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Feeding Your Garden

What do laundry detergent manufacturers have to do with gardening? What do gardeners have to do with weed growth in local streams? Are you confused about fertilizers? What do those numbers on the fertilizer package mean? Keep reading; here’s a crash course!

While there is no substitute for long-term soil building with yearly applications of organic matter, fertilizers can serve a purpose. They may be necessary for “heavy-feeder” crops (such as corn) or for soils that are imbalanced or depleted. For instance, container plants require fertilization due to a relatively small amount of soil that is constantly being watered, causing soil nutrients to continually leach out.

Fertilizers should never be added “just because.” If a little is good, a lot is NOT better! All fertilizers-both organic and synthetic-are harmful to aquatic environments, such as streams, lakes, and oceans. Excess fertilizer gets washed into ground water and surface water during irrigation and rainstorms, thus fertilizing algae and weed growth. That’s why many laundry detergents now say “contains no phosphates.”

Even a “balanced” fertilizer may contain nutrients not needed by your plants, thus becoming excess fertilizer. Before deciding to add any fertilizer to your soil or plants, take a good look at plant growth and see if there are any fertilizer-deficient symptoms (see below). Invest in an easy, inexpensive soil test kit from your local garden center and determine whether you really need fertilizer.

There are basic differences between natural (organic) and synthetic (chemical) fertilizers. Natural fertilizers are derived from dead organisms; they are available in many different forms. Most of them contain a lower level of nutrients than synthetic products; most release their nutrients more slowly. Rather than dissolving in water like synthetic fertilizers, they are broken down by microorganisms in the soil, releasing nutrients as they decay. That’s why a natural fertilizer is less likely to burn roots than synthetic fertilizers. However, even “natural” or organic fertilizers-compost tea, for example-may apply more nitrogen than a plant can use, thus contributing to fertilizer contamination of local water sources.

Synthetic fertilizers are produced by chemically altering raw materials (using high amounts of fossil fuels) to convert them into a form readily absorbed by plants. They’re usually very water-soluble and are immediately available to plants. If you apply too much of these fertilizers, or environmental conditions (like a lot of rain) make them available more quickly than your plants can use, your plants, soil organisms, and local water sources will suffer. Examples of synthetic fertilizers include urea, ammonium sulfate, super phosphate, muriate of potash and sulfate of potash.

The “primary” elements that plants require are nitrogen, phosphorus (phosphate) and potassium (potash). A fertilizer label will show the formula of major nutrients or the percentage by weight that the fertilizer contains. That formula of nutrients is also known as the N (nitrogen) P (phosphorus) K (potassium) formula. If, for example, the NPK formula reads: 5-10-5, there is five percent nitrogen content, ten percent phosphorus content, and five percent potassium content. Under the NPK formula will be the guaranteed analysis information, or specific nutrient sources. This information is especially important if you are concerned with using organic fertilizers.

What do the three major (macro) nutrients do for your plants? Nitrogen is required for growth and photosynthesis. Nitrogen is the least stable soil nutrient because it quickly leaches out of the soil (into water tables and surface water). Symptoms of nitrogen deficiency are pale green or yellow leaves, spindly or stunted plants, or small fruits. On the other hand, too much nitrogen will produce lush foliage instead of fruit and can burn plant tissues. Organic sources of nitrogen include aged animal manure (fresh manures may burn plants), dried blood, fish emulsion, guano, cottonseed meal, alfalfa meal, cocoa bean or peanut shells, bone meal, hoof and horn meal, feather meal, wool waste, human hair, fish meal or scraps, seafood meal and soybean meal.

Plants use phosphorus to flower, fruit and set seed. Phosphorus promotes strong root growth, makes stems strong and improves disease resistance. Most soils in Tuolumne County tested by Master Gardeners have proven to be high in phosphorus. If you determine that your soil needs phosphorus, it should be incorporated into the root zone of the plant. Symptoms of phosphorus deficiency include purplish leaves; pale, dull leaves with yellow, streaked margins; thin, hard stems; fruit that sets late and drops early; and stunted plants and roots. Organic sources of phosphorus include rock phosphate, colloidal phosphate, bone meal, dried blood, cottonseed meal, basalt rock powder and langbeinite (usually sold as SulPoMag).

Potassium helps plants grow strong root systems and promotes disease resistance. It also regulates the uptake of other nutrients. Most soils have an adequate reserve of potassium, but in a form that is unavailable to plants. About one percent of the total soil potassium (called exchangeable potassium) acts as an important source for plants. Like phosphorus, if potassium is needed, it should be incorporated into the root zone of the plant. Symptoms of potassium deficiency include spotted, mottled or curled lower leaves; scorched leaf margins; weak stems; shriveled fruit; and reduced disease resistance. Organic sources of potassium are seaweed extract or kelp meal, granite dust, greensand, leaf mold and wood ashes.

Erin O-Hare is a long-time Master Gardener and avid vermicomposter (that means worms, folks!).