Soil type dictates the frequency of fertilizer
application. Sandy soils require more frequent applications of
nitrogen and other nutrients than do clay-type soils. Other
factors affecting frequency of application include the type of
crop, the level of crop productivity required, frequency and
amount of water applied, and type of fertilizer applied and its
The type of crop influences timing and frequency of application since some
crops are heavier feeders of particular nutrients than others.
Root crops require less nitrogen fertilization than do leafy
crops. Corn is a heavy feeder of nitrogen, while most trees and
shrubs are generally light nitrogen-feeders. Corn may require
nitrogen fertilization every four weeks, while most trees and
shrubs perform nicely with one, good, well-placed application
every year or two. A general rule of thumb is that nitrogen is for
leafy top growth; phosphorus is for root and fruit production; and
potassium is for cold hardiness, disease resistance, and general
Proper use of nutrients can control plant growth rate
and character. Nitrogen is the most critical nutrient in this
regard. If tomatoes or squash are fertilized heavily with a
nitrogen fertilizer into the summer, the plants may be all vine
and no fruit. If slow-release fertilizers or heavy amounts of
manure are used on crops that form fruit or vegetables, leaf and
vine growth will continue into late summer, and fruit and
vegetable development will occur very late in the season.
Remember that a nitrogen application will have its
greatest effect for three to four weeks after application. If
tomatoes are fertilized heavily on April 1, there may be no flower
production until May 1, which will, in turn, delay fruit ripening
until late June. For this reason, it is important to plant crops
with similar fertilizer needs close together to avoid improper
rates of application.
Late fertilization (after July 1) of trees and shrubs
can cause new flushes of growth to occur on woody plants that are
normally adjusting themselves for the coming winter. This may
delay dormancy of woody plants and cause severe winter dieback of
the new growth.
The following suggestions about groups of garden plants
are given as general guides. Gardeners should be aware that
individual species within these groups vary considerably. After
each group of plants, the need for the primary nutrients
(nitrogen, phosphorus, and potassium) is indicated as high,
medium, or low.
Deciduous shade trees
Evergreen shade trees
Medium to Low
Medium to High
Medium to Low
Medium to Low
Medium to Low
Application MethodsThere are different methods
of applying fertilizer depending on its formulation and the crop
Broadcasting A recommended rate
of fertilizer is evenly spread over the growing area and left to
filter into the soil or is incorporated into the soil with a
rototiller, or spade, with irrigation water. Broadcasting can be
used over large garden areas or when time or labor is limited.
Banding Narrow bands of
fertilizer are applied in furrows 2 to 3 inches from the garden
seeds and 1 to 2 inches deeper than the seeds or plants. Careless
placement of the fertilizer band too close to the seeds will burn
the roots of the seedlings. The best technique is to stretch a
string where the seed row is to be planted. With a corner of a
hoe, dig a furrow 3 inches deep, 3 inches to one side, and
parallel with the string. Spread one-half the suggested rate of
the fertilizer in the furrow and cover it with soil. Repeat the
banding operation on the other side of the string, then sow seeds
underneath the string.
For widely spaced plants, such as tomatoes, fertilizers
can be placed in bands 6 inches long for each plant or in a circle
around the plant. Place the bands 4 inches from the plant base. If
used in the hole itself, place the fertilizer at the bottom of the
hole, work it into the soil, and place a layer of soil about 2
inches deep over the fertilized soil before putting the plant in
Banding is one way to satisfy the needs of many plants
(especially tomatoes) for phosphorus as the first roots develop.
When fertilizers are broadcast and worked into soil, much of the
phosphorus is locked up by the soil and is not immediately
available to the plant. By concentrating the phosphorus in a band,
the plant is given what it needs even though much of the
phosphorus stays locked up.
Starter solutions Another way to
satisfy the need for phosphorus when setting out transplants of
tomatoes, eggplant, peppers, or cabbage is through the use of a
liquid fertilizer high in phosphorus, as a starter solution.
Follow directions on the label.
Side-Dressing Dry fertilizer is
applied as a side dressing after plants are up and growing.
Scatter fertilizer on both sides of the row 6 to 8 inches from the
plants. Rake it into the soil and water thoroughly.
Foliar Feeding Foliar feeding is
used when insufficient fertilizer was used before planting; a
quick growth response is wanted; micronutrients (such as iron or
zinc) are locked into the soil; or when the soil is too cold for
the plants to use the fertilizer applied to the soil.
Foliar-applied nutrients are absorbed and used by the plant quite
rapidly. Absorption begins within minutes after application and,
with most nutrients, it is completed within 1 to 2 days. Foliar
micro-nutrient fertilization can be a supplement to soil nutrition
at a critical time for the plant, but not a substitute. At
transplanting time, an application of phosphorus spray will help
in the establishment of the young plant in cold soils. For
perennial plants, early spring growth is usually limited by cold
soil, even when the air is warm. Under such conditions, soil
microorganisms are not active enough to convert nutrients into
forms available for roots to absorb. A nutrient spray to the
foliage will provide the needed nutrients immediately, allowing
the plants to begin growth. Foliar application is also an
excellent method for applying micro-nutrients in high pH soils.
Micro-netrients applied in this manner are not tied up in the
soil, but are directly available to the plant.