BMPs for Nutrient Management

Nutrient management best management practices should promote the efficient use of fertilizer. These practices should also reduce nutrient loss by maximizing the amount of nutrients used by the plants or retained in the growing medium for potential use. This can be accomplished by:

• Matching fertilizer applications with nutrient needs as the plants grow.
• Stopping or limiting nutrient loss from the growing medium during top watering in an open system.
• Stopping or limiting nutrient and water loss from irrigation and leaching by containing the effluent.

Nutrient needs
It would be very useful when developing fertilizer programs to know the specific nutrient requirements of plants both in amount and in time. Unfortunately this type of information is very limited for greenhouse crops.

Plant nutrient requirements change as plants grow and enter 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 have distinct phases of growth. They show a pattern of increasing nitrogen 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.

Nutrient uptake patterns have been determined for only a few crops. Some information is available to enhance postharvest longevity and reduce nutrient runoff by reducing fertility in the latter stages of growth (Table 1).

Open, closed systems
An open system refers to a production system that allows any waste water from irrigation and leaching to escape from containers to the greenhouse floor. A closed system contains the waste water for treatment or reuse (e.g. an ebb-and-flood floor or bench system).

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. However, improper product selection and high application rates can lead to nutrient leaching from containers that is as high with CRFs as with water-soluble fertilizers.

Most of the excess nitrate leaching with CRFs occurs right after planting during the early stages of growth. CRFs are best used at a manufacturer recommended low labeled rate.

Limiting leaching
Stopping leaching is difficult to achieve when top watering with a hose because it requires precise control of the irrigation solution volume applied. Traditionally the recommendation has been to water until about 10-15 percent of the volume applied drains from the container to avoid excess soluble salts. In today’s terminology this is described as a 0.1-0.15 leaching fraction. Most growers likely exceed this leaching fraction. Leaching fractions of 0.4-0.6 or 40-60 percent of the volume are what more commonly occur.

The goal is to achieve a leaching fraction of zero. For many growers, being able to lower the leaching fraction to the recommended range of 0.1-0.15 would be a big step in reducing runoff.

The best way to stop or limit leaching with an open system is by using controlled drip irrigation. The irrigation solution should be applied slowly and in small volumes for the best results.

Achieving a zero leaching fraction with a hose is probably impossible to accomplish. However, reducing the leaching fraction is possible if the person doing the irrigating takes the time to observe how much water is applied or how much time passes before leaching begins as each container is watered. Employees should be trained and retrained on how to properly irrigate.

It is important to remember that any significant reduction in the leaching fraction should be accompanied by a reduction in fertilizer rate (parts per million) and/or frequency of application. If the leaching fraction is reduced or there is no leaching, more fertilizer remains in the growing medium and soluble salts can increase to a harmful level. Therefore, the fertilizer rate should be cut at least 25 percent. Also, soluble salts should be monitored more frequently when leaching is stopped or cut back.

Using water shut-offs on all hoses can save a lot of water and fertilizer, especially in retail greenhouses where irrigation may be interrupted frequently by customers.

Low phosphorus
The typical greenhouse fertilizer program provides significantly more phosphorus (P) than plants require. There are several fertilizers with low phosphorus 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 phosphorus enrichment of effluent.

Low phosphorus rates have been shown to have a desirable growth-retarding effect on many bedding plants without any foliar deficiency symptoms or major delay in plant development. Like chemical growth retardants, a low phosphorus level has the greatest growth inhibiting effect during the early stages of vegetative growth. Carefully test low phosphorus rates on a small number of plants before using it on the whole crop. Never try to grow a crop without any phosphorus.

Subirrigation and reuse
Subirrigation is the best and most expensive method for eliminating greenhouse runoff and increasing water and fertilizer efficiency because all of the liquid is contained by a water-tight growing area or in a supply tank. Since no leaching occurs, the fertilizer rate and soluble salts level must be carefully monitored.

The subirrigation fertilizer rate should be about 25-50 percent less than conventional top watering in an open system. Many growers who use subirrigation like the way the crops grow and the labor savings in irrigation time so much that they are able to justify the investment.

Container spacing
Water and fertilizer are lost from overhead systems as a hose or boom irrigator moves between containers. The irrigation solution lost between containers can account for as much as 60 percent of the water and nutrient loss during top watering.

Reductions in water loss can be made by staggering the rows of containers. Greater improvement can be made by using square pots and tray-to-tray spacing. While closer spacing reduces water and fertilizer loss, only certain crops finish at acceptable quality when spaced pot-to-pot.

Saucers and trays
Various types of saucers or collection tray systems can be used to reduce loss from spills between containers. 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 container and to capture the leachate.

Fertilizer rate (parts per million) and/or application frequency should be reduced in saucer and tray systems. Whatever solution is held in the saucer can be absorbed or reabsorbed as the growing medium dries. Also, the medium can become overwatered if the solution stands in the saucer for too long.


Capillary mats
Capillary mats have been used for many years for watering and fertilizing plants by subirrigation. They may also be used to irrigate container plants amended with controlled-release fertilizers.

A slightly different application would be to top water the plants and rely on the capillary mat to soak up and hold any leachate from the containers.


Tina Smith is extension floriculture specialist, University of Massachusetts, (413) 545-5306; tsmith@umext.umass.edu. Doug Cox is associate professor, University of Massachusetts, Plant Soil and Insect Sciences, dcox@pssci.umass.edu.