August 2002

Soil Fertility for Field-grown Cut Flowers By Douglas Cox

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

An effective fertility program begins with fall and earlyspring soil management practices, but right now, with the 2002 crop alreadyplanted, the most important practices involve providing adequate nutrition tothe plants as they grow and flower.

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

Fertilizing Cut Flowers After Planting

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

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

Unfortunately, nitrogen is required in the greatest amountby 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 maintainadequate levels for production. Leaching of nitrogen as nitrate can be aserious problem not only as a way of losing nitrogen for the crop, but alsobecause it can pollute groundwater and surface bodies of water. Too muchnitrogen fertilizer, high rainfall or excess irrigation, and sandy soil are allfactors favoring nitrate leaching.

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

Fertilization for Annuals

Germinating seeds and young seedlings are very sensitive tofertilizer 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 actualnitrogen per 1,000 sq. ft. at seeding either by broadcasting or banding. Bandsshould be placed about two inches below and to the sides of the seeds.

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

When transplants from the greenhouse are used to start thecrop, about 20-30 percent of the total nitrogen requirement should be appliedin a preplant application made just before planting, assuming the transplantsare healthy and have been hardened-off. Another 20-30 percent of the totalshould be applied as a sidedress application when the transplants are about8-10 inches tall.

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

Fertilization for Perennials

After flowering or harvest, most perennials continue togrow, building the root system and expanding the crown or other overwinteringstructures. So some perennials may benefit from a second fertilizer applicationafter flowering. About 1-2 lbs. of actual nitrogen per 1,000 sq. ft. isrequired by peony. One-half should be applied at emergence, one-fourth justafter flowering and one-fourth at dormancy. Asiatic and Oriental lilies grownas perennials should be fertilized by sidedressing with 2-3 lbs. of 10 percentnitrogen fertilizer per 100 ft. row as the shoots begin to emerge from the soiland again at the same rate right after flowering. Late-season fertilizerapplications to perennials are somewhat controversial. Some feel thatfertilizing in the fall may disrupt the plants’ normal preparations fordormancy and make them susceptible to cold injury, while others think there isa benefit to the roots and crowns. I recommend using caution and avoiding highnitrogen applications; however, applying phosphorus may be beneficial to rootand crown development. Also, it would be safe to apply limestone if it isneeded to adjust pH.

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

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

Less phosphorus than nitrogen is needed by plants, andphosphorus is more stable in the soil than nitrogen. Some agricultural soilstest “very high” or “excess” in phosphorus because ofthe regular use of NPK fertilizers over many years. Generally, the potentialharm caused by high phosphorus is not to the plants but to the environment. Toomuch phosphorus can lead to runoff to surface bodies of water. This encouragesexcess growth of algae and other aquatic plants, which leads to a seriousdecline in water quality. Ideally, nutrients should be applied based on need asdetermined by a soil test. Unlike nitrogen, an application of phosphorus maynot be needed every year.

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

Potassium deficiency causes marginal chlorosis and burningon the lower leaves first. Unless the deficiency is severe, it may not affectthe 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 andorganic matter.

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

Cover crops and organic fertilizers

Once a cut flower crop has been harvested, there may be somegrowing 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 covercrop. Cover crops absorb residual nutrients left in the soil, add organicmatter, may add nitrogen to the soil and may help protect the soil from erosionover the winter. In New England, winter rye, oats and hairy vetch are popularcover crops; however, different species may be recommended in other parts ofthe United States. Hairy vetch is a legume cover crop that fixes nitrogen fromthe atmosphere and may add as much as 100 lb. of nitrogen per acre when plantedin August and plowed down in the spring.

There are many “organic” alternatives tochemical fertilizers that can be used to fertilize outdoor cut flowers. Composts,animal manure, dried blood, bone meal, rock phosphate and seaweed products canall be used successfully, keeping in mind that most of these have much lowernutrient analyses and lower solubility than commercial fertilizers. With anorganic fertilizer, a much larger weight of material must be applied to supplythe same amount of nutrients compared to a chemical fertilizer. For example, tosupply 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 tonitrogen from both materials should be about the same.

A successful organic fertilization program for outdoor cutflowers requires a multi-pronged approach to maintain fertility and organicmatter. The program should include cover crops for the times when the soil isfallow; materials to build fertility and/or organic matter in the long-termsuch as rock phosphate, bone meal, composts, limestone and manure; and moresoluble fertilizers like dried blood (12-1.5-0.5) to meet the immediate needsof the crop.

Trickle irrigation

A trickle irrigation system can be used to efficientlyprovide water and nutrients through injection to annual or perennial cutflowers. Depending on the size of the field and the type of injection equipmentin use, cut flowers are fertilized either on the basis of pounds nitrogen peracre (large fields) or ppm nitrogen (small fields). Using a trickle system anda conventional greenhouse injector, 20 percent nitrogen water-solublefertilizer (e.g., 20-10-20 or 20-20-20) can be applied at either 300-400 ppmnitrogen weekly or 150-200 ppm nitrogen at every watering (constant liquidfeed). In the absence of a trickle system, these rates could be applied on asmall scale through a hose using a simple injector like a “Hozon.”Avoid fertilizers that contain calcium or sulfate because these could clogemitters or other parts of the system.

Soil Testing

An annual soil test for pH and nutrient levels should bemade in either the fall or spring. Since phosphorus, potassium, calcium andmagnesium are fairly stable in the soil, a fall test gives a good reading onthe status of these elements for the following spring. If needed, limestone orrock phosphate can be applied in the fall, giving these materials a head starton reacting with the soil before spring. Since the status of nitrogen is verychangeable, a spring test or one right before a planned fertilizer applicationor planting is best.

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

Lime Requirement

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

In addition to raising the pH of acid soils, limestoneserves as fertilizer for calcium and magnesium. As a soil becomes acid, it alsosupplies less and less calcium and magnesium. Use finely ground dolomiticlimestone or calcitic limestone containing 5-10 percent magnesium to get bothelements.

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

Preparing the Soil for Next Year

Some fertilizer materials are best incorporated prior toplanting. These would include limestone and superphosphate (fall or spring), astarter fertilizer (spring) and any compost or other organic material that mustdecompose 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 thenworked into the top 6-8 inches of the soil by disking or rototilling. Bandingphosphorus or starter fertilizers close to the rows may be preferable tobroadcasting when the soil fertility is low. Banding puts the fertilizer closeto the root systems of the plants as they begin to grow.