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Best Management Practices (BMPs) to Increase Fertilizer Efficiency and Reduce Runoff

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Nitrates and phosphates from fertilizer are potential environmental hazards if they enter groundwater or surface water by runoff or leaching. Some greenhouse practices or common materials used in plant production may lead to the creation of nutrient-enriched effluent. First, most potting mixes have little ability to supply or retain nutrients in amounts to sustain plants without application of fertilizer. Second, many irrigation practices, especially overhead watering with a hose, are very inefficient in terms of water and nutrient loss. Third, water-soluble fertilizers are often used at rates in excess of the plants' needs without regard for volume applied and frequency of application.

Fortunately most attention of government regulatory agencies is now focused on the greater threat of pesticides to water. This situation allows growers and researchers time to develop and trial different "best management practices" (BMPs) to reduce nutrient loss to the environment from greenhouses.

Nutrient management BMPs should promote the efficient use of fertilizer and reduce nutrient loss by maximizing the amount of nutrients used by the plant or retained in the plant container for potential use. Growers should attempt to meet three goals in developing a nutrient BMP program, they are:

  • Goal 1. Match fertilizer application with plant nutrient needs as the plant grows.
  • Goal 2. Stop or limit the loss of nutrients from the plant container during topwatering in an open system.
  • Goal 3. Stop or limit nutrient and water loss from irrigation and leaching by containing the effluent.

Goal 1. Match fertilizer application with plant nutrient needs.

It would be very useful when developing fertilizer programs to know the specific nutrient requirements of greenhouse plants both in amount and in time. Farmers are very lucky because they have extensive information on how much N, P, or K is needed to grow a field crop and at what time during crop development fertilizer will be most beneficial; unfortunately information of this type is very limited for greenhouse crops.

Nutrient balance sheets. One approach to studying nutrient needs is to construct a "nutrient balance sheet" showing where the applied element(s) go and where improvements in fertilizer efficiency can be made. Fertilizers or fertilizer programs can be compared as in the example of 4-inch seed geraniums (Table 1). Here the plants received the same amount of N and water as they grew and where the fate of the N was kept track of until flowering. The balance sheet shows that the largest amount of N was recovered by the plants fertilized with ammonium nitrate; N leaching was greatest with ammonium sulfate and calcium nitrate; unaccounted for N (presumably lost as ammonia gas) was highest for ammonium sulfate and urea; and the amount retained by the potting mix was about the same for all N sources. The balance sheet clearly shows the magnitude of N loss by leaching and the importance of N source in maximizing fertilizer efficiency. Interestingly, about 1.0 gram of N was required to grow the geraniums in this study; a level very close to estimates made for other floriculture crops. Complete balance sheets for floriculture crops are rare because the research needed to construct them is expensive and time-consuming.

Table 1. Nitrogen balance sheet for 4-inch seed geranium.z
  Percentage (%) of applied N
N Fertilizer Plant Medium Leached Unacct. for
Ammonium sulfate 27 19 43 11
Ammonium nitrate 47 18 32 3
Calcium nitrate 33 18 46 3
Urea 37 19 21 23
Osmocote
14-14-14
38 26 30 6
zCox, D.A. 1985. HortScience. 20:923-925.

Nutrient uptake patterns. Plant nutrient requirements change as the plant grows and enters new developmental stages (e.g., vegetative vs. reproductive). Ideally fertilizer should be applied during periods of highest demand and reduced or stopped at other times. Using this approach could reduce runoff and prevent harmful nutrient deficiencies or excesses. Some plants such as chrysanthemum and marigold {C}{C}{C}

have distinct vegetative and reproductive phases of growth and they show a pattern of increasing N uptake during vegetative growth and a leveling off or decline following the appearance of visible buds. Nitrogen is most critical during the vegetative phase and fertilization can be reduced after visible bud. On the other hand, New Guinea impatiens, which do not have distinct vegetative and reproductive phases and they show a continuous, gradual increase in N uptake as they grow {C}{C}{C}

. New Guineas do best with low fertility early on and fertilization becomes more critical as the plant gets older and larger.

Nutrient uptake patterns have been determined for only a few crops, but some information is already available for you to use to enhance postharvest longevity and reduce nutrient runoff by reducing fertility in the latter stages of growth (Table 2).

