Why use organic fertilizers? Most "organic" fertilizers (those derived from plant, mineral, and animal sources) release nutrients more slowly than do synthetic fertilizers. So they are less likely to burn plants, pollute groundwater, or kill soil organisms that are part of a healthy ecosystem. They also provide a small but steady supply of nutrients, which is more useful that the large, short-term supply that synthetic fertilizers provide. And many organic fertilizers supply plants with necessary micro-nutrients, or trace elements, not found in synthetic fertilizers.
Buying Organic Fertilizers Fertilizer labels list three numbers, separated by hyphens; for example, 3-5-3. The first number is the percentage of available nitrogen (N), the second is available phosphorus (P), and the third is available potassium (K). These numbers are sometimes called the NPK ratio or the nutrient analysis.
The nutrient analysis for organic fertilizers is often lower than those for synthetic ones. That's because labeling laws allow only the immediately available nutrients to be listed, not the total. The nutrients in most organic fertilizers aren't very soluble, so they're released gradually. For example, rock phosphate contains about 32% total phosphorus, but only about 2% is available at any one time. Over time, however, the total amount of nutrients released from organic fertilizers is higher than the label suggests.
An organic fertilizer may contain necessary nutrients besides N, P, and K. bonemeal, for example, is high in calcium as well as phosphorus. Liquid seaweed contains many micro-nutrients. Read the label carefully for information on other materials the fertilizer contains, what it's made from, and what effect the material has on plants and soil organisms. Applying Fertilizers Fertilizers can be solids or liquids. Solid fertilizers, which you apply dry, may be fine powders, large granules, or something in between. Fertilizers you apply wet may come as powders that you dissolve in water or as concentrates that you dilute with water.
Dry fertilizers work more slowly but last longer. You can spread a dry fertilizer evenly over a large area, a method called broadcasting. Or you can sprinkle dry fertilizer next to a plant, or beside a row of plants, which is called side-dressing. To get the nutrients to the roots faster when you side dress, lightly scratch the fertilizer into the soil, being careful not to nick the roots or stem. Another way to use dry fertilizers is to put them in the hole at planting time; because most organic fertilizers aren't very soluble, you don't have to worry about burning the roots.
To apply a liquid fertilizer, first dissolve or dilute it according to label instructions. You can pour it into the soil at the base of the plant (this is called a soil drench). Or you can spray the fertilizer on the leaves (this is called foliar feeding). Because plants close down their leaf pores when it's sunny or hot, apply foliar fertilizers early or late in the day, or during a cloudy spell. Spray them to the tops and underside of the leaves until the liquid runs off.
An amendment is something that improves the soil without necessarily adding nutrients. Amendments can make clay soil looser or sandy soil hold water better. Limestone and sulfur are soil amendments because the change the pH (the acidity) of the soil.
Organic Fertilizers Most of these fertilizers provide one or two nutrients, although some are more balanced. You can buy organic fertilizer blends that provide a range of nutrients plants need.
Bloodmeal Dried animal blood. About 15% nitrogen, readily available. Lasts 3-4 months. Apply 3 lbs. per 100 sq. ft. for soils low in nitrogen, 2 lbs. for soils with moderate nitrogen, and 1 lbs. for soils with adequate nitrogen. Rake it into the surface of the soil; it washes into the root zone. NOT FOR TEXAS. Finely ground and steamed animal bones, used mainly as a phosphorus source. 11% phosphorus, 1% nitrogen, and 24% calcium. Lasts 6-12 months. Use only on acidic soils (the calcium increases alkalinity). Phosphorus is available more quickly than with rock phosphate (described below). Apply before planting when the soil temperature is above 55 degrees F. Apply 3 lbs. per 100 sq. ft. on soils low in phosphorus, 2 lbs. on soils with moderate phosphorus, and 1 lb. on soils with adequate phosphorus. Work it into the top 6 inches of soil.
Fish meal Dried and ground fish parts. A good source of nitrogen. Analysis is 6-3-3. Lasts one season. Apply at planting. Broadcast 3 lbs. per 100 sq. ft. on poor soils, 2 lbs. per 100 sq. ft. on moderately fertile soils, or 1 lb. on fertile soils. If sidedressing, use the same rates per 100 foot of row.
Granite meal (Also called granite dust) Ground granite. 3-5% potassium, 67% silica (sand), and micronutrients. Slow release. One application can last 10 years. Improves soil structure. Broadcast 10 lbs. per 100 sq. ft. on soils low in potassium, 5 lbs. on soils with moderate potassium, and 2.5 lbs. on soils with adequate potassium. Rake into the soil surface.
