Soil Fertility for Field-grown Cut Flowers

August 9, 2002 - 09:44

Field-grown cuts can be a great bridge between spring bedding plants and holiday specialty crops, providing much-needed revenue during the slow summer months.

Growing annual and perennial flowers for cutting during the
summer is becoming a popular way for greenhouse operators to bridge the spring
bedding plant and holiday potted crop seasons. In many respects, outdoor flower
crops, particularly annuals, require similar soil fertility management
practices as vegetables and other outdoor field crops.

An effective fertility program begins with fall and early
spring soil management practices, but right now, with the 2002 crop already
planted, the most important practices involve providing adequate nutrition to
the plants as they grow and flower.

The major goals for an outdoor cut flower fertilizer program
are to provide adequate levels of nutrients for vegetative growth early in
development, promote the development of strong stems and encourage a consistent
yield of good flowers throughout the cutting season. In addition to producing a
good crop, another important goal is to use fertilizers in a responsible manner
to maintain soil health and avoid environmental contamination.

Fertilizing Cut Flowers After Planting

Generally, nitrogen fertilizer for cut flowers should be
applied on the basis of 1-2 lbs. of actual nitrogen per 1,000 sq. ft. Some
large and fast-growing annuals like sunflowers may need more, and a slightly
higher rate may be needed on sandy soils or where irrigation or rainfall rates
are high. Generally, nitrogen fertilization involves split applications or
applications targeted at the main growth period.

Adequate nitrogen is critical for cut flower production. A
nitrogen deficiency will result in poor plant growth, a reduction in flower
yield and the appearance of foliar chlorosis. Too much nitrogen may result in
too much vegetative growth, cause weak stems and delay flowering.

Unfortunately, nitrogen is required in the greatest amount
by plants and is easily lost from the soil by leaching or in the form of a gas.
For these reasons, nitrogen fertilizer must be applied each year to maintain
adequate levels for production. Leaching of nitrogen as nitrate can be a
serious problem not only as a way of losing nitrogen for the crop, but also
because it can pollute groundwater and surface bodies of water. Too much
nitrogen fertilizer, high rainfall or excess irrigation, and sandy soil are all
factors favoring nitrate leaching. 

Nitrogen fertilization also affects soil pH. Over time, the
continuous use of ammonium or urea fertilizers tends to lower pH, and limestone
may be required to neutralize the acidity.

Fertilization for Annuals

Germinating seeds and young seedlings are very sensitive to
fertilizer injury, so initial fertilizer applications should be very small.
Starter fertilizer should be applied at a rate of about 1/4-1/2 lb. of actual
nitrogen per 1,000 sq. ft. at seeding either by broadcasting or banding. Bands
should be placed about two inches below and to the sides of the seeds.

For annual species started from seed in the field, fertilizer
should be applied after the young plants are well-established, according to the
following recommendations for annual transplants.

When transplants from the greenhouse are used to start the
crop, about 20-30 percent of the total nitrogen requirement should be applied
in a preplant application made just before planting, assuming the transplants
are healthy and have been hardened-off. Another 20-30 percent of the total
should be applied as a sidedress application when the transplants are about
8-10 inches tall.

Sometimes, one application of fertilizer at planting can
grow a good crop of annual cuts. However, split applications of nitrogen
fertilizer are more efficient in fields receiving heavy irrigation or rainfall
or that have sandy, well-drained soil. Split application provides a more
consistent supply of nutrients than one large application made at planting and
comes closer to meeting the actual needs of annuals as they develop.

