Homestead's Education Coordinator Gene Sumi gave another great talk recently - about what's arguably THE most important subject for any gardener, especially those who like to garden organically. In a nutshell, if there's something wrong with your plants, the cause is probably a problem with your soil.
Traditionally, people used to check the pH of their soil by simply tasting it - sweet soil indicates a high pH and sour indicates low pH or acidic soil. And to test texture, they would simply squeeze soil into a ball and drop it on a hard surface. If it shatters completely it's too sandy and if it doesn't break up at all it's too clayey.
These days there are extremely good tests from actual laboratories available at a very reasonable cost, and Gene recommends testing at least three areas separately - the veg garden, the lawn, and the shade garden (where mostly acid-loving plants grow). The University of Maryland no longer does the tests themselves but provides this list of recommended labs. I used the University of Massachusetts and was very pleased with the analysis, and how quickly it arrived - via email just three days after I mailed the sample.
Azaleas are a bellweather plant in that they react quickly and dramatically if the soil isn't acidic enough for them - by turning yellow. To make the soil more acidic Gene suggests adding soil sulphur, which is organic and won't burn the plants. Chemical acidifiers work faster but are caustic and can burn plants. Acid-loving plants include azaleas, camelias, hemlock, holly, and hydrangea.
To raise pH, Gene recommends adding limestone, another organic material, and the Espoma brand's Garden Lime is a product he especially likes.
Here's an overview of soil texture but in a nutshell, texture determines the soil's ability to retain and drain water. A huge problem for many gardeners is the common construction practice of removing all topsoil and leaving the poor homeowner with subsoil, which cannot support plants. There's no way to create real topsoil, which takes nature eons to make, but we create a functioning substitute for it by amending the soil's texture. For lawns and other groundcovers, only 4-6" of topsoil is needed; for trees and shrubs, about a foot is required.
For soil that's too clayey, the best solution is to add gypsum, a surface-mined product that's pH-neutral. It permanently loosens the clay and is cheap. Gypsum also removes salt from soil, which is why when the Dutch were reclaiming land from the sea they used large quantities of the stuff - it effectively turned super-salty soil into soil that could support agriculture. (See the Gypsum Institute's online resources on the subject.)
Gene especially likes the Soil Perfector product by Espoma. It's a crushed slate that resembles puffed rice and permanently corrects soil structure.
This is without a doubt THE most important soil amendment, despite the fact that good soil (loam) only contains 5% organic matter. There are lots of sources of organic matter, and Gene gave this overview of the most common ones:
- Composted manure and peatmoss are both in diminishing supply these days, so are a "boutique item" - that means expensive. He also noted that peatmoss is rarely recommended anymore because it's nonrenewable.
- The best organic material - bar none! - is available in bags or bulk for a very reasonable price because it's SO local and renewable. That's the leaf compost produced by Montgomery County - Leafgro. It's simply decomposed leaves, and Gene thinks it's "the perfect, perfect thing to put in soil". Leaf compost has only become commercially available in the last decade or so, thanks to Dutch engineers who came up with a way to create it efficiently in great quantities.
- Montgomery County also offers Compro, a leaf compost product that's specially made for lawns, with the addition of some lime and Milorganite to increase fertility.
- Virginia Pine Bark and Fines is another local product - made from the native tree Pinus virginiana. It's great as a soil amendment that will break down over the season, or on top as an attractive mulch, too.
Gene was asked if we need to do anything to the soil regularly, like adding fertilizer. Assuming you have actual topsoil, not post-construction subsoil, he recommends a simple yearly application of an organic mulch, but no actual fertilizers unless you see a problem. (The exceptions are vegetable gardens and most annuals, which DO require regular feeding.)
And check out the University of Maryland's handout "Fertilizing Responsibly for a Healthy Chesapeake Bay." It explains that fertilizer run-off contributes to nutrient pollution of the Chesapeake Bay and calls on Marylanders with lawns to "drastically reduce or eliminate phosphorous fertilization if their soil test results show phosphorous (phosphate) levels that are adequate, optimum or excessive. "