By: Fred Hulme, Ph.D., Published on greenhousegrower.com, January 2012
Water-soluble fertilizers have been the standard for greenhouse nutrition, but here are some tips for using controlled-release products.
The ideal fertilizer program provides all the essential elements in required amounts and ratios to coincide with plant nutrient demands, minimizing waste and maximizing crop quality and profits. Unfortunately, there are inefficiencies, waste (nutrient runoff) and/or luxurious consumption (such as excess phosphorous and stretching) in many current fertilizer programs. Nutrient deficiency symptoms often are not obvious. Plants might not reach their full potential, appearing slightly off color or stunted, or requiring a longer production time.
By employing best nutrient management practices, growers can use fertilizers as efficiently and effectively as possible to achieve both high-quality growth and profitable return on investment with minimal environmental impact. These practices include:
- Proper fertilizer selection
- Proper fertilizer use – using the correct rates or concentrations at the right time
- Employing best management practices to minimize problems like leaching
- Measuring and monitoring crops
Using water-soluble fertilizers (WSFs) to feed greenhouse crops has been a standard practice for more than 50 years. WSFs were initially developed when growers grew cut flowers in beds of mineral soil and applied agriculture granules (e.g. 10-10-10). Over the years, most growers have shifted to soilless growing media, yet have not adapted their fertilizer practices accordingly. During this time, WSFs have demonstrated some excellent, positive benefits to growers. WSFs offer concentrated, quickly available nutrients that can easily be injected through irrigation systems. They give growers the flexibility to change nutrient content and ratios via different formulations and concentrations. Growers can also use WSFs to customize fertilizer programs and solve problems with blending and prescriptive additions.
On the other hand, WSF technology also has some shortcomings. It’s the equivalent of fertilizer “fast food” – single applications don’t persist for more than a few days. WSF nutrients can easily be leached from soilless media, even with drip tubes. Selecting the correct formulation can be confusing, and its handling, mixing, dosing and monitoring requires special care.
You can’t use WSF effectively in cool, cloudy conditions because plants can’t be fed when they can’t be irrigated frequently enough. It’s also difficult to use WSF when you’re producing multiple crops with different nutrient needs but using one single injection/irrigation system. WSF can cause dramatic fluctuations in root zone pH and EC over time, as EC rises and pH depresses when plants are fertigated.
Conversely, EC is reduced and pH goes up between fertigations as nutrients are used up by the crops. Furthermore, WSF can be expensive, particularly when used on longer-term crops. Finally, when crops leave the greenhouse, they no longer receive nutrients, often causing quality issues down the supply chain.
Another category of fertilizer technology, controlled-release fertilizers (CRFs), can be used in tandem with WSF (or as a replacement for WSFs for some crops). CRFs can provide solutions to some of the issues experienced when using WSFs alone. CRFs are polymer-coated fertilizers that can be incorporated into the growing media at planting and feed a crop for an extended period of time, both during production and after. CRFs slowly and continuously dose water soluble nutrients into the root zone at very low concentrations – the ultimate in constant liquid feed.
CRFs offer many benefits. Applying them to crops is easy and cost effective. Growers can easily vary CRF rates in the same growing space for different crops. CRFs provide slow, constant feeding even when water can’t be applied (e.g. cool and cloudy conditions). CRFs can substantially reduce nutrient runoff while also providing significant post-production nutrition at the garden center, and even in the homeowner’s landscape. While CRFs may cost more per pound than WSFs, their positive effects on plant quality makes them more economically beneficial.
Everris has conducted and sponsored a great deal of research on CRF performance over the past few years. The company’s goal was to explore the viability and practicality of CRF as a primary nutrient source for greenhouse crops in both production and post-production phases. Test crops included bedding plants, short-term pot crops (eight weeks or less), fall mums and poinsettias. Crops were grown in a variety of greenhouse conditions in spring, summer and fall. The research focused on CRF performance, nutrient leaching, nutrient efficiency, root zone environment and cost and return on investment. Here are a few of the overall trends uncovered by this research:
- It is easily possible to grow a variety of greenhouse crop types using CRF as a primary fertilizer source or in combination with WSF. Researchers were onsistently able to achieve superior or comparable plant growth with CRF compared to WSF.
- Available CRF longevities and rate selection offer flexibility in nutritional programs.
- Traditional overhead irrigation with WSF can be extremely wasteful. Drip systems will reduce nutrient leaching, but they still can be quite wasteful. CRF can reduce nutrient leaching significantly (by a factor of tenfold or more in some studies).
- CRF can be highly efficient, and it is possible to grow a crop with less fertilizer, reducing a grower’s fertilizer cost (especially for bigger, longer-term crops).
- CRF can simplify root zone management, minimizing the stress of pH and EC swings.
- CRF can provide excellent postharvest performance, leading to less crop shrinkage in the garden center and greater consumer acceptance, as plants are nourished by residual CRF nutrients in the landscape.
Switching from an entirely WSF-based fertilizer to using CRF in the greenhouse is a fundamental change. Everris recommends growers start at lower rates (3 to 6 pounds of a 100-percent-coated, homogenous, complete CRF that includes micronutrients per cubic yard for most greenhouse crops and situations.) This process will help growers become more comfortable with CRF before adapting rates upward to match their environments, growing systems and production goals. Rates will need to be higher for heavy-feeding plants like hardy mums. If uncertain, contact your fertilizer supplier for guidance with product selection and rates. It’s also best to trial CRF on a small scale before jumping into its use headfirst.
Combination Fertilizer Programs
Using WSF and CRF technologies together often produces excellent results with less dramatic change. Remember to reduce WSF concentrations to account for CRF. Growers who produce a wide variety of crops should use one concentration of WSF on all crops and supplement with CRF for heavy feeders like vegetative petunias. Combination fertilizer programs (low-to-medium rate of CRF), along with supplemental applications of lower-concentrate WSF as needed, can round out a potentially inefficient fertilizer program and provide numerous benefits.
- WSF gives growers options to react to specific situations that arise, such as pH issues, micronutrient deficiencies and nitrate drift from green bark.
- CRF provides continuous, low-level base feed to the root zone when you can’t use water-soluble fertilizers during cool, wet or cloudy weather or busy shipping times.
- CRF can make up for WSF applications lost to leaching from rainfall or excessive irrigation.
- CRF continues fertilizing for the garden center or homeowner, leading to good postharvest benefits, greater customer satisfaction and repeat customers for growers, retailers and the industry as a whole.
This article was originally published on January 6, 2012 on greenhousegrower.com.