Fertilization for Perennials

After flowering or harvest, most perennials continue to
grow, building the root system and expanding the crown or other overwintering
structures. So some perennials may benefit from a second fertilizer application
after flowering. About 1-2 lbs. of actual nitrogen per 1,000 sq. ft. is
required by peony. One-half should be applied at emergence, one-fourth just
after flowering and one-fourth at dormancy. Asiatic and Oriental lilies grown
as perennials should be fertilized by sidedressing with 2-3 lbs. of 10 percent
nitrogen fertilizer per 100 ft. row as the shoots begin to emerge from the soil
and again at the same rate right after flowering. Late-season fertilizer
applications to perennials are somewhat controversial. Some feel that
fertilizing in the fall may disrupt the plants’ normal preparations for
dormancy and make them susceptible to cold injury, while others think there is
a benefit to the roots and crowns. I recommend using caution and avoiding high
nitrogen applications; however, applying phosphorus may be beneficial to root
and crown development. Also, it would be safe to apply limestone if it is
needed to adjust pH.

What about phosphorus and potassium? Just like in the
greenhouse, complete nitrogen/phosphorus/potassium (NPK) fertilizers are
applied to outdoor cut flowers according to the nitrogen requirement. Ideally,
however, phosphorus and potassium should only be applied if their need is
indicated by a soil test. So rather than using 10-10-10, a grower could use
single element carriers such as ammonium nitrate, preplant superphosphate and potassium
chloride to fulfill the exact needs according to a soil test.

The effects of phosphorus on cut flower production are less
obvious than those of nitrogen. A phosphorus deficiency may result in smaller
plants and shorter flowering stems, but foliar symptoms may not be apparent.

Less phosphorus than nitrogen is needed by plants, and
phosphorus is more stable in the soil than nitrogen. Some agricultural soils
test “very high” or “excess” in phosphorus because of
the regular use of NPK fertilizers over many years. Generally, the potential
harm caused by high phosphorus is not to the plants but to the environment. Too
much phosphorus can lead to runoff to surface bodies of water. This encourages
excess growth of algae and other aquatic plants, which leads to a serious
decline in water quality. Ideally, nutrients should be applied based on need as
determined by a soil test. Unlike nitrogen, an application of phosphorus may
not be needed every year.

Superphosphate is a soluble phosphorus fertilizer and is best
used to correct a low-phosphorus condition before planting. Rock phosphate is
very insoluble and is best used for the long-term maintenance of phosphorus in
the soil.

Potassium deficiency causes marginal chlorosis and burning
on the lower leaves first. Unless the deficiency is severe, it may not affect
the leaves on the Á harvested stems, but it could reduce overall yield.
Potassium deficiency is most likely to occur on sandy soils low in clay and
organic matter.

It may not be necessary to apply potassium fertilizer to
some clay soils and soils high in organic matter every year. There is no harm
to the environment from excess potassium and no direct harm of high potassium
to most plants. However, too much potassium can depress the uptake of calcium
and magnesium, sometimes to the point that deficiencies of these elements

Cover crops and organic fertilizers

Once a cut flower crop has been harvested, there may be some
growing time left in the late summer and early fall in regions having cold winters.
Rather than leaving the soil fallow, this would be a good time to grow a cover
crop. Cover crops absorb residual nutrients left in the soil, add organic
matter, may add nitrogen to the soil and may help protect the soil from erosion
over the winter. In New England, winter rye, oats and hairy vetch are popular
cover crops; however, different species may be recommended in other parts of
the United States. Hairy vetch is a legume cover crop that fixes nitrogen from
the atmosphere and may add as much as 100 lb. of nitrogen per acre when planted
in August and plowed down in the spring.

There are many “organic” alternatives to
chemical fertilizers that can be used to fertilize outdoor cut flowers. Composts,
animal manure, dried blood, bone meal, rock phosphate and seaweed products can
all be used successfully, keeping in mind that most of these have much lower
nutrient analyses and lower solubility than commercial fertilizers. With an
organic fertilizer, a much larger weight of material must be applied to supply
the same amount of nutrients compared to a chemical fertilizer. For example, to
supply 2 lbs. per 1,000 sq. ft. of actual nitrogen using horse manure
(0.6-0.3-0.5) 333 lbs. would be needed versus 20 lbs. of 10-10-10. In theory,
assuming that all the nitrogen in the manure is soluble, plant response to
nitrogen from both materials should be about the same.

