Spring is almost upon us, and with the season comes a rush of new plant growth and the urge to spend time in our gardens. Although fertilizer might seem a dry topic, give it some thought before you visit your favorite nursery.
Fertilizing plants is often equated with “feeding” them, but plants produce their own food through photosynthesis, using the sun’s energy to convert carbon dioxide and water to sugars and oxygen. Fertilizers do, however, provide plants with essential nutrients for cell development, function, and growth.
The basic purpose of fertilizers is to replace soil nutrients that deplete over time. Soil composition and pH have a direct effect on what nutrients can be absorbed by plants and how efficiently, so it’s wise to do a basic soil test before choosing or using a fertilizer to remedy any apparent nutrient deficiencies.
Fertilizers come in two basic types: organic (those derived from natural sources, including plant compost, animal manure, fish emulsion, and bone meal) and inorganic (composed of synthetic chemicals). Organic fertilizers have many benefits: they release nutrients over a long period; they improve the structure and water-holding capacity of soil; and they have a complex profile of macro- and micronutrients. The downside is that they’re more expensive and can vary in content or quality. Inorganic fertilizers release nutrients quickly, are consistent in composition, and are less expensive, but they don’t improve soil quality or contain as many nutrients.
Every fertilizer label shows something called the “N-P-K” ratio, which indicates the percentage by volume of nitrogen (N), phosphorus (P), and potassium (K). A “balanced” fertilizer has equal amounts of each of these three plant macronutrients; for example, a product labeled 16-16-16 has 16% N, 16% P, 16% K, and 52% other ingredients. A “complete” fertilizer contains all three major nutrients; an “incomplete” fertilizer has only one or two of them.
You might think, “If I use more fertilizer than recommended, my plants will grow even better.” No! Many serious problems can result from overuse of fertilizers. Too much fertilizer can burn plant roots and foliage; surplus nitrogen can leach into and pollute water; and excess nutrients can over-stimulate plant growth, leading to an unnecessary cycle of frequent pruning and stressed, disease-susceptible plants. Fertilizers should always be applied according to the instructions.
It’s also tempting to try shortcuts, and rationalize, “One kind of fertilizer will be fine for all my plants.” Wrong! Plants have unique nutrient requirements, and different fertilizers are formulated for different purposes. For example, many plants native to Australia can be harmed or killed by phosphorus-containing fertilizers, while other plants need phosphorus to thrive. A fertilizer intended for citrus trees is different from that designed for acid-loving azaleas and camellias… and so on.
Timing is another key consideration: season, rainfall, planting dates, and other factors are important in determining the right time to fertilize. If deciding when to fertilize shrubs and trees, their age, maturity, and species should be considered. Lawns are still another matter, and homeowners tend to apply fertilizer on lawns unnecessarily and wastefully.
An effective alternative to commercial lawn fertilizers is the easy practice of “grasscycling.” Grass clippings become natural fertilizer when they’re allowed to remain in place after mowing – they decompose and return nutrients to the soil. Grasscycling is often thought of as a relatively new sustainable practice, but its benefits have been known for many decades. I recently stumbled across a 1924 article from a Midwestern newspaper that cited this advice from the U.S. Department of Agriculture: “Cuttings should begin early with the lawn mower set as high as possible and should be repeated frequently. The clippings should all remain upon the lawn. The more of these clippings that can be retained about the roots of the grass the better the chances for a good lawn.” Nowadays, lawnmowers can be fitted with special mulching blades to make the process more efficient and the clipping size small.
By now, it should be clear that fertilizing is a very complex topic with far more detail than can be covered here. Before you fertilize, make sure that you—or those you hire to care for your garden—fully understand the specific goal of fertilizing; the product that will best meet that goal; and the proper rate, method, and timing for applying the chosen fertilizer.
Two excellent online resources are “A Gardener’s Guide to Fertilizing Trees and Shrubs” (NC State University) and “The UC Guide to Healthy Lawns” (UC IPM). You can also consult the California Master Gardener Handbook (Chapter 3 – Soil and Fertilizer Management) for in-depth guidance.
For advice on gardening related questions, call the UC Master Gardener office at 209-953-6112, or use our website.