May 2002

Maximizing Fertilizer Use and Minimizing Runoff By John M. Dole and Janet C. Cole

Controlling fertilizer and water use and runoff is anecessity for some businesses due to their proximity to environmentallysensitive natural areas or water sources. Reducing water and fertilizer use formany businesses, however, is becoming increasingly important to controlexpenses. Research at North Carolina State University and Oklahoma StateUniversity is shedding new light on how to manage fertilizer and water use.

The Integrated Fertilizer and Irrigation Management (IFIM)system is a comprehensive approach to runoff control from container plantproduction. IFIM is similar to IPM, the integrated pest management systemalready being used by the nursery and greenhouse industries. Through the use ofIFIM, runoff from container nurseries and greenhouses can be eliminated whilemaintaining or improving plant quality. IFIM recommendations have been arrangedin levels of increasing sophistication of water and nutrition management andrunoff control. In addition, higher-level practices will often require moreexperience in growing practices or increasing cost. The following is a list ofactions you can take to control fertilizer and water use and runoff.

Level I

Secure the highest-quality water source possible. The most important considerationin setting up an irrigation plan is water quality. Water should be tested priorto selecting a site for a new business and periodically, over time anddifferent locations, after the business is established because the quality canchange seasonally. Electrical conductivity (EC), a measure of the soluble saltlevel or salinity of water, is one of the most important factors. Water with alow EC — 0.1 to 0.5 dS/m — will give the grower the greatest numberof irrigation options and will reduce future problems caused by high-solublesalts accumulating in the medium. Plant species vary in their tolerance tosoluble salt levels, which can stunt plant growth and cause marginal leaf burn.High salt levels frequently can be managed by leaching, which, unfortunately,increases water use.

Alkalinity should be between 40 and 100 ppm, and pH shouldbe 6.0-8.5. Typically, water pH determines if the media pH will change afterpotting, while water alkalinity determines how quickly the media pH willchange. At an excessively high or low pH, some plant nutrients will beunavailable for the roots to absorb. For growers who use basic fertilizers,such as calcium and potassium nitrate, water pH and alkalinity should be at thelow end of the recommended range to prevent media pH from climbing duringproduction. Growers who use acidic fertilizers, especially in the southeast,can readily use water with pH and alkalinity at the center or upper end of therecommended range.

Finally, the nutrient content of the water should bechecked. While low levels of some nutrients can be beneficial, high levels ofone or more nutrients may indicate that the nutrition program should beadjusted. If the water has high levels of nitrogen, calcium or magnesium, lessof these nutrients can be added as fertilizers. High nitrogen levels can beespecially prevalent in areas with sandy soil, shallow wells or intensiveagriculture. Unfortunately, high levels of calcium, magnesium and iron can beantagonistic to other nutrients such as manganese or boron and reduce theiruptake. Also, very high boron levels, greater than 1 ppm, can be toxic.

Use media with a high water- and nutrient-holdingcapacity. Using amedia that retains the maximum amount of water, yet retains sufficientaeration, will reduce the frequency of irrigation required. Commercial premixedmedia vary in water-holding capacity. For self-mixed media, the percentage ofpeat moss can either be increased, or a water-absorbent material can beincorporated. Remember, increasing the time needed between irrigations willalso help customers better maintain the plants in the retail environment or inthe home. Note that too much water retention may reduce aeration and increasethe likelihood of root rot.

Fertilize at or below recommended rates. Relative to other costs, such aslabor, fertilizer is inexpensive. However, in highly competitive markets, everypenny may be necessary to obtain a profit. Plants are often fertilized atunnecessarily high levels as insurance against substandard growth. Lower ratesmay be used in many cases. For example, if several crops are being grown withonly one injector, use constant liquid fertilization rates set for thelowest-nutrient-requiring species, and supplement those species requiringhigher rates with controlled-release fertilizers. In addition, many beddingplant species may grow too tall if given high fertilizer rates. Excessivefertilizer rates can also cause foliar damage, increased susceptibility to rootrot and stunting.

