Gardeners who want to grow organically face a few extra challenges when selecting fertilizer, although with a little extra care the nutrients that plants need can still be provided at a reasonable cost.

The biggest challenges are understanding the need for fertilization (or lack of), recognizing organic sources, and deciphering the supply chain.

Why fertilize plants?

The reasons to fertilize really depend on plant species, the gardener’s goals, and the site. For example, a gardener who wants to produce an abundance of tomatoes in poor soil will likely be more successful by fertilizing. A gardener who wants a nice crop of apples year after year can better sustain the tree by fertilizing. In contrast, an herb gardener may have healthier plants without adding extra fertilizer.

Generally speaking, plants that produce heavily need more nutrients to sustain production. Vegetables, fruit trees, small fruits such as blueberries and strawberries, and annual flowers produce larger fruit, flowers, pods, leaves, etc., when fertilized. (Although there is a fine line — too much fertilizer can encourage heavy foliar growth and spindly, rapidly growing plants.) Plants that produce very little, only produce for a short period of time, grow slowly, or are mostly ornamental generally have much lower nutrient requirements.

Organic fertilizer can be useful but isn't always necessary.

Organic sources and the supply chain

Fertilizer is something that is added to the soil to provide nutrients. “Organic” just means the fertilizer is derived from a natural source instead of being man-made. Composted manure, other composted materials, plant meal products, seaweed products, and rock phosphates are common examples.

Manure and compost are the most readily available and provide many benefits to the soil in addition to adding nutrients. If getting manure or compost from a non-commercial source, ask lots of questions regarding practices. Manures and compost can carry unwanted residues and byproducts, especially if passed along without being properly finished.

Plant meals, seaweed or kelp, and even commercially bagged manures and compost are becoming more common options in retail centers. One thing to keep in mind is that the products may be shipped in from hundreds of miles away, somewhat defeating the purpose of purchasing organic.

Best bets

Base choices on the crop/plant and its nutrient needs. Have the soil tested to determine what nutrients are already there. Soil testing is available through local county Cooperative Extension offices, private regional labs, and home testing kits. Look for local sources of fertilizer that provide the nutrients needed and apply just enough to enhance plant growth. Look for products with actual nutrient content listed instead of just advertising soil conditioning or improvement.

For nitrogen, plant meals and animal byproducts like bat guano and fish emulsion typically provide the most nutrient. Bone meal and rock phosphate are the best organic sources for phosphorus. Alfalfa pellets and some kelp products are the best bet for supplementing potassium.

Organic fertilizers often improve drainage and microbial activity in the soil in addition to providing nutrients, which also leads to healthier plants.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation. She is the host of “The Garden Show” and has been a gardener since childhood. Send your gardening questions and feedback to features@ljworld.com.