How to Improve Your Garden Soil
Use any of these DIY soil tests to improve your garden soil. These tests will tell whether you have good soil, or if you have to make improvements.
When and How to Fertilize Plants
May is a month when many gardeners perform their annual fertilization of the entire landscape because, well, that’s just what you’re supposed to do and when you’re supposed to do it.
Ask a horticulturist, though, and the practice of fertilizing becomes a lot less straightforward – and maybe not needed at all. Like so much in gardening, fertilizing is a "depends" situation.
The rift boils down to a difference between the nutrients that plants need vs. what gardeners think they need.
Many gardeners have been taught to believe that all plants need regular doses of supplemental fertilizer to maximize growth and keep plants healthy. In other words, they need to be "fed" like botanical pets.
The soil-science truth is that plants grow well when they have access to the mix of needed nutrients in the amounts they need them. Since soils often already have adequate amounts of those nutrients, adding more is potentially wasteful, potentially polluting, and maybe even detrimental to plant performance.
Here’s a graphic example in a lawn of what excess fertilizer can do (in this case, a spill) to plants. Photo by George Weigel
The trouble with too much
One way excess nutrients can harm plant growth is by encouraging lush, succulent growth that’s more prone to wind damage and diseases such as powdery mildew.
Both manures and most synthetic fertilizers also contain salts that can build up in the soil through overuse, leading to drought-like symptoms and stunted growth.
And a third potential problem is that unneeded fertilizers can throw off the soil’s delicate balance of the 17 most important elements used to fuel plant growth.
Examples: Too much phosphorus in the soil can block plants from taking up enough zinc. Too much magnesium makes it harder for plants to use calcium. And if gardeners are overusing fertilizers that also acidify the soil, overly acidy conditions can raise available iron, aluminum, and manganese to plant-toxic levels.
The bottom line is that plants need enough nutrients, not more than enough. More of even a good thing is not better.
Here’s a graphic example in a lawn of what excess fertilizer can do (in this case, a spill) to plants. Photo by George Weigel
What do plants need?
Of the 17 elements plants need to grow, three of them are needed in relatively large amounts. These "macronutrients" are nitrogen, phosphorus, and potassium, and their amounts are required by law to be listed on fertilizer packages. (These are the three-digit “N, P, K” numbers you’ll see on bags, such as 20-10-10 or 5-10-15.)
Three other "secondary" elements are needed in moderate amounts (calcium, magnesium, and sulfur), and eight more are needed in "micronutrient" amounts: boron, chlorine, copper, iron, manganese, molybdenum, nickel, zinc.
The remaining three elements are carbon (delivered from air), hydrogen (in water), and oxygen (in air and water).
Different plants need different blends of that lineup, and different soils have varying levels. Even more complex, the nutrients available can change over time based on soil temperature, rainfall, and changing microbe populations – in addition to what gardeners are doing (tilling, fertilizing, applying pesticides, etc.)
In an effort to tailor fertilizers to the many different scenarios, companies make a dizzying array of options. Organic gardeners also often apply regular amounts of natural fertilizers such as manures and composts.
A soil test is the most accurate way to tell if your plants need fertilizer, and if so, what kind and how much. Photo by George Weigel
How to tell what your plants need
One good clue to whether particular plants need fertilizer or not comes from observing the plants. If they’re growing well and not displaying any signs of trouble, they’re getting the nutrition they need.
On the other hand, stunted growth, leaf discoloration, and similar trouble signs might indicate that a plant is deficient in one or more nutrients. But even then, those trouble signs could be the result of plant disease, compacted soil, or even cultural issues such as excess mulch or too-deep planting.
The most accurate way to tell if a garden needs fertilizer is to do a soil test. Otherwise, you’re just guessing.
DIY tests can be done by digging and drying a small bag of soil from the root zone throughout a garden, then sending it off to a test lab for analysis.
Most state land-grant universities have such labs and do tests inexpensively, typically $10 to $20. The results tell you what levels of the main nutrients your soil needs, and more importantly, give recommendations on the exact kind and amount of fertilizer needed – if any.
The tests also gear the recommendations to the kinds of plants you’re growing.
A landscape of trees, shrubs, grasses, and perennials, for example, may not need any supplemental fertilizer, whereas a vegetable garden or flower pots may need two or more fertilizer applications during the growing season.
The University of Maryland Extension suggests soil tests every two to three years – or whenever a plot is showing signs of potential nutrient deficiency.
“A soil test takes the guesswork out of fertilization and is extremely cost effective,” says Maryland Extension. “It not only eliminates the expense of unnecessary fertilizers but also eliminates overuse of fertilizers and helps to protect the environment.”
