going evaluation. Soil is alive (or it should be filled with life), which means it is ever-changing and requires a check-up now and again.

Tags: bacteria, cabbage, clay, dirt, Fungi, garden coach, mineral, mycorrhizae, npk, nutrient capacity, organic material, pH, plant testing, potting soil, roots, sand, silt, soil, soil interface, soil lab, soil microbia, soil structure, soil test, soil texture, Watering

This entry was posted on Tuesday, September 1st, 2009 at 9:14 am and is filed under soil. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.