Many gardeners, especially those in warm areas, are still planting bulbs, trees, and shrubs, and adding soil amendments such as compost and fertilizer when planting. But the most critical additive may be fungi.
Mycorrhizae are fungi that colonize plant roots, and it's estimated that 90 percent of the plant species in the world have them, including vegetables, flowers, bulbs, trees, and shrubs. Mycorrhizae aid plant growth in several ways: they help plants absorb and hold major and minor nutrients, increase tolerance to stress and drought, and inhibit disease organisms in the soil.
While mycorrhizae are naturally occurring in healthy soils, they are deficient in many disturbed soils, soils on construction sites, and in plants grown in nurseries. You can buy special mycorrhizae formulations, and fertilizer manufacturers have been adding mycorrhizal fungi to their products as a way to boost plants' performance.
A few things should be kept in mind when using these products. Before sprinkling mycorrhizae around plants, check the soil's phosphorus levels. High phosphorus in soils inhibits mycorrhizae formation, making the application a waste of money. Also, mycorrhizae are inhibited by chemical fertilizers but not by organic fertilizers, so don't apply chemical fertilizers when planting. Well-decomposed compost doesn't harm mycorrhizae, but fresh compost may inhibit their growth. Only one application of mycorrhizae is necessary per plant for its lifetime.
Finally, there are different strains of mycorrhizae; trees and shrubs benefit from a different strain than annuals and perennials, for example. And plants in the Ericacea family, such as blueberries and rhododendrons, need their own specific strain. So check the label to make sure you're getting the right product for whatever you're growing.