Where to Feed, With an Exception

To get the most out of any organic fertilizer, keep in mind how plants feed and how these fertilizers act in the soil. The bulk of plants’ feeder roots — whether the plants are midget marigolds or mighty oaks — lie just beneath the surface, so generally there is no need to dig fertilizer deep into the soil. And anyway, low oxygen levels there would retard the microbial growth necessary to unlock nutrients from most organic fertilizers.

Make exception to that “no dig” rule if phosphorus levels are low, as indicated by a soil test or stunted plants that are purplish when young or late to ripen. Phosphorus moves very slowly in the soil, so the only way to get it quickly into the root zone is to mix it into the top six inches of soil. Once a soil is up to snuff with phosphorus, periodic surface applications can trickle down through the soil profile fast enough to maintain adequate levels throughout the root zone.

Because phosphorus is only slightly soluble in water, plants take it up by root intercept, that is, by growing to the phosphorus. (Most other nutrients are soluble and moved about within the soil by water.) In spring, cool soil temperatures might slow root growth enough for a plant to show that it’s hungry for phosphorus, even though enough phosphorus is present in the soil. Usually, you and your plants can just wait things out until the soil warms and the temporary symptoms subside.

Meal Times

As far as when to apply organic fertilizers, keep in mind that the nutrients in most of them are initially insoluble and in forms that plants cannot use immediately. Account for the time lag between application and nutrient release by spreading organic fertilizers a few weeks before planting. Or a few months before planting! You could even have applied organic fertilizer way back in late autumn.

Over time, even wood chips nourish the soil and plants, here applied in fall

Because soil microorganisms need time, warmth, and moisture to release nutrients from organic fertilizers, plants may have to wait to “eat” if the ground is dry. In this case, watering not only quenches a plant’s thirst, but also makes food available.

  Occasionally, you may have to tailor your fertilizer to special conditions. For instance, a spell of unseasonably cool weather in spring slows microbial activity. If you must spur plant growth at that time, apply a light application of some soluble organic fertilizer whose nutrients are quickly available — blood meal or fish emulsion, for example.

A quick-acting fertilizer might also be needed when a plant is so hungry that it actually shows symptoms of starvation, such as yellowing, older leaves, and needs some fast food. Leaves can absorb nutrients directly, and for a really quick effect, spray seaweed extract or fish emulsion right on leaves. Avoid plant injury by reading label directions and following specified rates for leaf sprays carefully.

Compost tea is a tea for plants made just the way it sounds, by steeping a porous bag of compost (in burlap, for example) in water until the water turns dark brown. Like seaweed extract, compost tea is a relatively weak fertilizer. Some gardeners value microbes in the tea and spray it on leaves mostly for pest control. The jury is out on this use of compost tea.

Straining compost-water mix for making compost tea

Consider use of quick-acting or foliar fertilizers as quick-fixes only. Build up good reserves of nutrients in your soil and such applications will be unnecessary.

Looking Long Term, With a Couple More Exceptions

Spreading an inch of compost, or a few inches of leaves, wood chips, or some other organic mulch over the ground each year is a great way to build up that nutrient reserve and might provide all the nourishment your plants need, depending on what you grow. The hungriest parts of the garden are vegetable and formal flower beds; give them compost. Less needy are woody plants, mixed flower borders, and wildflowers; give them any organic mulch.

Over the years, the compost or other organic mulches will enrich the soil to offer a spectrum of nutrients available to plants. That’s in addition to providing other benefits, such as improving water flow through and air flow into the ground, and nurturing beneficial microorganisms for pest resistance.

After decades of applications, mulch provides all the fertility my blueberries need

In naturally poor soils, some additional, more concentrated, nitrogen fertilizer might — just might — also be needed. If that’s the case, my go-to fertilizer is soybean meal. Small bags are available at garden centers; larger ones at feed stores. (It’s mostly used as an animal feed.) Sprinkled over the ground once a year just before mulching, is convenient, nourishing, and organic. Apply a couple of pounds per hundred square feet.

After a few years of building up levels of organic matter in your soil, continued additions of organic matter should provide all your plant’s needs. Of course, I’ve got to mention a couple of exceptions here also: One especially rainy season where rainfall was 8 inches (16 percent!) more than usual, many garden plants looked hungry for nitrogen, possibly because much of the soluble nitrogen had washed out of the ground; so I sprinkled some nitrogen-rich, organic fertilizer over the beds. And I always add soybean meal to my home made potting mixes, which have to provide nourishment and other benefits to root growth limited by the size of the pot.

(More details on using fertilizers can be found in my book WEEDLESS GARDENING.)