Slow Release

Controlled-release fertilizers (CRFs) use various coating technologies to achieve a patterned release of nutrients into a plant's root zone. The rate of nutrient release from the prills is strongly tied to temperature. An average daily temperature of 70°F is what the labeled release times are based on. Warmer temperatures lead to quicker release and slower temperatures slow down nutrient release.

CRFs have been readily adopted by the nursery industry for decades. This is ironic because in outdoor growing situations very little can be done to control temperature or nutrient leaching from rainfall. CRFs deserve a second look for the greenhouse. The greenhouse environment allows a greater degree of temperature and irrigation control which may be even more conducive to using CRFs - a "controlled environment" for a controlled-release fertilizer.

There are several potential benefits of CRFs. They need to be applied only once, which can save on labor as compared to the periodic stock solution preparation and constant applications required for liquid fertilizers. Liquid fertilizers are readily available by the plant, whereas CRFs are available sparingly as they are released from the prill. This may help to limit excessive lush plant growth, thereby reducing the need for plant growth regulators. Finally, because most of the fertilizer salts are not soluble, they do not leach out of the container as easily as liquid feed representing potential benefits to the environment. A couple of reasons why CRFs have not been readily adopted in greenhouse production are: growers have little experience with which application rates should be applied to which crops; and guidelines for monitoring the root-zone (pH/EC) using liquid fertilizers must be revisited for CRFs. Experiments conducted at Cornell University shed some light on application rates and root-zone monitoring.
 

CRFs and poinsettias
In fall 2010 we conducted an experiment at Cornell University to see how CRFs would stack up in poinsettia production. Rooted cuttings of 'Prestige Red' and 'Peterstar Red' were transplanted in mid August, pinched after two weeks and grown on for 10 more weeks. Throughout the 12-week production period, plants received either liquid feed (21-5-20 at 250 ppm nitrogen) or Osmocote Plus 5-6 month applied at rates of 4, 6, 8 and 10 pounds per cubic yard. We applied this as a top dress treatment at rates of ¾ to 2 teaspoons per 6-inch pot (this covered the range from a low label application label rate up to a medium to high rate).
 

‘Peterstar Red’ that received L to R: liquid feed and 4 or 10 pounds per cubic yard Osmocote.

For 'Peterstar Red' we found that liquid-fed plants were about the same size as those receiving 6 pounds per cubic yard Osmocote. Plants receiving 4 pounds per cubic yard Osmocote were a bit smaller than liquid fed, and plants receiving the 8-10 pounds rate were 20 percent larger than liquid fed plants. However, for 'Prestige Red,' 10 pounds of Osmocote per cubic yard was required to equal the size of liquid-fed plants, 6 to 8 pounds led to slightly smaller plants and 4 pounds per cubic yard gave significantly smaller plants.
 

CRFs in bedding plants
In summer 2010 we conducted an experiment to look at the effect of CRF rates on finish size of petunia 'Fame Blue,' lantana 'Landmark Citrus' and coleus 'Electric Lime.' Rooted liners of the three species were transplanted into 6-inch pots with a commercial potting mix (Lambert LM-111). The plants were grown on for six weeks and received liquid fertilizer or CRF. The liquid fertilizer was 21-5-20 at either 100 or 200 ppm nitrogen applied daily in the irrigation water. The CRF treatments were from Osmocote Plus 3-4 month; it was top dressed at rates of 2, 4, 6 and 8 pounds per cubic yard (this roughly correlates to a low label application rate up to a medium to high label rate).

After six weeks, plants were deemed marketable and saleable regardless of fertilizer treatment. However, some treatments produced a larger size plant by the end. For petunia, a moderately heavy feeder, 200 ppm N from liquid feed produced the largest plant. The other treatments, including 100 ppm N liquid produced markedly smaller plants. On the other hand, lantana and coleus are less vigorous feeders and responded well to CRF. For these two species, 8 pounds per cubic yard Osmocote Plus produced plants as large as liquid feed at 200 ppm N. Liquid feed at 100 ppm N or 6 pounds per cubic yard produce intermediate sized plants and the lower application rates of Oscomocote Plus (2 or 4 pounds) led to somewhat smaller plants.
 