Growers should vigilantly monitor the nutritionalstatus of their crops.The natural variations in weather, irrigation frequency, leaching, waterquality, media nutrient content and plant growth will combine to make each cropunique. The simplest monitoring method is to visually assess the crop. This isbest done by an experienced grower. Visual monitoring is not, however,effective by itself in the long term. By the time symptoms are noted, damagehas already occurred to the crop, resulting in reduced quality and decreasedsales. The best monitoring method is to track media pH and EC levels using pHand EC meters. A variety of pH and EC meters are available from greenhousesupply firms.

Reduce water and nutrition at the end of the cropcycle. This will”harden” plants and increase post-harvest life. For many pottedflowering plants, such as poinsettias, fertilization should be reduced orterminated 1-3 weeks prior to sale. For bedding plants, such as petunia,marigold and ageratum, the fertilizer rate should probably be reduced by one-halfat visible bud. Completely eliminating fertilizer applications is not advisablefor bedding plants because the small amount of media provides little reservoirfor the plants. Also, bedding plants often dry out rapidly in retail areas andare watered frequently, leading to much nutrient leaching.

Use mechanized irrigation systems. Systems such as drip or microtubedeliver water directly to the plants and eliminate runoff between them.Research at Oklahoma State University showed that in poinsettia production, 32percent of water applied using handwatering was lost as runoff compared withonly 23 percent for microtube (drip) irrigation (see Figure 1, page 36) Overtwo years of poinsettia production, hand-irrigated plants required 2.5 to 5.3gallons per plant while microtube irrigation required 2.1 to 4.8 gallons perplant, respectively. In-line drippers should provide similar water savings.Other irrigation systems, such as ebb-and-flow, are even more water- andnutrient-efficient and are included under level II.

Grow cultivars or species that require the least waterand nutrients. Thisrecommendation is easy to suggest but hard to practice, as customers dictatemost of the species and cultivars grown. However, floriculture crop speciesvary greatly in their water and fertilizer requirements, and high water- andfertilizer-requiring crops may be avoided in some cases. For example,chrysanthemums require much water and fertilizer, while Easter lilies havelower requirements.

Shade greenhouses promptly. Reducing the light intensity inthe warmest part of the year will decrease temperature and irrigation needs.Delaying installation of shading will increase water demand and irrigationneeds in the spring. There are two common ways to reduce the light intensity ina greenhouse: shade cloths and shading compounds. External shading is moreeffective than internal shading because in the latter case, the light hasalready entered the greenhouse and is absorbed by the screen, raising theinternal greenhouse temperature. However, internal shading can be automated andallows growers to regulate light levels by opening the shade cloth duringcloudy days and closing it during sunny days.

Another type of shade cloth is made of spun fiber, such asRemay or Vispore, which are white and lightweight. Spun fiber cloths are notstrong enough to be used externally and are often used to cover specificbenches. The cloth is so lightweight that it can be laid directly on the plantsfor a temporary light screen, such as immediately after transplanting cuttingsor after cutting back plants.

Shading compounds can also be used to reduce lightintensity. The white-colored compounds can be applied to the outside of theglazing in one heavy layer or 2-3 thin layers.

Repair leaking hoses and water lines promptly. Leaking linescan waste large amounts of water and fertilizer if connected to the injector.Conduct regular maintenance checks to detect and repair leaks. Look for wetareas on the greenhouse floor that might indicate leaking pipes underground.

Use water shut-offs on all hoses. This straightforwardrecommendation can save a lot of water, especially in retail operations whereirrigation may be interrupted frequently by customers.

Optimize plant production. Improper production practices mayslow plant growth, delay harvest and increase total water and nutrientsapplied. Delayed production typically adds to production expenses and should beavoided. Optimize production temperatures and other practices to ensure thatthe highest-quality crops are produced in the shortest period of time. Inparticular, growing crops at lower-than-optimum temperatures can greatlyincrease the production time of some species and may be more costly in the endthan using extra heat to get a shorter crop time.

Level II

Reduce or eliminate leachate. Leaching is frequently used toprevent the build-up of soluble salts in the media and to ensure that allplants are well-watered. Leachate can be readily reduced by not irrigating toolong. Train and retrain employees regularly on how to properly irrigate. Plantscan be grown with no leaching, but high-quality water, low fertilizer rates andan experienced grower are required. Using a no-leach production system onhanging baskets also eliminates water dripping on crops below the baskets. Forno-leach production, low constant liquid fertilizer rates or controlled-releasefertilizers are used to prevent excessively high media EC.