Greensand (also called glauconite) Sand-based fertilizer mined from dried ocean deposits. Slow-release source of potassium (an application can last up to 10 years). Contains 5-7% potassium, trace minerals, and silica. Helps loosen clay soils. Apply in autumn. Broadcast 10 lbs. per 100 sq. ft. on soils low in potassium, 5 lbs. per 100 sq. ft. on soils with moderate potassium, and 2.5 lbs. on soils with adequate potassium.
Kelp/Seaweed Comes as solid or as liquid concentrate. High in potassium and micro-nutrients. Solid form adds organic matter. For the solid, work about 10 lbs. per 100 sq. ft. into the soil. For the liquid, follow label directions for dilution.
Langbeinite (also called sulfate of potash magnesia) Quick-release potassium. Also adds magnesium and sulfur. Broadcast up to 1 lb. per 100 sq. ft. The easily dissolved potassium can burn plants if you apply too much. Sold as Sul-Po-Mag and K-Mag.
Manure Solid animal waste. Provides nutrients and organic matter. Nutrient content varies with the type of animal and its diet. Fresh manure's high nitrogen content can burn plants, so either apply several months before planting or let it age in a compost pile for several months before applying to the soil around plants. Apply 10-20 lbs. per 100 sq. ft.
Rock Phosphate NOT FOR TEXAS. Crushed and washed rock. 33% total phosphorus; only about 3% is available at any time. High in calcium, so use only on acidic soils. Apply directly to the soil and work into the top 6 inches, or add to compost pile in autumn. Apply 6 lbs. per 100 sq. ft. for soils low in phosphorus, 2.5 lbs. for soils with medium phosphorus, 1 lb. per 100 sq. ft. for soil with adequate phosphorus.
Wood Ash NOT FOR TEXAS. About 12% potassium; varies with the type and age of the wood. Contains large amounts of calcium and micro-nutrients. Apply to acidic soils only (do not use on soil with a pH above 6). Apply no more than 2 lbs. per 100 sq. ft. every 3-4 years, because excess amounts can cause nutrient imbalances. Turn it under the soil in the early spring, avoiding young plants.
Soil Amendments Some soil amendments add small amounts of nutrients to the soil, but their main purpose is to improve the soil in some other way. Some change the soil's pH (its acidity) making many nutrients more available to plants. Others add organic matter, which loosens clay soils and makes sandy soils hold water and nutrients better. Compost Decayed and partially decayed plant waste, either from the garden or kitchen. Adds nutrients and organic matterto the soil, and stimulates soil microorganisms. Nutrient analysis varies with what materials are in it and how long it's aged (range is 1/2-1/2-1/2 to 4-4-4). Grass clippings Adds organic matter to the soil and stimulates worms and other soil microorganisms. Fresh clippings also add nitrogen. Analysis averages 0.5-0.2-0.5. Work fresh clippings into bare soil before planting, using 50 lbs. per 100 sq. ft. for soils low in nitrogen, 30 lbs. for soils with moderate nitrogen, and 20 lbs. to maintain soil nitrogen. Or spread a 2-inch layer around the base of existing plants. Gypsum (Also called land plaster) Calcium sulfate powder. Contains 22% calcium and 17% sulfur. Loosens clay soils and neutralizes excessive sodium or magnesium in soils. Broadcast 4 lbs. per 100 sq. ft. over soils low in calcium, 2 lbs. per 100 sq. ft. to soils with a moderate level of calcium, and 1/2 lb. to soils with adequate calcium. Work it into the soil. Limestone NOT NEEDED IN TEXAS. Decreases the acidity of soils, making them more alkaline. Effect lasts several years. Apply 8 lbs. per 100 sq. ft. on clay soil, 6 lbs. on loams, and 2 lbs. on sands. Use calcitic limestone on most soils; use dolomitic limestone on soils low in magnesium. Have the soil retested a year after application and reapply if necessary. Sulfur Sulfur decreases the soil pH, making alkaline soils more acidic. The results are short-term. Broadcast 1 lb. per 100 sq. ft. to lower the pH by one full point (for example, from 8 to 7. Mix the sulfur into the top 3 inches of soil. If you need to apply more than a pound, apply half in the late winter or early spring and half in autumn after harvest. To lower the pH around existing plants, scrape the sulfur into the soil at the base of the plant, as far out as the end of its branches. Have the soil retested a each year and reapply if necessary. Worm castings Worm manure. All-around soil improver. Source of organic matter and small amounts of plant nutrients. Apply 25 lbs. per 100 sq. ft. to soils low in organic matter, 10 lbs. to soils with a moderate amount of organic matter, and 5 lbs. to soil with adequate organic matter. Work into the soil before planting or apply to the surface after planting.