Looking at the roots
When averaged across the experimental period, PourThru electrical conductivity (EC) was markedly higher for the liquid fertilizer treatments (1.3-2.2 mhos/cm) as compared to the CRF treatments (0.4-0.5 mhos/cm). For liquid fed crops, an EC of 1.3-2.2 mhos/cm would indicate sufficient fertility for seedlings and most bedding plants, while a measurement of 0.5 mhos/cm would indicate very low fertility. However, our findings highlight that the standard EC guidelines for liquid-fed crops cannot be extended to crops fertilized with CRF. For coleus and lantana, an EC of 0.5 for CRF plants led to the same growth as an EC of 2.2 mhos/cm for liquid fed crops.

The reason that standard EC guidelines can't be followed for plants fertilized with CRF is that unlike with liquid fertilizers, most of the fertilizer salts with CRF are held inside the individual prills rather than dissolved in water in the root zone. The prill releases fertilizer gradually over time so it does not build up to levels as high liquid fertilizer. For now, EC monitoring can still be used to check that salt levels don't reach damaging levels that can burn roots (for example, PourThru EC values higher than about 5).

In the future, researchers may establish better EC guidelines for CRFs or find other alternative testing methods. Growers using CRFs can still assess fertility visually. Poorly fertilized plants often exhibit nitrogen deficiency first; visible as yellowing of lower leaves. Commercial laboratories can analyze leaf tissue to check whether the plant has had fertilizer in sufficient quantities. Note that laboratory substrate testing is likely to yield lower than expected values (again similar to the case with EC) because most fertilizer salts are held inside the prill. In the bedding plant experiment, container leachate was collected throughout the six week experiment. CRF led to much lower losses of nitrogen from leaching. The average nitrogen concentration in leachate was 220 ppm N for the highest liquid feed treatment compared to 20 ppm N for the highest CRF treatment.
 

Where to start
As with any new practice, growers should trial CRFs under their growing conditions before going full scale. When implementing CRFs, there are many things to consider, but here are some suggestions as to where to start:

• Begin with a low to medium label rate. This will work well with many short-term bedding plants that are moderate feeders. Heavier feeders can be supplemented with added liquid feed.
   
• CRFs rarely burn plants due to high soluble salts (as evidenced from the low EC values reported above). Occasionally salt burn is a problem when temperatures are hot (leading to excessively fast nutrient release) and containers are not leached.
   
• To save on labor for fertilizer application, CRFs can be incorporated into the potting mix in-house or by potting mix suppliers. Either way, make sure that the potting mix is used up as soon as possible. Don't hold for more than a month, otherwise fertilizer salts may build up to damaging levels.
   
• Detached greenhouse structures that don't have their own injectors for liquid feed are a great place to begin with CRFs to save on labor. Portable injectors can be brought in occasionally to supplement fertility of heavy feeding crops.
   
• Hand watering is not an efficient method of delivering liquid fertilizer to crops; much fertilizer is wasted from water not hitting pots or leaching out of pots. Initially you might target CRFs for crops that are difficult to water efficiently by hand, such as pots spaced far apart on the bench or hanging baskets.
   
• CRFs provide insurance that plants are getting fertilizer. With liquid fertilization, stock solutions must be prepared correctly and the injector must be working properly. Two springs ago I asked a greenhouse to test a CRF regime against their standard liquid feed. Visiting the greenhouse, the CRF plants were much larger than liquid feed – it turns out the injector wasn't working correctly. The CRF plants were large and marketable while the liquid fed plants were not.
   

Neil Mattson is assistant professor and floriculture extension specialist at Cornell University. You can reach him at nsm47@cornell.edu.