Table 2. Fertilization recommendations for improved postharvest performance of selected greenhouse crops.z
Crop Nitrogen recommendation
Ageratum, marigold, petunia Cut nitrogen rate in half at visible bud stage.
Celosia Cut N rate in half 1-2 weeks before sale.
Snapdragon Reduce fertilization when flower spike starts to elongate.
Begonia Reduce fertilizer rate at the end of the production period.
Poinsettia Stop fertilizing 2 weeks before sale to reduce leaf drop.
Potted chrysanthemum Stop fertilizing at visible bud, or use 150 ppm for entire production (instead of 450 ppm) to increase postharvest life 7-14 days.
Easter lily Stop fertilization prior to marketing of lilies are to be stored at 35 F. This will improve postharvest foliage color.
Azalea Stop fertilizing 2-4 weeks before cooling to reduce leaf browning.
Exacum Reduce fertilizer levels during production to increase postharvest longevity.
zMcAvoy, R.J. 1995. Connecticut Greenhouse Newsletter, Issue 184, February/March.

Goal 2. Stop or limit the loss of nutrients from the plant container during topwatering in an open system.

The term "open system" refers to crop systems which allow any effluent from irrigation and leaching to escape from the pot to the greenhouse floor. "Closed systems" are those which contain the effluent for treatment or reuse (e.g. an ebb and flood subirrigation bench). Most growers in New England use an "open system" to grow plants.

Controlled-release fertilizers. The major advantage of using controlled-release fertilizers (CRFs) is that the loss of nutrients from spills during fertigation is completely eliminated, but nutrient leaching from the pot can still be as high with CRFs as water-soluble fertilizers. In a typical study, the same amount of N and water was applied to 4-inch marigolds from water-soluble 20-10-20 and Osmocote 14-14-14 (Table 3). Contrary to expectations, more N leached by 30 days after planting and at the end (60 days) with CRF incorporated in the mix at planting than with regular application of water-soluble fertilizer. The performance of CRF, in terms of N leaching, was improved when the fertilizer was applied to the surface of the mix or when CRF was applied in two smaller doses 30 days apart. So to use CRF most effectively, it is best to make surface and split applications.

Table 3. Nitrogen leaching by 4-inch pots of 'First Lady' marigoldz.
Osmocote 14-14-14 N leached (mg/pot) % of TOTAL leached 30 days
At planting (gm/pot) 30 days later (gm/pot) 30 days TOTAL
60 days
3.6 (incy) None 71 80 89
3.6 (surx) None 42 50 83
1.8 (inc) 1.8 (sur) 32 38 84
1.8 (sur) 1.8 (sur) 22 28 77
Water-soluble 20-10-20 41 60 69
z Cox, D.A. 1993. J. Plant Nutr. 16(3):533-545.
yinc = incorporated in the potting mix.
xsur = applied to the surface.

Notice that in all fertilizer treatments about 70% or more of the N leached by 30 days. Clearly none of the treatments did a good job of matching the plants needs early on.

Stop or limit leaching. This is tough to achieve when topwatering with a hose because it requires precise control of the volume of irrigation solution applied. Traditionally the recommendation has been to water until about 10-15% of the volume applied drains from the pot to avoid excess soluble salts. In today's terminology this is described as a 0.1-0.15 "leaching fraction" (LF). Most growers probably greatly exceed this LF; probably LFs of 0.4-0.6 are more common. The goal is to achieve a LF of zero, but for many getting the LF down to the recommended range of 0.1-0.15 would be a big step in reducing greenhouse runoff. The best way to stop or limit leaching with an open system is by the use of a carefully-controlled spaghetti-tube irrigation system with drip emitters. Irrigation solution should be applied slowly and in small volumes for the best results. Also, researchers have found that "pulse irrigation" - brief periods of fertigation - is best for efficient application of water and nutrients.

Achieving 0 LF with a hose is probably impossible, but reducing LF is possible if the waterer takes the time to observe how much water is applied or how much time passes before leaching begins as each pot is watered. The "fire hose" method of indiscriminate watering is a definite "no-no" with zero leaching!

It is important to remember that any significant reduction in LF should be accompanied by a reduction in fertilizer rate (ppm) and/or frequency of application. If LF is reduced or there is no leaching, more fertilizer will stay in the pot and soluble salts could increase to a harmful level. Therefore, fertilizer rate should be cut at least 25%. Also, soluble salts should be monitored more frequently when leaching is stopped or cut back.

Low phosphorus fertilizer programs. Most researchers agree that the typical greenhouse fertilizer program provides significantly more P than crops require. There are several fertilizers with low P analysis (e.g., 15-0-15, 20-1-20, 20-2-20) on the market which could be included in a routine fertilizer program to reduce P enrichment of effluent. Also, carried a little farther, incipient P deficiencies have been shown to have a desirable growth-retarding effect on many bedding plants without any foliar symptoms or major delay in plant development. Like chemical growth retardants low P has the greatest growth inhibiting effect during the early stages of vegetative growth. Carefully test this method on a small number of plants before committing the whole crop. Never try to grow a crop without any P!

