UNDERSTANDING FERTILIZERS
By Bob Chapman
Gardeners spend a lot of time and money spreading fertilizers on home lawns and gardens, often doing it because we were told it was necessary, but without understanding the reasons why. Why do we need to add fertilizers to the lawn and garden? What is the best fertilizer to use? What is the cheapest? What about organic fertilizers? These and other questions will be reviewed and answered. It is a very brief outline of a very complicated process.
WHY ADD FERTILIZERS?
We need to add fertilizers to the soil because, although there are usually enough nutrients in the soils, it may not be in a form that plants can use or not available in enough quantities when the plants need the nutrients. To keep our lawns and plants growing at optimum levels and producing leaves, flowers or fruit, we need to supplement these nutrients by adding fertilizers at various times to our soils. There are 16 identified elements that plants need to grow and live. In California, only two are usually in short supply, nitrogen and iron (iron is in the soil but may not be chemically available).
ESSENTIAL ELEMENTS NEEDED
Plants need the following elements in the soil and readily available in order to grow, produce flowers, fruit and seeds. The name of the element and their chemical designations are listed below. The first three, carbon, hydrogen and oxygen, are taken primarily from the air and water.
The "building blocks"
• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
Macronutrients
• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)
Secondary nutrients
• Calcium (Ca)
• Magnesium (Mg)
• Sulfur (S)
Micronutrients
• Boron (B)
• Chlorine (Cl
• Copper (Cu)
• Iron (Fe)
• Manganese (Mn)
• Molybdenum (Mo)
WHAT HAPPENS AFTER ADDING FERTILIZERS?
So, we add fertilizers to the soil. What happens then? First, technically speaking, we are adding nutrients that we call fertilizers. When added to the soil, most fertilizers are not available to the plants' roots. The exceptions are synthetics, or inorganic fertilizers. The organic fertilizers must first be digested by bacteria in the soil. They break down and release the nutrients found in organic matter. This is accomplished in a two-step process known as ammonification and mineralization. Bacteria use some nutrients for feeding themselves but do leave some for roots to use. These nutrients can then be absorbed by the plant roots. For a plant to use these nutrients they must be in a liquid solution. Through ionic exchange and the process of osmosis, nutrients are absorbed by the roots. It makes no difference to the roots how the fertilizer got down to them or whether it was natural, organic or inorganic in nature. All must be in a liquid solution to be of use to the plants. The process of converting fertilizers to nutrients is shown below in a simplified form.
Fertilizer (organic or inorganic) applied? converted to ammonia by bacteria? to nitrites by bacteria ? to nitrates by bacteria? are now nutrients in solution? absorbed by osmosis and ionic exchange into roots? to leaves (converted to food by photosynthesis)? to all parts of the plant.
So when a fertilizer is added to the soil, it is converted to nutrients by bacteria to a form that the roots can absorb and then is transported to the leaves. The manufacture of food takes place in the leaves through the process of photosynthesis, and the final distribution of these foods to the various parts of the plant completes the circle. We can readily appreciate the wonders of nature at work.
Gardeners involved in organic gardening believe that only natural products will benefit plants, making fruits and vegetables taste better. They believe that inorganic chemicals are harmful to man and his environment, that only organic materials should be used in the garden. There several organic fertilizers currently available, such as steer manure, blood meal, fish emulsion, chicken manure, soybean meal, bat guano, cottonseed meal, etc.
HOW MUCH FERTILIZER SHOULD BE USED?
Fortunately, California soils have usually enough phosphorus and potash readily available. Most California soils are deficient in nitrogen and sometimes manganese, one of the micronutrients. Iron, which causes chlorosis, or yellowing of the leaves, is in plentiful supply in the soil but is locked up chemically and unavailable to plants. The knowledgeable gardener needs only to be aware of the amount of nitrogen needed to provide optimum growth for plants in the garden. The University of California at Davis states that you need to apply five pounds of actual nitrogen per 1000 square feet per year in our crowded landscapes. The trick is to determine how many pounds of actual nitrogen there are in a pound of fertilizer you purchased. Read the label on your fertilizer bag or box. It states the percent of nitrogen in the mix and also the form the nitrogen is in. A common formula is in the 20 to 23 and up to 31 percent range. Using rough estimates, you can figure that for every four to five pounds of fertilizer you apply about one pound is actual nitrogen. Applying fertilizer five times per year would give plants the needed nutrients for optimum growth.
