The home gardener can choose between organic fertilizers and synthetic, or chemical, fertilizers. But before even considering these decisions, we must first ask a more basic question: does our soil actually need fertilizing? The only way to get that answer is to test the soil.
What the Soil Needs
In addition to nitrogen, phosphorous and potassium (N-P-K), there are 13 other elements essential to plant growth. N-P-K are considered macronutrients because plants usually require them in larger quantities. Calcium, magnesium and sulfur are secondary macronutrients but are usually either present in sufficient quantities in the soil or are added coincidentally with other materials (e.g., lime). The other 7 nutrients are called micronutrients because they are needed in very small quantities and most soils contain sufficient supplies.
Easy-to-use soil testing kits are available from the Piedmont Master Gardeners at the Virginia Cooperative Extension office. A routine soil test will provide results for soil pH plus plant-available levels of P, K, Ca, Mg and various micronutrients. Soil testing for nitrogen has limited use because the nitrogen level constantly changes in response to soil organic matter additions, soil microorganism activity, temperature, moisture levels, leaching, and nitrogen consumption by plants and other soil life. These soil tests will also provide recommendations of what could or should be added to your soil to get it to acceptable levels if needed.
Fertilizer and Compost
There are two important ideas to consider at this point: First, fertilizer, although commonly referred to as plant food, is not really food. Fertilizer provides the nutrients necessary for plants to produce their own food. The three macronutrients, N-P-K, all play distinct roles: nitrogen is essential for plant growth; phosphorous helps with root formation, cell division, flowering and other functions; and potassium helps plants regulate water and use sunlight to make food. Second, compost is not considered to be fertilizer. Although compost may contain a small amount of nutrients, they are generally not readily available to plants.
If compost is not fertilizer, why is it considered so important?
Compost, along with other organic matter, improves the capacity of soil to hold nutrients. Although compost will add some amount of nutrients, its main function is to condition the soil through a complex process called cation exchange capacity or CEC. Very simplistically put, this process describes the relative ability of soils to store a particular group of nutrients. In addition, compost attracts worms, thus indirectly providing nutrients for plant use; earthworms and other organisms digest the organic matter, producing nutrient-rich castings, or excrement. Compost also increase bacterial and fungal activity, particularly the mycorrhizal fungi, which makes other nutrients more available to plants. (For more information on mycorrhizal fungi, see Mycorrhizae Part I and Mycorrhizae Part II in The Garden Shed.) Adding organic material to the soil increases its ability to hold water; improves the physical structure of the soil; reduces erosion from water and wind; decreases compaction and crusting of the soil; and raises soil pH. Organic matter helps soil particles bind together into aggregates, or clumps, which makes it much easier to work the soil. This soil quality is referred to as tilth. Studies have also indicated the possibility of reduced soil-borne pathogens in soil that has been amended with organic matter.
Gardeners often refer to compost as black gold because of its contribution to enriching the soil, but this enrichment is not achieved in one application or even several. Compost should be added to the soil on a regular basis as part of a long-term, soil-building program. The goal in soil management is to increase the organic content to 4-5%, over a period of years. For those new to composting, or for those who wish to remind themselves of composting guidelines, see an excellent article in The Garden Shed on making compost.
If you purchase compost, be aware that the term compost is not regulated, so a wide range of products can be marketed under that name and can vary considerably in quality. Purchase from a provider whom you trust. Also, make sure that the compost is mature. If immature compost is added to the garden, its bacteria compete with plants for nitrogen in the soil.
Let’s assume that you have decided to add compost to your garden on a regular basis. How much should you add? Fortunately (according to guidelines provided by Cornell University), mature composts can be used in most planting situations without serious concern for precise amounts. As a general guideline, to apply 1″ of compost over a 10′ by 10′ area, you’ll need about 8 cubic feet or about 300 lbs. of compost. Compost in planting beds for perennials is often applied at a rate of 1-2″ incorporated into the soil about 6-8″ deep. To use compost as a landscape mulch, apply 1-3″ deep over the soil. For top-dressing an established lawn, spread a light layer of about ¼” at a time, so as not to smother the grass. For new lawns, apply 1-2″ of compost, and incorporate into 5-7″ of soil, with a final volume of about 30% compost.
What about organic fertilizers other than compost? The term organic as applied to fertilizers means that the nutrients contained in the product are derived solely from the remains or by-products of once-living organisms.
Plant-tone and Hollytone are organic, granular fertilizers. Cottonseed meal is a by-product of cotton manufacturing. Formulas vary slightly but generally contain 7% nitrogen, 3% phosphorus and 2% potash. Cottonseed meal is frequently used for fertilizing acid-loving plants, such as azaleas, camellias and rhododendrons. Blood meal is dried, powdered blood collected from cattle slaughterhouses. It is a rich source of nitrogen, and it supplies some of the essential micronutrients, including iron. Fish emulsion, a well-rounded fertilizer, is a partially decomposed blend of pulverized fish. The odor is intense but dissipates within a day or two. Although organic, these fertilizers add nutrients but do not condition the soil.
Manure is a complete fertilizer (i.e., it provides nitrogen, phosphorous and potassium) and also conditions the soil. Manures differ from each other because of their source (horse, cow, poultry), their age, how they were stored, and animal bedding material which may be mixed in. Manure with straw will have a different nitrogen component than pure manure and the rate of nitrogen release may be slower in manure with bedding due to the higher carbon content of straw. You should inquire about whether the animal bedding was treated with pesticides and/or herbicides. Fresh, raw manure or “hot” manure activates and builds up soil microbial activity to the extent that the nutrients could volatilize or burn up before plants use them. Fresh manure can also damage plant tissue, kill seedlings and contain weed seed. There is also a low chance of pathogenic bacteria such as E.coli, Listeria or Salmonella. It is recommended that composted manure be used rather than raw manure. Composted manure should be applied in the fall or after harvest. Try to leave it on at least 120 days before harvest of a crop.