Use fertilizers that release low levels of nutrientsin the runoff.Typically, a greater percentage of the nutrients in controlled-releasefertilizers are absorbed by the plants and not lost as leachate. The nutrientefficiency (retention) of greenhouse irrigation systems was increased if 50 or100 percent of fertilizer was supplied by controlled-release fertilizers ascompared with 100 percent constant liquid fertilization. Fertilizing with 100percent constant liquid fertilizer caused higher concentrations of nutrients tobe released to the environment with no increase in growth or quality (picturedon page 36). The nutrient retention of controlled-release fertilizers wasgreatly increased with the use of ebb-and-flow irrigation, which producedlarge, high-quality plants and released small volumes of runoff with lownutrient content.

Reuse water and nutrients. In work with potted poinsettias,ebb-and-flow systems produced only 12 percent runoff compared to 32 percent forhand-watering and 23 percent for drip irrigation (see Figure 1, page 36).Overall, poinsettias grown with ebb-and-flow irrigation also required onlytwo-thirds the water of hand irrigation and only three-fourths the water ofmicrotube irrigation. Not surprisingly, recirculatory ebb-and-flow irrigationhad the highest water-use efficiency; recirculatory subirrigation systemsproduce the greatest amount of plant material per gallon of water compared withhand watering, drip irrigation and capillary mats (see Figure 2 below).

Use high-humidity chambers to reduce water used duringpropagation. Tentsand propagation chambers have become popular for seed germination and cuttingrooting. High-humidity tents are generally preferable to misting systemsbecause tenting eliminates over-misting, algae growth on floors and concernsover malfunctions, and it ensures a uniform propagation environment.High-humidity tents also reduce leaching of nutrients from the leaves, freemoisture on the foliage and disease incidence compared to misting. Tentingeliminates the need for a mist or fog system, which can make direct propagationin the final container more feasible.

The major drawbacks of tent propagation are the additionallabor required to set up, remove and dispose of the plastic. Tenting may not befeasible in areas with high light intensity or high temperatures due to heatbuild-up under the tent. Generally, clear plastic is used in the winter underlow-light conditions and white plastic in summer when light levels are greater.

To construct a tent, cover a bench with plastic sheetingsuch that at least one foot hangs over the side of the bench. After propagatingthe plants, cover the bench with white or clear plastic held above the plantsby hoops; place a water seal between the two plastic sheets to hold thehumidity in the tent. The tent should be closed for 24 hours. After this, thetent can be opened for increasing amounts of time to allow fresh air to enterand harden the cuttings after they have begun to root. Ultimately you canremove the plastic.

Use irrigation indicators to optimize irrigationfrequency. Timeclocks can result in over-watering depending on environmental conditions. Moreaccurate irrigation indicators include tensiometers, accumulated light, vaporpressure deficit (VPD) and gravimetric scales. Tensiometers measure the tension(suction) within the medium on a column of water caused by evaporation.Irrigation is triggered when sufficient tension has been reached. In theaccumulated light system, irrigation occurs when the sum of light measurementstaken at regular intervals reaches a predetermined amount. VPD determines howmuch water can be absorbed by the air (deficit). Gravimetric scales measureweight loss due to water evaporation, and irrigation is initiated whenmonitored plants lose sufficient weight. Labor efficiency occurs when theirrigation process is completely automated. Computerized irrigation controlsystems are available that allow the grower to choose from one of severalmethods for determining when to irrigate.

Level III

Install containment ponds. Properly constructed containmentponds trap irrigation runoff from the entire operation and prevent it fromescaping the property. Ponds can be the ultimate solution if all water iscollected, thus preventing any contamination of the surrounding environment.They can, however, be expensive to properly construct and require sufficientland to hold them. The latter can be a major limitation for operations withlimited land in urban or suburban locations. In areas of limited water, pondscan serve as a water source. If used for irrigation, regularly check pH, EC andnutrient content.

Install artificial wetlands. In natural ecosystems, wetlandstrap and clean water. Research is being conducted at several universities onusing artificial wetlands in greenhouse and nursery operations. Artificialwetlands more actively remove contaminants from wastewater than containmentponds and can be used to allow water to drain off the property.