WHEN TO APPLY FERTILIZERS
The general rule is to apply fertilizers when plants are growing, not to make them grow. More applications are needed early spring to late fall then in the winter. One of the facts about the bacteria in the soil that do all of the hard work is that they need warmth and moisture to survive. When the soil temperature drops to 50 degrees or lower, bacterial action is inhibited. Some gardeners use inorganic fertilizers in the winter that needs little bacterial action. These fertilizers are best used on lawns. We don't want to initiate tender growth on plants just before freezing weather sets in.
SOURCES OF NITROGEN
Some are organic wastes, compost, mulches, plant residues and animal waste products. We will not dwell on these sources, as they are well known. Other sources of nitrogen are manufactured: sulfate of ammonia, IDBU (isobutylidene diurea), ureaform and ammonium nitrate. The manufactured fertilizers are almost always a product of a reaction of ammonia and oxygen to form acids that are chemically manipulated to produce the various fertilizers. Some of these fertilizers are in a form that eliminates one step in the conversion to nutrients by bacteria in the soil. These are used extensively by nurseries growing potted plants and hanging baskets. They are also found in fertilizer mixes. Manufacturers can regulate the availability of nutrients by adding some that are relatively quickly available to those that require bacterial action. The slow release of nutrients can be regulated by the size of fertilizer pellet as it takes a lot longer for the bacteria to break down a big granule or pellet to nutrients. Thus on the fertilizer label you might see three sources of nitrogen, each with varying degrees of availability to plants' roots.
PROGRESS IN USING FERTILIZERS
We have come a long way since early man discovered that spreading animal manures in the garden increased the health of his plants and the production of crops. Now a gardener can apply fertilizer that is a resin-coated pellet that releases nutrients slowly. Spread this product in early spring and you won't have to add another fertilizer in the normal growing season. It is expensive but does "meter" the nutrients for up to nine months. How can a fertilizer applied just a single time equal five applications of a regular fertilizer? It can, because, unfortunately, some nitrogen is leached out of the soil before it can be used and is wasted. One commonly known source of nitrogen, sulfate of ammonia is non-leachable. It also has an acidifying effect on alkaline soils. Sulfate of ammonia requires bacterial action and warmth for the bacteria to work so it takes about 10 days to three weeks for the nitrogen to become available. Many organic fertilizers are also in the category of a slow release fertilizer. It takes time for bacterial action to take place. As a general rule, about half of the nutrients in organic fertilizers are released in a growing season.
FERTILIZERS NOW AVAILABLE
To summarize, fertilizers now on the market are:
- Simple or single-nutrient fertilizers such as sulfate of ammonia or muriate of potash
- Slow release fertilizers ( one application per season)
- Fertilizers that are combined with herbicides (weed and feed)
- Soluble complete fertilizers (Miracle-Gro, Schultz's "Instant Soluble Plant Food")
- Natural organic fertilizers (manures, blood meal, fish emulsion, etc)
- Partially soluble complete fertilizers (Superfine Lawn Fertilizer, Scott's)
- Liquid lawn fertilizers (slightly less actual nitrogen per 1,000 square feet, applied every other month during the growing season).
- Specially formulated fertilizers for specific plants (Citrus, azaleas, roses, etc.)
Combination fertilizers (systemic insecticide and fertilizer for flowers and vegetables)
BUYING FERTILIZERS
The wise gardener will shop and compare quality and price when buying fertilizers. What product will give your soil and plants the amount and kind of nutrients at what price? Look for the brand that will give you the major nutrients (including a high percentage of nitrogen) and the minor nutrients all plants need. When found, use the product on all plants in your garden and lawn. Doing so will give your lawns, flowers, shrubs and trees their needed nutrients and will lead to a healthy, sturdy landscape able to survive nicely.