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University
of Rhode Island GreenShare Factsheets
Fertilizing
Vegetable Garden Soils
Vegetable plants, like all other green plants, require several nutrient
elements for growth, development and productivity. Among the necessary
plant nutrients are carbon, hydrogen and oxygen, available from
the atmosphere and from water, and nitrogen, phosphorus, potassium,
calcium, magnesium and sulfur supplied from the mineral portion
of the soil. These elements are used by plants in relatively large
amounts and are referred to as macronutrients. Micronutrients, or
trace elements, used in small quantities, include boron, chlorine,
copper, iron, manganese, molybdenum and zinc. These 16 plant nutrients,
whether used in large or small quantities, are absolutely essential
for plant vigor and productivity. A deficiency of any of these nutrient
elements can limit plant growth and development and, ultimately,
yield. Most soils contain sufficient amounts of the micronutrients
needed to support plant growth. However, soils may be lacking in
some of the macronutrients, particularly nitrogen, phosphorus and
potassium. It is the job of the gardener, therefore, to ensure the
presence of all the essential elements supplied by the soil in the
right quantities and the right chemical forms for plant use. This
is done by supplying organic matter and by the judicious use of
fertilizers to maintain or increase soil fertility. Soil
pH is the level of acidity or alkalinity of the soil. Most vegetable
crops prefer a slightly acid pH of 6.0 to 6.8, although this varies
with each vegetable crop. Nutrient elements in the soil may be chemically
tied up or bound to soil particles and unavailable to plants if
the pH is outside of this range. Soil pH can be raised by applying
ground agricultural limestone containing calcium and some magnesium.
Lowering soil pH is achieved by adding sulfur, either in its elemental
form or as a component of some fertilizers. The
gardener cannot assume fertile soils will not require a periodic
fertilizer application. Plant nutrients are lost from the soil over
time through use by plants and by leaching with water. It is important
that these lost nutrients be replaced. Low-fertility soils will
require not only maintenance fertilizer applications to replace
lost nutrients, but also applications that will improve the overall
soil fertility and support plants at a productive level. It is impossible
to determine the fertility level and pH of garden soil by looking
at it--fertilizer and lime applications are best made based on the
results of a soil test. Contact the URI Cooperative Extension for
information on soil testing, or see the GreenShare Factsheet on
soil testing for an order form in PDF
Format.
Nitrogen is the plant nutrient most often in short supply
in the soil. It is usually necessary to supply additional nitrogen
each season. Phosphorus and potassium are also required in large
amounts. Nitrogen is essential for vigorous vegetative growth and
development. Phosphorus is necessary for good root development and
for fruit and seed production. The role of potassium is not as well
understood, but is important for overall plant development. These
are the three nutrients that are almost always present in commercial
fertilizers. Commercial
fertilizers, whether organic or inorganic, always have three numbers
printed on the bag or container. These numbers or analysis represent
the percentage of nitrogen (N), phosphorus (P2O5)
and potassium (K2O) in the product. For example
an 8-16-16 analysis fertilizer contains 8 percent nitrogen, 16 percent
phosphorus and 16 percent potassium. The rest of the product is
an inert carrier material. A complete fertilizer will have a number
for all three nutrients. Some fertilizers are incomplete, meaning
that they contain only one or two of the nutrients. Urea, for example,
has an analysis of 45-0-0, and contains 45 percent nitrogen with
no phosphorus or potassium. Inorganic or chemical fertilizers usually
have a higher analysis than most organic fertilizers and are less
expensive. The nutrients in chemical fertilizers are in a form that
is readily available to plants. Organic fertilizers contain nutrients
in forms that must be chemically changed in the soil before the
plants can use them. These nutrients, although not immediately available,
are usable to the plants over a longer period of time. Some chemical
fertilizers will contain nutrients in both readily available and
slow-release forms. Organic fertilizers are less likely to burn
plants and often contain several micronutrients in addition to nitrogen,
phosphorus and potassium.
Fertilizers are often applied over the garden area before planting
and then are tilled or spaded into the soil. Calibrated spreaders
used for lawn fertilizer application can be effectively used for
applying garden fertilizer. Do not use fertilizers containing herbicides
in the vegetable garden. Row or band applications make the most
efficient use of small amounts of fertilizer. This method allows
for fertilizer placement near the plants where it will be of the
most use. Make small furrows about three inches to each side of
the row and two to three inches deep. Use caution, however, so that
seeds or roots do not come into direct contact with the fertilizer.
In lieu of a soil test recommendation, apply two to three pounds
of fertilizer per 100 feet of row. Select the fertilizer analysis
according to the type of vegetable grown. For leafy vegetables,
high nitrogen fertilizers such as 12-12-12 or 15-15-15 analyses
are appropriate. For vegetables grown for their fruits, seeds, roots
or bulbs, fertilizers such as 6-24-24, 6-12-18 and 8-16-16 or equivalent
are satisfactory. Growing
healthy, productive plants in the vegetable garden involves attention
to many cultural details. Since fertilization is just one of these
details, fertilizers should not be considered a cure-all for all
gardening problems. When properly used, fertilizers are extremely
valuable in obtaining good gardening results. Fertilizers cannot
correct or improve poor soil structure that requires additions of
organic matter, nor can it compensate for an incorrect pH or unfavorable
weather conditions. Choosing inappropriate vegetable cultivars and
not practicing weed or pest control cannot be corrected by applying
fertilizers. The use of fertilizers is just one of a number of integrated
practices that lead to gardening success. Adapted
from Marianne Riofrio, Ohio State University Extension, 2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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