AIKEN MASTER GARDENERS: May perfect month to apply fertilizer for productive gardens

·5 min read

May 9—10-10-10. That's fertilizer, right? Wait, what? One can tailor uses of fertilizers by using chemistry and math. As my 10th-grade algebra teacher could attest, I'm not particularly strong in mathematics. I am always wary with formulas but by understanding the chemical symbols of N-P-K on a fertilizer bag, and then applying basic math, I can usually figure out when and how to use fertilizer. Research and note-taking help me to use fertilizers. May is perfect to apply amendments for a productive gardening season. April showers bring May flowers, and May applications will prolong the satisfactions of a garden.

Sixteen nutrient elements are essential for growth and reproduction. Plants obtain the three most abundant nutrients — carbon (C), hydrogen (H) and oxygen (O) — from water and air. The other 13 elements fall into three categories: Primary, secondary and micronutrients. Nitrogen (N), phosphorus (P) and potassium (K) are primary nutrients needed in fairly large amounts. Calcium (Ca), magnesium (Mg) and sulfur (S) are secondary nutrients required in lesser quantities. Zinc (Zn), manganese (Mn), iron (Fe), boron (B), copper (Cu), molybdenum (Mo) and chlorine (Cl) are micro-nutrients required in trace amounts.

Fertilizers that contain all of the primary elements are considered complete, but formulations vary widely, so a wise gardener reads labels on amendments to avoid over-application and overpaying.

Clemson's Home and Garden Information Center is a useful source of information. Fertilizers can be divided into two broad groups: Organic and inorganic. Organic fertilizer such as compost is derived from a living plant or animal source. Inorganic or synthetic fertilizers are usually manufactured but have the advantage of lower cost. The most commonly used synthetic fertilizers consist almost entirely of nitrogen, potassium and phosphorus in readily utilized form. In contrast, organic fertilizers likely have significant micronutrients, but the macronutrients may not be readily absorbed. Synthetic fertilizers are often deemed "fast" while organic fertilizers tend to be "time-release."

One significant drawback with organic fertilizers is that they may not release enough principal nutrient such as nitrogen quickly enough for a plant's best growth. Organic fertilizers depend on soil organisms to release nutrients, so are more effective when soil is moist and soil temperature is warm enough for soil organisms to be active. Whether synthetic or organic, each bag of fertilizer displays primary nutrients as a ratio of N-P-K.

All plants need nitrogen. This element is not reliably stored in soil. Nitrogen is easily absorbed in water and thus highly variable depending on moisture and factors like temperature. Plants need nitrogen to kick start growth, and plants like corn, tomatoes and roses benefit from regular applications of nitrogen throughout the growing season. Nitrogen accounts for rapid growth and the lovely green color of plants. Nitrogen deficiency can cause yellow leaves. Because nitrogen absorption is temperature sensitive, apply fertilizers with high rates of nitrogen no earlier than May — especially to lawns — and no later than mid-August. When temperatures climb above bearable levels, applying nitrogen will not help growth.

In contrast, phosphorus binds with and stays in soil. This chemical element is responsible for root growth and fruit set. Super-phosphate — with a ratio of 0-46-0 — is recommended when planting bulbs and new roses, helping to establish new roots. On turf grass and in most established gardens, phosphorus accumulates over time. Check that middle number signifying P on the fertilizer bag. When fertilizing an established garden or bed, select a formula like 16-4-8, with a lower P level, unless phosphorus is desirable, such as on roses. I use a systemic formula of 6-9-6 on my rose bed every six weeks.

The third essential element of potassium helps plants with photosynthesis at the cellular level, adding strength to cell walls, aiding in water movement, and providing disease resistance and resilience to stress. This element's symbol of K comes from the Latin word kalium. Potassium gives plants support for the long haul of the growing season.

Now that we have covered soil chemistry, let's apply some math. I can usually figure out the amount of fertilizer needed when I want to cover a 100- or 200-foot area. In a 100-pound bag of fertilizer labeled 10-10-10, there are 10 pounds of N, 10 pounds of P, and 10 pounds of K along with filler material. A 10-pound bag will contain 1 pound of N, 1 pound of P, and 1 pound of K. For another example, the ratio of 16-4-8 fertilizer is 4:1:2 or 4 parts nitrogen to 1 part phosphorus to 2 parts potassium. Fertilizer labeled 16-4-8 typically costs more but will cover more area than a lower analysis.

In the vegetable garden I often start with that familiar ratio of 10-10-10. For tomatoes, a handful in each hole will kick start plant growth with nitrogen and root growth with potassium. An added handful of well-decomposed compost will slowly release secondary elements and micro-nutrients. Too much fertilizer — either synthetic or organic — can burn young plants, so I am judicious. Remember that fertilizers can contain weed killer or insecticides. For instance, I use fertilizer containing insecticide and fungicide on my roses with that 6-9-6 formulation to promote root growth and discourage beetles and blackspot, but I would never apply that particular fertilizer to vegetables. Always read labels, especially for turf grasses. To make the task easier, before application, rely on a soil test's recommendations. Fertilizers are pricey, so do some research before heading out to the store. Rely on the advice of good nursery staff and call the Aiken Master Gardeners if more advice is desired.