Tidewater Gardening - May 2011


Organic vs. Synthetic Fertilizers
K. Marc Teffeau

With the continuing interest and emphasis on nutrient loading into the Chesapeake Bay and its tributaries, we need to understand that how we manage our landscapes and lawns has an impact on water quality. One area where homeowners can reduce potential nutrient run-off from their property is the use of “natural organic” fertilizers (those derived from plant, mineral, and animal sources).
There is much confusion over the terms organic, natural, inorganic and synthetic when applied to fertilizers. A basic principle of soil chemistry is that nutrients are available to plants in only one or two forms, regardless of the source. Plant roots do not distinguish between nitrogen in the nitrate form that come from a synthetic 10-6-4 fertilizer, or from an organic form such as blood meal. The benefits of using natural organic materials go beyond just the simple provision of a specific element like nitrogen, phosphorus or potassium.
There are some guidelines that need to be followed in the use of these materials, and you need to understand their characteristics. One major advantage to using natural organic fertilizers is that 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 natural organic fertilizers is higher than the label suggests. Because of their slow release, they are less likely to burn plants, pollute groundwater, or negatively impact soil organisms that are part of a healthy ecosystem. They also provide a small but steady supply of nutrients, which can be more useful than the large, short term supply that synthetic fertilizers provide.
Another advantage of these materials is that many organic fertilizers supply plants with necessary micronutrients, or trace elements, not found in synthetic fertilizers. These “natural” fertilizers do cost more per pound of actual element supplied than synthetic fertilizers, and the nutrient analysis for organic fertilizers is often lower than those for synthetic ones.
Critical to the success of any gardening effort is the development of a healthy, biologically active soil. Natural organic fertilizers are beneficial for improving the soil’s physical structure and increasing bacterial and fungal activity. Because natural organic fertilizers depend on soil organisms to break them down to release nutrients, it is important to develop and maintain good soil health.
Through the use of composts, natural organic fertilizers, and green cover crops (especially in the vegetable garden), homeowners can create and enhance a vibrant soil biological component which will decompose the organic fertilizers and reduce their need for artificial chemical inputs, while still getting an abundance of production.
Natural organic fertilizers can come in a solid or liquid form. 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 or apply it to a plant, or beside a row of plants. To get the nutrients to the roots faster when you side dress, lightly scratch the fertilizer into the soil, being careful not to disturb the roots or stem of the plant.
Another way to use dry natural source fertilizers is to put them in the hole at planting time. This is a benefit in using natural organic fertilizers because most of them aren’t very soluble and do not have a high salt index, so the possibility of burning the plant roots is less.
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 (soil drench) or you can spray the fertilizer on the leaves (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 on the tops and underside of the leaves until the liquid runs off.
Most of the natural organic fertilizers provide one or two nutrients, although some are more balanced. You can buy organic fertilizer blends that provide a range of nutrients that plants need.
There are a number of “organic” or “natural” fertilizer sources. Blood meal is dried animal blood which contains about 15% nitrogen. It is a readily available nitrogen source that can last 3 to 4 months. A general recommendation would be to apply 3 lbs. per 100 sq. ft. for soils low in nitrogen, 2 lbs. for soils with moderate nitrogen, and 1 lb. for soils with adequate nitrogen.
Bone meal is finely ground and steamed animal bones and is used mainly as a phosphorus source. It contains 11% phosphorus, 1% nitrogen, and 24% calcium and will last 6‑12 months. If your soil pH is above 7.0, I would not use bone meal as the calcium will raise the soil pH even higher. It is one way, however, to raise the pH on acidic soils. The phosphorus is available more quickly in bone meal than with rock phosphate. Apply before planting when the soil temperature is above 55°.
Fish meal is dried and ground fish parts. It serves as a good source of nitrogen and its analysis is usually 6‑3‑3. This material will last one season. Apply at planting and 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 side dressing, use the same rates per 100 foot of row. One disadvantage of any fish based products is that the smell may attract cats into the garden.
Granite meal, also called granite dust, is made from ground granite. It contains 3‑5% potassium, 67% silica (sand), and micronutrients. It is a slow-release material and one application can last 10 years. Granite meal will improve soil structure. A general recommendation is to 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) is a sand-based fertilizer mined from dried ocean deposits. It is a slow-release source of potassium where an application can last up to 10 years. Greensand contains 5‑7% potassium, trace minerals, and silica. It helps loosen clay soils. Apply this material in the 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. It is high in potassium and micronutrients. The solid form adds organic matter to the soil. For the solid material, work about 10 lbs. per 100 sq. ft. into the soil. For the liquid, follow the label directions for dilution. No matter what nutrient source you use it is recommended that you have a soil test done to determine fertilizer needs before applying.
Warm season crops can be planted in the vegetable garden in May. These vegetables include squash, sweet corn, tomatoes, peppers, eggplants, green and lima beans, and cantaloupes. You can squeeze in a late planting of cool-season crops like spinach, lettuce and peas, but do it the first week of May.
In the flower garden, now is the time to set out marigolds, petunias, ageratums, and fibrous begonias. All are good border plants. Multiflora petunias withstand heat much better than other types and are more attractive throughout the summer. They are more resistant than other types to botrytis, a disease that cripples petunias, especially in damp weather, and they branch more easily, meaning less maintenance. Multifloras are most useful for massed effects in beds.
You can also set petunia plants among fading tulips or daffodils to hide the unsightly wilting leaves. After the bulb foliage begins to fade, you can tie the leaves in gentle knots to neaten them, but don’t remove them until they have dried completely.
Happy gardening!