Soils may have either physical or chemical properties that limit plant growth. These problems can be corrected with the addition of an appropriate amendment, fertilizer, or by adjusting the soil pH.
Amendments
Amendments are materials that change the physical or chemical properties of the soil and indirectly improve plant growth. You can use:
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- Compost to improve soil structure, drainage and aeration.
- Sulphur to make alkaline soil more acidic and improve nutrient availability.
- Gypsum to improve soil drainage if poor drainage is caused by an excess amount of sodium on clay particles.
Changing the properties of the soil with organic matter or chemical amendments can help:
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- Make the soil surface more water permeable – especially silty soils.
- Enlarge pore spaces so water and air can move more freely – important in clay soils.
- Prevent soil cracking – making the distribution of roots, water, and air more uniform.
- Make soil easier to work – important for managing annual plants.
- Transition plants from container soil into heavier native soil.
Non-living organic matter in soil consists of dead plant and animal matter in various stages of decay. A soil rich with organic matter supports a thriving population of beneficial organisms, from microscopic bacteria and fungi, to larger life forms like earthworms. Decomposing organic matter acts like a slow release fertilizer and helps improve the soil structure. Soil microorganisms feed on organic matter and release nitrogen and other nutrients they contain in a form that roots can absorb.
Compost is an excellent organic amendment. Good compost, either homemade or purchased, is the most beneficial all-purpose amendment for soil. The top 6 or 8 inches of garden soil should be amended with composted organic matter. Organic matter continually breaks down and decomposes, so you must periodically add new material. Successful gardeners incorporate organic matter into their soil before each planting season. NOTE: Making compost is covered thoroughly later in this section.
In addition to compost, other amendments may be helpful. Amendments should be free of weed seeds, toxic materials, and disease organisms. Wood ashes should not be used on soils in the San Diego area. Ashes are very alkaline and have a high salt content. Go to the Soil Amendments table for information on specific amendments and their benefits.
Fertilizers
Vegetables and flowers usually grow better if fertilizer containing nitrogen, phosphorus and potassium is mixed into the soil prior to planting. Apply fertilizer uniformly over the soil, then dig or till it in to a depth of 6 to 12 inches.
Phosphorus (P) and potassium (K) are not very mobile in soil and should be placed where roots will grow. A pre-plant application of “P” and “K” will nourish plants for the entire growing season. Crops that take several months to mature usually need additional fertilizer applied periodically. Nitrogen is very mobile in soil and most annuals will need additional applications of nitrogen fertilizer during the growing season.
Fertilizers are available in both organic (natural) and in-organic (chemical) forms. Many successful gardeners use a combination of organic and inorganic fertilizers. Each type has certain advantages and disadvantages.
Organic Fertilizers
Organic fertilizers are derived from plant, animal, and natural mineral sources – They are not made synthetically. They “feed” microorganisms in the soil that break down the organic matter and release nutrients in a form that plant roots can absorb. They are recommended for school gardens by the USDA and by some school districts. Common Organic and Natural Fertilizers.
Limitations of some organic fertilizers are their bulk, availability, low nutrient content, and unpleasant odor. They are also potentially contaminated with weed seeds and harmful microorganisms (manures). Organic fertilizers:
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- Are derived from plants and animals. They are natural and have carbon containing compounds (carbohydrates and proteins).
- Contain several plant nutrients, often in a low concentration.
- Require decomposition by microorganisms and earthworms to release nutrients in a form that plant roots can absorb.
- Release nutrients gradually over a period of several months – faster when soil is warm and slower when it is cold. In winter some plants may not obtain enough nitrogen for optimum growth.
- Are not very soluble in water and resist leaching from soil by rain or irrigation.
- Are less likely to burn plants unless they contain a high level of salts such as manure.
- Provide a food source for soil microorganisms which produce humus and improve soil structure.
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Inorganic Fertilizers
Inorganic, chemical, or synthetic fertilizers are often referred to as commercial fertilizers. These fertilizers are usually water-soluble and immediately release nutrients in a form that plant roots can absorb. They act quickly and can produce good yields, but do not contain organic compounds that “feed earthworms and other soil organisms that create humus”.
Ammonium fertilizers can acidify (lower pH) the soil with long term use. Some disadvantages of the inorganic fertilizers are their salt content, and their potential to leach and burn crops if mismanaged. Slow release inorganic fertilizers that release nutrients gradually over several months can be useful, especially for container plants. Overuse of chemical fertilizers (containing nitrogen and phosphorous) can cause serious water pollution problems due to an overgrowth of algae, fish deaths, and other adverse effects on natural ecological systems. NOTE: Some school districts prohibit the use of inorganic/chemical fertilizers. Inorganic fertilizers:
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- Are processed minerals and synthetically produced compounds. They typically do not contain carbon (except for urea).
- Contain one or more plant nutrient which may be in a high concentration.
- Are usually very soluble in water and have nutrients in a form that plant roots absorb.
- Release nutrients immediately from soluble products, and over a period of several months, from slow release products.
- Make nutrients available quickly for plants to use in warm and cold soils.
- Can leach from soil with heavy rain or irrigation.
- Can damage plant roots if too much soluble material is applied or soil is not watered well.
- Do not provide a food source for soil microorganisms and may decrease their population.
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Dry vs. Liquid Fertilizers
Dry fertilizers are usually more concentrated than liquid fertilizers and are usually applied directly to garden soil. (Some can be mixed with water, then applied around plants.) There are some slow release products that have small beads filled with soluble, chemical fertilizer that release a little fertilizer every time you water for a period of 3 to 4 months. Scratch them into the surface of the soil in a circle around the plants foliage, so the nutrients will be absorbed by roots as they grow outward.
Liquid fertilizers can be applied to the soil or sprayed on the foliage. Some come in a concentrated form to be diluted and applied with a sprayer, or poured on the soil around plants. They are particularly appropriate for a short-term solution to a nutrient shortage, and can be applied every two weeks if needed.
Soil pH – Acidity or Alkalinity
Soil acidity or alkalinity is expressed as a pH number ranging from 1.0 (extremely acid) to 14.0 (extremely alkaline). A pH of 7.0 is neutral, neither acid nor alkaline. Most plants grow best in slightly acidic to neutral soil (a pH between 6.0 and 7.0). San Diego soils that are cultivated tend to be alkaline because our irrigation water is alkaline. Soil pH affects the chemical form of some nutrients like phosphorus and iron and makes them more, or less, available for roots to absorb. When the pH is too high or low, plants may suffer a nutrient deficiency even in a nutrient rich soil.