Hartley Magazine

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Fertilizing Greenhouse Plants

Plant fertilizer

Because potting mediums rarely provide enough nutrients for long-term growth, most greenhouse plants need fertilization. Plants three main macronutrients (those needed in relatively large amounts) are nitrogen (N), phosphorus (P), and potassium (K). On the fertilizer package, the proportion of each is given by a numerical ratio, with the order always being N to P to K. For example, a ratio of 10-5-5 means that the fertilizer has twice as much nitrogen by weight (10%) as phosphorus (5%) and potassium (5%).

The particular N-P-K ratio depends on the fertilizer’s purpose. A fertilizer high in N is intended to enhance chlorophyll and stimulate leaf growth, thereby aiding photosynthesis, while one high in P is designed to promote root growth as well as flower, fruit, and seed development. A fertilizer high in K (also known as potash) is meant to make a plant more vigorous because K is involved in many essential plant processes. Receiving sufficient K will make a plant less susceptible to environmental stresses that could stunt its growth or increase its vulnerability to disease. For example, a rose fertilizer might be a more balanced 3-4-3, while an orchid fertilizer 11-35-15 is intended to aid root growth in plants usually potted in wood bark chips. Of course, beyond N, P, and K, plant micronutrients are important, too, but they’ll be covered in the next issue.

Plants should be fertilized only during their active growing season, and always read the package instructions to make sure you use the right amount. Many people think that if a little fertilizer is good, a lot must be better, but it isn’t. In fact, an overload of fertilizer, especially a synthetic one, can actually harm plants. This is because the fertilizer’s soluble salts that deliver the nutrients became present in the soil in such a high concentration that the plant roots can no longer take up water. They may even release water. The aboveground symptoms are bottom leaves that turn yellow and wilt, while others leaves get brown and dry, especially at the margins. Beneath the soil surface the roots are also browning and eventually may rot. Letting the soil get drier isn’t a solution because that will only increase the soluble salt concentration. The best remedy at this stage is to repot the entire plant.

While no fertilizer is good for plants in very high doses, organic fertilizers can have certain benefits over synthetic ones. Organic fertilizers such as blood meal (12-0-0) and bone meal (2-14-0) don’t just feed plants. They also promote the activity of microorganisms, which over time creates the soil structure in which plants can thrive. Granted, organic fertilizers feed plants much more slowly than synthetic ones do. They don’t give plants the quick boost of growth and color that synthetic fertilizers provide. But if you don’t plan to keep renewing your soil by periodically repotting your plants, organic fertilizers may be the better long-run option.

Another choice is between fertilizers in liquid or dry form. For smaller plants in smaller pots, I usually use dry fertilizer dissolved in water. For larger plants, such as my citrus trees, I typically apply granular fertilizer to the soil and water it in. Not all the granules dissolve at once, thus the plant gets fertilized a little more slowly. But this is simply a personal preference. Either way, the plants receive the same nutrients.