Garden soil fertility determines the ability of the soil to grow healthy plants. A soil that lacks essential nutrients will produce weak, disease-ridden or small plants. Improving a garden soil requires adding organic matter and nutrients to replace what is used by the plants growing in the soil.
A measurement of the acidity or alkalinity of the soil is called a pH test. Soil pH, according to Clemson University, affects what nutrients are available in the soil. Plants require a supply of nutrients to stay healthy. Soil is alkaline when the pH test indicates a reading over 7, while acidic soil is under 7. Slightly acidic or slightly alkaline soil is acceptable, depending on what plants are being grown in the soil.
Increasing Soil pH
The addition of lime to garden soil raises its pH. There are four kinds of lime products: pulverized, granular, pelletized and hydrated. Pulverized is lime supplied as a fine powder. Granular and pelletized lime are slightly larger and are less likely to clog a fertilizer spreader during application, says Clemson University. Application of lime is determined according to a pH test's results and is best applied during the fall or winter.
Decreasing Soil pH
For plants that require a slightly acidic soil, adding ammonium sulfate or sulfur to garden soil will lower the pH. Both are available at gardening centers. Aluminum sulfate changes the pH of the soil as soon as it is applied, while sulfur products take time to work as the sulfur needs to make a chemical change to sulfuric acid. Conversion of sulfur to sulfuric acid depends on the freshness and quality of the sulfur to begin with. Sulfur is applied by pounds per square foot .
A fertile soil requires a healthy structure so that it retains water and nutrients essential to plant growth, says the Virginia Cooperative Extension. Organic matter inside the soil improves soil structure, making it both porous and a good absorber of water. Organic matter is inert material such as the remains of crops, leaves, grass clippings, compost or animal manure. Vegetable and animal matter decay into a substance called humus, a dark brown or black substance that is rich in nutrients and slightly acidic in quality, according to the Oklahoma State University Cooperative Extension.
Adding Organic Matter
Organic matter is laid on top of the garden and tilled into the top layer of soil. Cover crops, which are crops grown during the winter such as rye grass to prevent erosion and maintain soil fertility, are also tilled directly into the soil, adding nutrients and good soil structure. Organic matter will not supply all the nutrients needed by plants, so a complete fertilizer purchased from a garden center, either organic or chemical, should be applied with the organic matter.