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Choosing seedling fertilizers

© Kenneth Joergensen

Growing seedlings indoors require an active fertilizer program to produce healthy transplants.

What makes a good seedling fertilizer? When selecting fertilizers it is important to understand the goal with fertilization. When you buy petunias at your local garden center a primary goal will be to boost flowering and induce rapid growth so that the plants in no time can fill your entire flower bed. Indoors when fertilizing seedlings the goal is somewhat different, however.

The plants should: 1) have all essential nutrients to grow, 2) develop a good large, healthy root system, and 3) remain healthy, toned, and compact which makes handling indoors easier and provide for better garden performance outdoors later.

A Complete Fertilizer When choosing fertilizer we tend to discuss only the 3 macro nutrients: Nitrogen, Phosphorus and Potassium (N-P-K) but plants need a complete range of fertilizer nutrients including Magnesium, Calcium, Sulfur, and other trace elements. No single nutrient is less important than another, some nutrients are simply needed in greater quantities. Plants can suffer equally from a boron deficit as a nitrogen deficit despite boron only represents a fraction of the dry weight of a plant.

Seedlings are also typically grown in so-called "soil less" planting mediums (mixtures of peat moss sphagnum, perlite and vermiculite) which have no trace elements unlike soil. When choosing a fertilizer it is important to ensure it contains a complete range of nutrients (tip: the label may say "contain all essential trace elements").

N-P-K Fertilizer Ratios. Traditionally a high phosphorus fertilizer has been recommended for seed starting. This advice is reinforced by the fertilizer industry through the marketing of the so-called "Starter Fertilizers" also with high phosphorus. The mistaken assumption is that specific nutrients are responsible for isolated tasks such as phosphorus being responsible for root growth, potassium for flowering, etc. The nutrient interaction is much more complex, however. Just because phosphorus plays an important part in root development it does not mean that plants automatically require very high levels.

Researchers have noted that bedding plants generally require less than 10 ppm phosphorus for healthy seedling growth. This can be adequately provided using fertilizers such as 20-5-20, 13-2-13, etc. Much lower levels than the typical 15-30-15 or 20-20-20 fertilizers used by home growers.

Indoors in low light and moderate temperatures high levels of phosphorus can lead to stretching making the plants look untidy and difficult to handle. Phosphorus also induce premature flowering which can lead to poor performance later.