Using sewage sludge, now branded as biosolids, as fertilizer on food crops is a hotly debated food safety issue, although most consumers are not aware of the debate. Milorganite is a well-known brand of sewage sludge. One of the main objections is the presence of heavy metals, such as cadmium. Another serious objection is that more study is needed to understand how the contaminants found in sewage sludge, even at low levels, will affect the environment and human health over the long term.
Another source of organic soil improvement is the cover crop, also called green manure. Cover crops, such as annual rye, ryegrass, and oats, are planted in the garden in the fall. Sow seed about one month before the first killing frost. Till under in the spring at least two weeks before vegetables are planted. Cover crops provide additional organic matter, hold nutrients that might have been lost over the winter, and help reduce erosion and the loss of topsoil. Legume cover crops can increase the amount of nitrogen in the soil. Deep-rooted cover crops can grow for a season in problem soil to help break up hardpan.
Finally, there are some cautions to consider when using organic fertilizers. In general, organic fertilizers release nutrients over a fairly long period. A study at Virginia Tech comparing organic with inorganic fertilizer determined that organic fertilizer is the better choice for conditioning the soil. However, because organic fertilizers are slower acting than synthetics, they may not supply plants with nutrients quickly enough for best growth. Because organic fertilizers depend on soil organisms to break them down to release nutrients, most of them are effective only when soil is moist and soil temperature is warm enough for the soil organisms to be active. Gathering natural materials, such as seaweed, grass clippings and leaves, to add to the compost pile is labor-intensive and time-consuming. Distribution of nutrients in organic fertilizer varies. Organic materials break down at different rates, so the composition and content of organic fertilizer is never consistent. Cost is also a consideration. If organic materials must be purchased, the total cost of the material, transportation, and labor to apply may be significantly more than buying synthetic fertilizer. However, the regular addition of compost, manure, cover crops and other organic materials can raise the soil nutrient levels and improve physical characteristics so that the need for adding synthetic fertilizers is greatly reduced over time.
Now let’s consider inorganic fertilizers. Inorganic fertilizers such as ammonium sulfate or ammonium phosphate are often called commercial, chemical, or synthetic fertilizers, because they go through some manufacturing process, although many of them come from naturally occurring mineral deposits. Synthetic fertilizers are relatively pure chemicals and offer the advantages of predictability and reliability. Formulations are blended with accuracy and you can buy different blends for different types of plants. Fertilizers are identified by the analysis given on the package, which refers to the amount of an element present in a formulation based on percentage of weight. By law, all analyses provide three numbers giving the percentage by weight of nitrogen (N), Phosphate (P205) and Potash (K20). Although not entirely accurate, many people simply interpret the numbers as N-P-K, nitrogen, phosphorous and potassium.
There are two kinds of fertilizers: quick release and slow release. Quick-release fertilizers, also known as “fast-acting” fertilizers, are water-soluble chemicals that once applied are readily available to the plant. These materials are easily leached with rain or over-irrigation and require frequent application. They are the least expensive fertilizers and are always synthetic products. Slow-release fertilizers, sometimes called water-insoluble types, release nitrogen over time. These products are a little more expensive and include certain synthetic fertilizer products. All natural organic fertilizers are slow release.
Just as with organic fertilizers, there are pros and cons to using synthetic fertilizers. Synthetic fertilizers may contain ingredients that may be toxic to the skin or respiratory system. They must be mixed and measured accurately. If used in excess, they can kill plants. Chemical fertilizers can build up in the soil, causing long-term imbalances in soil pH and fertility. If applied incorrectly, synthetic fertilizers can be detrimental to earthworms because most synthetics use salt formulations. Always follow label instructions.
A Place For Both Organic and Inorganic Fertilizers
The organic farming movement, i.e., gardening without synthetic fertilizers and pesticides, has made us very aware of taking a more responsible approach to both agriculture and home gardening. The movement also seems to promote the idea, or at least is interpreted as such in the popular media, that there is a good way to fertilize (organic) versus a bad way (chemical). We need to be aware that organic gardening is not about adding organic fertilizers in some way that equals bags of chemicals. It is a philosophy of gardening that supports the health of the whole system. In an organically managed yard or vegetable garden, the emphasis is on cultivating an ecosystem that sustains and nourishes plants, soil microbes, and beneficial insects.
So, is there any place for inorganic fertilizers? To answer that question, let’s assume that we’ve had our soil tested and the results show that the pH of 8.0 is outside our acceptable range of 6.5-7.5. The amount of P is acceptable but the results show that K is a little low. The organic content is only at about 1.5% as compared to the long-term desired level of 4-5%. What would be do?
The low organic content is a longer-term problem. We’ll need to start adding compost or composted manure on a regular basis to improve the soil tilth. That will take some time. But in the short run, we’ll need to address the high pH factor by adding some sulfur to increase the soil acidity. Potassium sulfate contains 50% potassium and 16% sulfate. Since the results for potassium are also low, potassium sulfate would probably be a good choice, following the recommendations from Virginia Tech based on the soil test.
In summary, there is a place in the garden for both organic and inorganic fertilizers, but care must be taken to make sure that each is used responsibly. Organic materials, such as compost and composted manure, should be added to the garden as part of a regular, long-term, soil-conditioning effort. Chemical fertilizers should be used as supplements in direct response to the requirements of a soil test. Each growing season, we learn more about these complex issues and how to follow best practices as responsible stewards in our own home gardens.