Start with the Soil

The most important thing you can do to ensure the success of your herb garden is to care for the soil. Soil provides nutrition, water, and structural support for plants.

Most herbs prefer loose, well-drained soil that contains some organic matter, is moderately fertile, and has a pH of 6.5 to 7.0 — although some herbs have other requirements. (See “Best Herbs for Special Sites.”)

If your soil hasn’t been tested recently, contact the Cooperative Extension Service to obtain a soil testing kit. A soil test conducted by a lab will give you a detailed analysis of your soil’s pH, organic matter, and nutrient content.

Soil pH can range from 4.0 or less (acidic) to 9.0 and above (alkaline); neutral soil pH is 7.0. Soil pH affects the availability of nutrients within your soil. The nutrient nitrogen, for example, is readily available in soil with a pH above 5.5. Below a pH of 6.0, the availability of phosphorous, potassium, calcium, and magnesium decreases, and the availability of the metallic micronutrients zinc, manganese, copper, and iron increases — a condition that can be harmful to plants. A very high pH, on the other hand, can result in deficiencies of these micronutrients, as well as of phosphorus. Soil microorganisms are also affected by pH; a pH of 6.6 to 7.3 is favorable for microbial activities that contribute to the availability of nitrogen, sulfur, and phosphorus in soils.

Soil pH can be raised by adding ground agricultural limestone, sold as either calcitic or dolomitic limestone — natural materials available from most garden supply stores. Calcitic limestone supplies mostly calcium; dolomitic limestone also provides magnesium. Apply limestone at the rate suggested by your soil test. Because limestone does not dissolve easily in water, it’s important to mix it thoroughly into the top 8 to 10 inches of soil.

If a soil test shows your soil to be very alkaline (a pH of 8.0 or above), add elemental sulfur to a depth of 6 inches to lower the pH. Because soil bacteria are needed to help lower pH, the soil should be moist, well aerated, and warm. Elemental sulfur works fast, but be careful not to add too much. It will work over a period of several weeks to months.

Compost is also an excellent buffer for soil pH, bringing both acid and alkaline soils closer to neutral over time, while also adding organic matter, beneficial microbes, and trace minerals.

Organic Fertilizers Made Easy

Plant nutrients such as nitrogen, phosphorus, and potassium are essential to plant growth, root development, and disease resistance. If a soil test indicates a deficiency of a key nutrient, such as phosphorus or potassium, add an organic fertilizer for peak performance. One of the main advantages of organic fertilizers is that they release their nutrients slowly, providing more nutrients over time. Chemical fertilizers are highly soluble and wash through your soil quickly. If your plants don’t use them, they can end up as pollutants in groundwater, and they can be harmful to earthworms and other beneficial soil organisms.

Here’s a quick overview of essential plant nutrients and the organic fertilizers that provide them.

Nitrogen. Promotes leafy growth, but too much of it can inhibit flowering. Plants respond quickly to this nutrient, so use it sparingly. Manure, fish meal, and bloodmeal are organic sources of nitrogen.

Phosphorus. Helps plants develop strong root systems and promotes flowering. It does not move easily in the soil, so dig it into the soil where plant roots can reach it. Organic sources include bonemeal and rock phosphate.

Potassium. Helps plants withstand disease, drought, and temperature extremes. It also helps with fruiting and seed formation. Greensand and sulfate of potash-magnesia (a mined mineral) supply potassium.

How much fertilizer should you use? Soil test reports usually advise adding a specific number of pounds of a nutrient for every 1,000 square feet of garden soil. The label on the fertilizer will tell you the percentage of nutrients it contains. Many high-quality organic fertilizer blends are available. Apply fertilizer in small amounts when the soil is moist, and follow up with a light watering.

The Beauty of Compost

Compost is made from plant matter that decomposes into a porous, spongy substance called humus. Indispensable to your garden, compost benefits soil and plants in so many ways.

  • Helps soil retain air and moisture around plant roots, offering protection during periods of drought
  • Allows water to circulate freely around plant roots, decreasing the chance of root rot
  • Buffers pH and nutrient imbalances
  • Protects plants from disease
  • Releases a slow, steady supply of nutrients
  • Darkens soil so that it warms earlier, extending your growing season
  • Supports beneficial bacteria, which break down organic matter and make nutrients more available to plants
  • Supports beneficial insects that help control plant pests and worms that burrow through soil to keep it well aerated.

Recycling yard and kitchen waste as compost creates fertile growing material for herbs and other plants

Recycling yard and kitchen waste as compost creates fertile growing material for herbs and other plants.

The Compost Advantage

There are several ways to change and improve soil quality. The very best thing you can do to improve soil structure is to add compost. Like the spongy, dark humus that covers the forest floor, compost is black gold: It is nature’s perfect recycling method for transforming organic waste, such as leaves and decaying bark, into an almost magical substance for soil and plants. Compost benefits all kinds of soils — wet, dry, sandy, heavy clay, low pH, high pH, depleted, and imbalanced.

Basil and many other herbs thrive in the loose, well-drained soil of a raised bed

Basil and many other herbs thrive in the loose, well-drained soil of a raised bed.

Compost can be made from yard waste, such as grass clippings, pine needles, wood chips, and leaves, as well as from straw, manure, and kitchen waste. Coffee grounds, fruit and vegetable peels, and eggshells are all excellent for composting, but avoid using meat and dairy products, which can attract insects and animals.

To make a simple compost pile, combine high-nitrogen materials (sometimes called “greens”), such as grass clippings, kitchen scraps, and manure, with carbon materials (sometimes called “browns”), such as dry leaves, wood chips, and sawdust. For best decomposition, the pile should be a minimum of 27 cubic feet (3 feet tall by 3 feet wide by 3 feet deep). Keep the materials damp but not saturated, and turn the mixture periodically with a shovel or pitchfork to admit air into the center of the pile. The more often you turn your compost, the faster it will break down. Mature compost is dark brown and sweet smelling — like humus.

Add finished compost to garden beds and containers before planting, or spread it on top of your soil as mulch.

Boosting Results with Raised Beds

Raised beds provide ideal growing conditions for many herbs. In raised beds, soil not only drains more freely, but it also thaws and warms faster — allowing you to plant earlier in the season. The soil in raised beds also tends to remain looser and more friable because it is never walked upon.

A raised bed can be as simple as a mound of soil in any shape. But many gardeners prefer enclosed, rectangular raised beds — basically a bottomless box filled with soil. Garden suppliers sell a variety of premade enclosures for raised beds, or you can easily construct your own from wooden boards, stones, concrete blocks, or bricks.

Don’t use pressure-treated lumber, which could contain heavy metals that can leach from the wood. Chromated copper arsenate, creosote, and pentachlorophenol — chemicals used as timber preservatives — are not only toxic to insects and fungi that attack wood, but also to people, pets, and garden plants. And as attractive and “green” as recycled wood might sound, stay away from it because you won’t be able to tell whether it’s pressure treated or not. Cedar is a good choice for constructing raised beds because it is naturally resistant to rot and insect damage.

Over time, the soil can push apart the sides of wood-framed beds if the corners are not securely fastened. Building your beds with prefabricated metal corners, available from many garden supply companies, can solve this problem.