Soil & Fertilizer Basics

“Feed the Soil” is the rallying cry of the organic gardening movement and it’s a big change from the Miracle-in-a-Box school of gardening, which is all about putting plants on performance-enhancing drugs. Sustainable gardeners are always going on about the benefits of adding organic matter to their soil because it creates the perfect growing medium for producing vigorous plants that can best resist disease, pests and drought. It’s not just about drenching the soil with nutrients; it’s about feeding slowly, creating good drainage, and encouraging earthworms and microorganisms. And when plants need something more than soil amendments, we choose slow-acting fertilizers because they don’t kill our beloved soil critters or turn our plants into drug addicts.

What is soil, anyway? It’s made up of particles of:

• Sand, which drains fast, usually too fast, leaching too many nutrients in the process. However, it’s sure easy to dig in.
• Clay, which does a great job of retaining moisture, sometimes too great. It can be red, brown or gray.
• Silt, which combines qualities of both sand and clay.

Loam, the ideal soil, is a mixture of sand, clay and silt and it’s the growing medium we’re told that most plants require. It drains well but still retains moisture and nutrients, and it’s easy to dig in.

About 5 percent of soil volume is or was living matter, meaning roots and other plant parts, fungi, bacteria, algae, viruses, insects, earthworms, nematodes, protozoa, and so on. (The rest is air, water and rocks.) Those billions of microflora and fauna in the soil improve drainage and aeration. Unfortunately, this whole soil-food web is easily damaged by rototilling of the soil, or by walking on it when it’s wet.

Organic Matter — the Great Cure-All

To quote gardening guru Paul James: “Adding organic matter is the single most important and effective thing gardeners can do to improve garden soil.” That’s because it does all these miraculous things:

• Correct for excess clay OR sand.
• Correct drainage that’s too fast OR too slow. Amazing!
• Correct for the compaction after construction or after being walked on while wet.
• Moderate and stabilize pH extremes of pH (soil acidity or alkalinity).
• Improve the soil’s ability to retain nutrients.

Yearly Mulching of the Gardening

At least yearly, add organic matter as a mulch (1 inch of finely chopped or 3 inches of bulky). If you’re starting with really lousy soil, do this twice a year for the first year, at least. (This is in addition to adding some every time you plant.) This is THE single most important maintenance that you can perform for your garden I know it’s a big job but it happens only once a year and without it, your garden probably isn’t sustainable. And here’s a bright note: after mulching the first 2-3 years, the shady parts of my garden only need mulching every other year. Yaaaay!

When? I mulch in the spring, but I know that northern gardeners often prefer mulching in the fall because it can help prevent heaving of the soil over the winter.

Visit Mulch and Mulching on this website.

Adding Organic Matter Every Time You Dig

Every time you dig to plant something, add some organic matter to the soil (try one part organic matter to two parts soil).

• For my average soil I add compost or leafmold when I plant, then spread leafmold around the plant as a mulch.
• If your soil is sandy, add fully composted or partially composted (decayed) organic matter, plus some that decay slowly, like bark or pine needs.
• Clayey soil benefits from the addition of bulky organic stuff, like leafmold mulch, shredded pine bark, straw and cover crops.

For a Boost, add a Dressing of Compost

Whenever I notice signs of nutrient deficiency (like small or discolored leaves, stunted growth, few or no flowers), the following spring I’ll add an inch of compost around the root zone of the plant. But I notice that some experts advise such top-dressing every spring throughout the whole garden, followed in late spring by the application of 1-2″ of organic mulch on top of the compost. Then in fall they say to “fluff up” the old mulch and (especially in cold climates) even add more. Well, if all that doesn’t promote good plant performance for you, you just may be trying to grow the wrong plants. (Unless you’re killing them by not watering them adequately — the numero uno method of plant murder by homeowners and even experienced gardeners alike.)

Actual “Fertilizers”

The major nutrients needed by plants but not available in most soils are:

• Nitrogen (N) for growth of dark green stems and leaves
• Phosphorus (P) promotes rapid growth and seedling growth, stronger stems and roots, and flowers.
• Potassium (K) or potash is good for overall plant vigor, flowers, and resistance to disease and drought and extremes of temperature.

While there are special fertilizers on the market for flowers, vegetables, shrubs, etc, most gardeners use the all-purpose fertilizer for all their plants. A happy side effect of this practice is fewer bags to buy and store.

Plants that Need Them Regularly

Roses, especially the hybrid teas, are hungry, or “heavy feeders,” meaning they just won’t produce flowers without regular fertilizer application. That said, my old Meidiland shrub roses bloom like crazy with absolutely no feeding, so consider using easy-care shrub roses instead of the hungry, high-maintenance hybrid teas.

• The first feeding should be two weeks after their new leaves appear in early spring, about 1/2 cup of 5-10-5 or 10-10-10 around the base of each plant (out to 12-18″), worked into the soil lightly and watered well.
• Continue feeding every month or 6 weeks until late summer.

Annuals need regular feedings — every week or two — around the root zone, not directly onto the stem.

Plants in pots (including houseplants) need regular feeding, with every watering or every other watering.

Even hungry plants like roses can be damaged by overfeeding, which can produce too much foliage and not enough blooms, and even kill the plant altogether. So get out your magnifying glasses and read those instructions.

Fertilizer versus Compost

NEW! According to the winter 2008 issue of Master Gardener Magazine, organic fertilizers are 4-6 percent nitrogen and will release most of that to the soil and plants over one growing season. BUT, composted organic materials (from manure or yard waste, for example) “decompose much more slowly, releasing only about 5 percent of their total nitrogen during the growing season. Use the nitrogen-rich materials as a source of nutrients for garden plants, and use composts as a soil amendment to increase organic matter.”

Visit the Compost home page on this website.

Fast v. Slow is More Important than Organic v. Synthetic

While it’s true that eco-savvy gardeners usually choose organic products over synthetic ones, in the case of fertilizers it’s far more important that the product be slow-release than what they’re made of. Plants respond the same to nutrients from either type of source, but here’s what fast-acting fertilizers do:

• Produce excessive growth that make the plant more vulnerable to insects and disease.
• Disrupt earthworm and microbial life in the soil.
• Pollute our waterways, big-time.

There are a few plants that benefit from liquid, fast-acting fertilizers — tomatoes, annuals, and houseplants — but even then it’s recommended that they be sprayed on the leaves, not dumped into the soil, or used only at 1/2 to 1/4 the recommended strength, every two weeks. And always water well before using any liquid fertilizers.