Water soluble fertilizer (WSF) is the principal fertilizer type used by greenhouse growers today. It is made from soluble nutrient raw materials mixed in stock tanks and delivered by injectors through an irrigation system. Controlled release fertilizer (CRF) consists of coated nutrient cores that are mixed into the media and release over a defined period of time, regulated by temperature. CRFs can be a highly efficient and effective means to deliver nutrients to color crops in combination with WSFs, or even as the sole fertilizer source. If the right products and rates are utilized, CRF can provide many benefits that WSF alone cannot including reduced fertilizer and labor costs, improved quality, simplified production, reduced nutrient leaching and higher post-production value—both at the point of sale and in the consumer’s backyard. Here are a few key guidelines for greenhouse growers to use when adding CRFs to their fertilizer programs. 

Consult your fertilizer provider

If you have never used CRFs in color crop production, it’s a good idea to consult a representative from your fertilizer provider. For best results, select a 100% coated, homogenous product that delivers NPK and other essential elements in a controlled release over the full growing cycle. While the up-front costs can be higher than other blended fertilizer formulations, they provide a high margin of safety, deliver a sustained and consistent supply of nutrition, and are a great value in the long run.

Select the correct longevity

Select CRF products with longevities based on your average daytime temperatures and the length of time you want to provide nutrition to the plant. Longevity claims are usually based on set reference temperatures, but these reference temperatures may vary between brands. Read the label to select the correct longevity for your environment. Most applications will last longer at cooler temperatures and shorter at higher temperatures (see table below for estimated longevities of Osmocote CRF based on average soil temperatures).

Select the correct rate

The rates usually seen on CRF bag labels are for outdoor container nursery crops and may be too high for salt-sensitive color crops. The best rule of thumb is to start with a very low CRF rate in combination with WSF. In a combination feed program, CRFs can provide valuable nutrient insurance for times when WSF isn’t an option due to cool, cloudy weather or busy shipping times. Many color growers who take this approach often increase their CRF rate and reduce WSF use over time as they gain experience in managing CRFs. 

Find the best application method

Since many color crops are grown in small containers, it is critical to achieve a uniform dose of CRF from pot-to-pot to ensure consistent nutrition. Incorporating CRF into growing media is the best and easiest application method. Done correctly, this virtually eliminates the labor costs associated with fertilizer application, especially if you buy mix from a professional potting soil supplier. If you buy your media well in advance of the planting season, it is best to hand-mix CRF into the media before planting or even top dress onto pots after liners or plugs are planted. This is often done on larger combination containers and hanging baskets. Some growers do not want CRF to interfere with their WSF during the production phase. These growers apply CRF onto containers just before shipping or even at the point of sale to provide nutrition to their crops in the retail and consumer phases.

Conduct trials

By trialing CRF products and rates on a smaller scale before adopting them, you can gain insight into how the technology performs in your growing systems. This will help you gain confidence before attempting broader application. If you bracket CRF rates and product types, you can assess how CRF can fit into your production without excessive risk.  

Consider mini-prills

For propagation and smaller cells, you may want to employ a mini-prill CRF. The main benefit of this product is more fertilizer particles per unit weight. This can provide a more uniform distribution of fertilizer across smaller root zone cells and containers.   

Use a combination of CRF and WSF

Many growers use a base level of CRF supplemented with WSF. By using different delivery systems, you can avoid “putting all your eggs in one basket.” CRF is a great way to back up a WSF program in case of inefficiencies or breakdowns. Some growers supply a CRF supplement to heavy feeders and a base WSF feed to the entire crop. With this method, all crops can thrive on a single source and concentration of nutrition.

Remember the consumer

Even the highest quality crop may be subjected to inadequate feed levels in the post-production retail phase. This can lead to reduced plant quality, shrinkage and poor consumer acceptance. CRF is an excellent way to provide salable crops a nutrient care package to keep them healthy and attractive through this period. We also know that many consumers provide inadequate or no fertilizer after they purchase a plant. Garden soil may provide some nutrition, but large containers and hanging baskets fed water soluble fertilizer alone don’t have the nutrient reserves to sustain them over time. By using a long-term CRF at the start of production or by top dressing close to the point of sale, growers can provide their customers with a healthy, beautiful and colorful plant that only needs water to continue looking its best.