With the right amendments, you can transform nutritionally poor soil into a rich garden bed. Soil that is low in fertility is usually either too sandy to hold nutrients or so dense with clay that plants can't access its nutrients. Sandy soil is made up of large particles. Water drains through those particles fast, taking nutrients with it. Clay soil is high in nutrients, but the excess water that gets trapped in the soil makes the nutrients difficult for plants to access. Loamy soil is made up of sand, clay and organic matter. Although loamy soil usually is already fertile, adding amendments to improve even it helps give new plantings a good start.
Time of Year
In U.S. Department of Agriculture plant hardiness zone 7 and lower, amend soil before planting in spring. Spring planting starts after the location's last average annual frost date. You can also amend the soil before summer and fall planting. Planting lasts from summer through fall up to one month before the area's first average annual frost date. In USDA zone 8 and higher, you can plant and amend soil any time of the year.
Soil texture is important to consider before adding amendments or working the soil. The soil should be crumbly and slightly moist when you add amendments. Sticky, muddy soil that cakes on shovels and other work tools is too wet to work. Wait until it dries slightly before adding amendments.
Improving fertility, while increasing drainage, in a bed full of clay soil requires creating space between the soil's dense particles. You can achieve that by incorporating bulky organic materials with the soil. Materials that work well are leaf mold, shredded leaves, decomposed straw and decomposed wood chips or bark. Also, adding sand helps clay soil drain well.
Add the materials to the soil one full season before planting so that they have time to break down. If, for example, you intend to plant in spring, then add the amendments in fall; likewise, get the bed ready in spring for a fall planting. Spread a 3- to 4-inch-thick layer of bulky organic materials evenly across the bed, and then till those materials into the soil to a depth of 12 inches. Add a 3- to 4-inch-thick layer of clean sand on top of the soil, and blend it with the soil to a depth of 12 inches.
The main challenge with sandy soil is its overly fast drainage, which leads to poor fertility. Use a fine amendment that will slow drainage and help add and retain nutrients. One amendment to start with sphagnum peat moss. Spread the sphagnum peat moss 1 inch deep over the bed, and then work it into the top 4 inches of soil. Both plant-based compost and seasoned manure also amend sandy soil well. Use a 2- to 3-inch-thick layer of compost. If you use seasoned manure, then add a 1-inch-thick layer of it per year. More manure than that amount can increase the soil's salt to a level that damages plants. Incorporate compost or seasoned manure into the top 6 to 8 inches of the soil. You can add amendments to sandy soil right before planting.
Loam contains a blend of small and large particles mixed with organic matter. Use seasoned compost to add nutrients to loamy soil. Spread a 1- to 2-inch thick layer of seasoned compost evenly across the entire planting area, and then work it into the top 6 to 8 inches of the soil. The compost helps improve soil nutrients, structure and drainage. Compost can be added to loam right before planting, a few weeks before planting or even the season before planting. Ensure the soil is moist and crumbly when you add the compost. Working loam soil when it is wet compacts it and damages its structure.
Add a balanced fertilizer to the bed before planting. A balanced, all-purpose fertilizer has equal amounts of nitrogen, phosphorous and potassium, three nutrients that are critical to strong plant growth. Look for a granular, dry fertilizer that has a 10-10-10 nutrient ratio listed on its package. Sprinkle the fertilizer over the soil, and blend it into the soil to a depth of 3 to 6 inches. Use 1 1/2 cups of the fertilizer for each 50 square-foot bed area. A balanced fertilizer helps prepare all soil types for planting annuals, perennials, shrubs, vegetables and hedges.