A successful organic fertilization program for outdoor cut
flowers requires a multi-pronged approach to maintain fertility and organic
matter. The program should include cover crops for the times when the soil is
fallow; materials to build fertility and/or organic matter in the long-term
such as rock phosphate, bone meal, composts, limestone and manure; and more
soluble fertilizers like dried blood (12-1.5-0.5) to meet the immediate needs
of the crop.

Trickle irrigation

A trickle irrigation system can be used to efficiently
provide water and nutrients through injection to annual or perennial cut
flowers. Depending on the size of the field and the type of injection equipment
in use, cut flowers are fertilized either on the basis of pounds nitrogen per
acre (large fields) or ppm nitrogen (small fields). Using a trickle system and
a conventional greenhouse injector, 20 percent nitrogen water-soluble
fertilizer (e.g., 20-10-20 or 20-20-20) can be applied at either 300-400 ppm
nitrogen weekly or 150-200 ppm nitrogen at every watering (constant liquid
feed). In the absence of a trickle system, these rates could be applied on a
small scale through a hose using a simple injector like a “Hozon.”
Avoid fertilizers that contain calcium or sulfate because these could clog
emitters or other parts of the system.

Soil Testing

An annual soil test for pH and nutrient levels should be
made in either the fall or spring. Since phosphorus, potassium, calcium and
magnesium are fairly stable in the soil, a fall test gives a good reading on
the status of these elements for the following spring. If needed, limestone or
rock phosphate can be applied in the fall, giving these materials a head start
on reacting with the soil before spring. Since the status of nitrogen is very
changeable, a spring test or one right before a planned fertilizer application
or planting is best.

Soil should be collected from about 10 randomly selected
spots in the cut flower field and from a depth of about 6-8 inches. The 10
subsamples should be mixed together into one sample to send to a lab.

Lime Requirement

Checking pH and adjusting it by liming is an important
practice in many parts of the United States. For most cut flowers, a pH range
of 5.5-7.0 is best, but some species may have particular optimum pH levels for
best performance. A soil’s pH affects the availability of trace elements
and the activity of many beneficial soil microrganisms. Phosphorus availability
is highest between pH 6 and 7, so liming an acid soil may help free some
plant-available phosphorus.

In addition to raising the pH of acid soils, limestone
serves as fertilizer for calcium and magnesium. As a soil becomes acid, it also
supplies less and less calcium and magnesium. Use finely ground dolomitic
limestone or calcitic limestone containing 5-10 percent magnesium to get both

When the pH is not in the desired range, a lime requirement
test can be run to determine how much lime is needed. In some cases, pH may be
too high, and an acidifying material like elemental sulfur may be required.
Lime requirement tests take into account the soil texture — sandy soils need
much less limestone to cause a pH change than soils high in clay or organic
matter. Table 1 on page 24 gives a rough estimate of the amount of lime needed
based on the desired pH change and the texture of the soil.

Preparing the Soil for Next Year

Some fertilizer materials are best incorporated prior to
planting. These would include limestone and superphosphate (fall or spring), a
starter fertilizer (spring) and any compost or other organic material that must
decompose to release nutrients.

Superphosphate should not be applied if the soil tests
“high” or “very high” for phosphorus. Generally,
limestone and preplant fertilizers are broadcast on the soil surface and then
worked into the top 6-8 inches of the soil by disking or rototilling. Banding
phosphorus or starter fertilizers close to the rows may be preferable to
broadcasting when the soil fertility is low. Banding puts the fertilizer close
to the root systems of the plants as they begin to grow.

About The Author

Douglas Cox is an associate professor in the department of Plant and Soil Sciences at University of Massachusetts, Amherst, Mass. He can be reached by phone at (413) 545-5214 or E-mail at

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