Goal 3. Stop or limit nutrient and water loss from irrigation and leaching by containing the effluent.

Subirrigation and reuse. This is the best method for eliminating runoff from the greenhouse and increasing water and fertilizer efficiency because all of the liquid is contained in the system by a water-tight growing area or in a supply tank. Unfortunately this is also the most expensive approach - a 6x12 foot ebb and flood subirrigation bench costs (without discounts) about $600 (incl. shipping). Since no leaching occurs fertilizer rate and soluble salts must be monitored very carefully. Fertilizer rate should be about 25-50% less than conventional topwatering in an open system. Many growers who have had an opportunity to trial an ebb-flood bench like the way the crops grow and the labor savings in irrigation time so much that they are able to justify the investment and have started adding them to their greenhouses.

A lot of water and fertilizer is lost during fertigation from overhead systems as the hose or boom moves between pots. These "spills" may account for as much as 60% of the water and nutrient loss during topwatering. Large improvements can be made here at relatively low cost.

Pot spacing. Take some empty round pots and space them pot to pot. Even at this spacing there is a lot of space for irrigation solution to spill as the pots are watered. Some improvement can be made by staggering the rows of pots. Much greater improvement can be made by using square pots and tray to tray spacing of bedding plants. But, did you ever grow poinsettias pot to pot all the way to flowering? Probably not! So, while close spacing reduces water and fertilizer loss, there are only certain crops that will end up of acceptable quality when spaced pot-to-pot.

A new product, called "Hydro-Miser," allows wider spacing and still helps reduce losses by spills. It is a simple plastic piece supported by the pot rims which covers the gaps between the pots and channels the water into the pots. The manufacturer conservatively claims a 20% reduction in water loss between pots. There is extra labor involved in putting the Hydro-Misers out, but they can be reused indefinitely.

Saucers and trays. Various types of saucer or collection tray systems can be used to reduce loss from spills between pots and the use of saucers is also an inexpensive way to learn about subirrigation. Some saucers are designed to cover most of the space between the pots, channel the water to the base of the pot, and capture leachate. Simple round saucers hold leachate but are much less effective at capturing spills. Round saucers could be filled with fertilizer solution for subirrigation.

Fertilizer rate (ppm) and/or application frequency should be reduced in saucer and tray systems because whatever is held in the saucer can be absorbed or reabsorbed by the pot as the growth medium dries. Also, plants could become overwatered if the solution stands in the saucer for too long.

Absorbent matting. Capillary mats have been used for many years for watering and fertilizing plants by subirrigation. They may also be used to irrigate potted plants amended with CRF. A slightly different use would be to topwater the plants and rely on the capillary mat to soak up and hold any spills or effluent from the pots. Of course cap mats can absorb only so much water before they start to drip, so watering must be done carefully. Perhaps this is a way of learning to efficiently apply water with a hose and reduce LF.

Reference books

Two books published in the mid 1990s will help growers control greenhouse runoff and provide general guidance in the use of fertilizer and water in the greenhouse, they are:

Water and Nutrient Management for Greenhouses, NRAES-56, is available for $20 each from NRAES, Cooperative Extension, 152 Riley-Robb Hall, Ithaca, NY 14853-5701. For more information call (607)255-4080.

Grower's Guide to Water, Media, and Nutrition for Greenhouse Crops is $55 per copy and can be ordered from GrowerTalks Bookshelf, PO Box 9, 335 N. River St., Batavia, IL 60510. For more information call 1-800-456-5380.

Many of the methods of reducing greenhouse runoff I have proposed require some adaptation to new methods of growing plants, varying amounts of capital investment, and certainly more attention to detail than current practices. However, there is still time to experiment and learn and in the long run a proactive, incremental strategy will be the least difficult way of implementing runoff control measures.

References

  • Cox, D.A. 1985. Nitrogen recovery by seed geranium as influenced by nitrogen source. HortScience 20:923-925.
  • Cox, D.A. 1993. Reducing nitrogen leaching-losses from containerized plants: The effectiveness of controlled release fertilizers. J. Plant Nutr. 16(3):533-545.
  • McAvoy, R.J. 1995. Managing nitrogen in greenhouse crops: Nitrogen sources, crop fertility, and water quality. Conn. Grnhse. Newsletter 184:18-29.
Prepared by:
Douglas Cox
Plant and Soil Sciences
University of Massachusetts
Amherst
Topics: 
Commercial Horticulture
Commercial Horticulture topics: 
Plant Nutrition