The bulk of a plant's feeder roots — whether it's a midget marigold or a mighty oak — lie just beneath the surface, so generally there is no need to dig fertilizer deep into the soil. Anyway, low oxygen levels there would retard microbial growth, which is necessary to unlock nutrients from most organic fertilizers.
TO DIG OR NOT TO DIG
An exception to that "no dig" rule is when phosphorus levels are low, as indicated by a soil test or stunted plants that are purplish when young or late to ripen. (Cold soil in spring also can cause a phosphorus deficiency, a temporary one that abates as soon as soil warms and roots start reaching out.) 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 6 to 12 inches of soil.
Once a soil is up to snuff with phosphorus, periodic surface applications can trickle down through the soil fast enough to maintain adequate levels throughout the root zone.
TIME YOUR FEEDING
When should you apply organic fertilizers? Remember that the nutrients in most of them are initially insoluble and in forms that plants cannot use. Account for the time lag between application and nutrient release by spreading organic fertilizers a few weeks before planting. Even a few months before planting, or way back late last fall.
Because soil microorganisms need time, warmth and moisture to release nutrients from organic fertilizers, plants may have to wait to eat in dry soil. Of course, plants grow but little in dry soil, so their fertilizer needs are less. 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 then, 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. Leaves can absorb nutrients directly, and for a really quick effect, you could spray a soluble organic fertilizer such as seaweed extract or fish emulsion right on leaves. Avoid plant injury by reading label directions and following specified rates carefully.
Consider using quick-acting fertilizers as quick fixes only. Build up good reserves of nutrients in your soil and such applications will be unnecessary.
Consider the slow action of organic fertilizers as a benefit. You only need to apply them once a year and, because heat and warmth spur microbial activity and plant growth, the nutrients are released in sync with plant needs.
As I point out in the fertilizer section of my book "Weedless Gardening" (Workman Publishing), spreading an inch of compost or a few inches of leaves, wood chips or some other organic mulch over the ground each year will usually provide all the nourishment your plants need.
The hungriest parts of the garden are vegetable and formal flower beds, so I like to feed the ground there with compost, which is an organic material relatively rich in nutrients. Less needy are trees and shrubs, informal flowers and wildflowers; here, any organic mulch, from wood ships to straw to pine needles, will suffice. Over the years, the compost or other organic mulches will enrich the soil to offer a spectrum of nutrients available to plants, a much wider spectrum that would be available from any chemical fertilizer.
In naturally poor soils, some additional, more concentrated, nitrogen fertilizer might — just might — also be needed for a year or more until the soil is up to snuff. Soybean meal or alfalfa meal is usually sold as an animal feed, but sprinkled over the ground just before some organic mulch is applied it's a convenient, nourishing and organic "feed" for plants also. Apply a couple of pounds per hundred square feet.