Fertilizer 101

By Jan Cashman • Posted on February, 27th 2009

by Jan Cashman

There are 16 elements which plants require for survival, growth, and normal development called “essential elements”. Three of them-carbon, oxygen, and hydrogen-are found in the air and water. The other thirteen are provided by soil and/or fertilizers. Six of these thirteen are listed as “macronutrients”, required in large amounts by plants; the other seven are called “micronutrients,” or trace elements, required in much smaller concentrations by plants. Micronutrients include iron, chlorine, manganese, zinc, boron, copper, and molybdenum.

Nitrogen (N), phosphorus(P), and potassium(K), essential macronutrients, are the three plant nutrients which are most commonly deficient in soils. Every fertilizer label states the percentage by weight that the product contains of these three macronutrients. They are always listed in the order N-P-K.

Nitrogen is important for vegetative growth. Plants deficient in nitrogenare typically yellow, spindly, and lack vigor. Ample phosphorus is required for normal root development and is involved in cell division, flowering, fruiting, and seed formation. Potassium is essential for plants’ resistance to certain pests and diseases. Too little potassium can cause weak stems and poor root development. Calcium, magnesium, and sulfur, called secondary macronutrients, are also essential but are less likely to be deficient in plants.

You can buy fertilizer in either chemical or organic form. Chemical fertilizers, which are less expensive, provide higher levels of nutrients and are faster acting. Organic fertilizers include fish emulsion, manure, and blood, bone, or grain meals. Most of these natural products contain only one nutrient-blood meal and manure contain only nitrogen; bone meal only phosphorus. Organic fertilizers contain lower levels of nutrients than chemical fertilizers, and release their nutrients more slowly, therefore are less likely than chemical fertilizers to burn the plant. Milorganite, an organic fertilizer made from biosolids with added iron, has been used by golf courses for years because it will not burn or pollute and is slow release.

Not all plants in your yard and garden require supplemental fertilizer. Mature trees and shrubs need little or no fertilizing if they are healthy specimens. On the other hand, fast growing lawns, annual and perennial flowers, and vegetables are more likely to require supplemental feeding. Different kinds of plants need fertilizing at different times of the year and with different types and different analyses of fertilizer.
I fertilize my annual flowers throughout the growing season with water soluble Miracle-Gro fertilizer through a spray feeder every ten days to two weeks. Miracle Gro’s “All Purpose” formula is 20-20-20 but I prefer the “Bloom Booster” formula 15-30-15, with higher phosphorus, to promote flowering. Miracle-Gro’s formula also includes a number of trace elements. Osmocote is a slow release fertilizer we recommend for your flowers in pots. It lasts up to four months so you only have to fertilize once at the beginning of the season.

I use a granular fertilizer for my vegetable garden, dressing the rows with a light application when the seedlings emerge and again when the plants are about half grown. A chemical fertilizer such as16-16-16 in 40# bags is economical and works for vegetables, however, I prefer Lilly Miller’s Tomato and Vegetable Food-5-10-10–that is released slowly and contains a number of trace elements. Do not use a fertilizer too high in nitrogen on your garden vegetables and flowers; too much nitrogen results in lots of vegetative growth but fewer flowers and fruits or vegetables. One exception to this might be sweet corn, which is a heavy user of nitrogen.
Lilly Miller’s slow release Rose and Flower Food-5-8-4–works well to fertilize perennial flowers. In the spring, I sprinkle a small handful of it next to each plant to give them a good start.

Some shrubs, especially roses, do best with regular fertilization. I use a rose food that contains both fertilizer and a systemic insectide. Flowering shrubs may also benefit from an annual application of fertilizer, but many shrubs grow well with no supplemental nutrients. If a shrub puts out strong new growth with good color each year, it is doing well without feeding. If new growth is scant, pale or weak, fertilize your shrubs.

Be careful of overfertilizing any newly planted shrub or tree. Use mild, water soluble fertilizers that are made for transplanting. Do not use strong, chemical fertilizers that could burn the roots of a young tree or shrub.
We sell easy-to-use fertilizer spikes for trees. If your tree is growing satisfactorily, they may not be necessary, but, if your tree’s new growth is weak, sparse, or pale, maybe fertilizer will help. Never use more tree fertilizer spikes than recommended. Pound them into the ground at least 2 feet from the tree’s trunk. I do not recommend fertilizer spikes on trees whose diameter is less than 2 inches because they are too strong. Never fertilize trees after July 1. Late fertilization encourages new growth when we want the trees to slow down and start to go dormant before winter.

Lawns, usually made up of Kentucky bluegrass in Northern climates, need fertilizer to grow well, maintain a deep green color, and stay thick to resist weeds. Use a fertilizer high in nitrogen for your grass such as 25-10-10 in May and again in July. Apply one pound of actual nitrogen per 1000 square feet of lawn area each time, using a broadcast spreader. (If the first number on the fertilizer bag says 25, it contains 25% nitrogen so a 40# bag has 10 pounds of actual nitrogen in it. On a 10,000 square foot lawn, you would use the whole bag.) Then use an analysis higher in phosphorus for your fall fertilization to promote root growth during fall and winter. We recommend 16-20-0-14 (the last number is sulphur).

Our area’s clay, alkaline soils bind up iron and make it less available to plants. A fertilizer that contains added iron helps avoid chlorosis in your plants, a condition caused by lack of iron where the leaves yellow between the veins and weaken.

Whatever fertilizers you are using on your lawn and garden, be sure to follow label instructions carefully. We have many stories of customers who have overfertilized–tree spikes placed too close to the trunk of a productive plum tree, killing it– newly planted potentillas dying because the customer put handfuls of undiluted Miracle Gro in the holes. Remember, for all types of fertilizers, more is not better. Too much fertilizer can burn or kill your plants.