Water-Soluble Fertilizer Vs. Granular Slow-Release Fertilizer

June 3, 2021
Category: Growing Tips

As you plan your fertilizer program for your greenhouse or outdoor crops, you’ll want to know the difference between water-soluble and granular slow-release fertilizers. 

There are advantages and disadvantages to each. And which you choose will depend on the needs of your soil and plants throughout the growing season. You may choose to keep both for use on different plants or at different times.

Water-Soluble Fertilizer

Water-soluble fertilizer, or liquid fertilizer, is available in crystal form to be dissolved in water or in liquid form to be diluted in water. Fish emulsion and compost tea are the most common organic liquid fertilizers. There are many types of constant liquid feed fertilizers on the market. 

Liquid fertilizer can be applied prior to planting as well as mid-season. Both ground and foliage applications are effective for providing nutrients to your plants. 


Some of the significant advantages of using liquid fertilizer include:

  • Easy application and can be paired with liquid insecticide or fungicide plant protectors as part of your constant liquid feed program
  • Easy to blend
  • Uniform application across crops, with the same nutrient content in every drop of liquid
  • Useful for starter and mid-season fertilizing
  • Offer quickly available nutrients to the plants, which is especially helpful to give undernourished plants a boost


The major disadvantages of using liquid fertilizer include:

  • Difficult to store and more likely to become volatile over time
  • More expensive than Granular Fertilizers
  • Requires specific equipment, which is a major investment of money and time for greenhouses who aren’t already equipped with a constant liquid feed system
  • May require more applications as water-soluble fertilizer can leach out of sandy soil or containers

Granular Slow Release Fertilizer

Granular fertilizer is a dry fertilizer packed in granules. Standard blends of the most commonly needed nutrients are available, or you can mix granular fertilizers to create the perfect blend for your crop. 

Slow-release granular fertilizer has a coating specially designed to cause each granule to release its nutrients over a 2-3 month period. While it is often more expensive than regular granular fertilizer, the ability to feed only once per season makes up for the extra cost. We focus on slow-release fertilizer in this article. 


  • Requires fewer feedings, often only one prior to planting
  • Easy to store without any loss of efficacy
  • Preplant application goes hand in hand with tilling
  • Using slow-release granular fertilizer allows for custom blending to improve nutrient content and efficiency for crops
  • Does not require any additional equipment


  • Less mobile encapsulated nutrients (especially phosphorus) may not reach plant roots
  • High salt content can burn roots or repel them from the nutrient content
  • While liquid fertilizer contains consistent nutrient levels throughout, the available nutrient content varies from granule to granule in slow-release granular fertilizer

Which Fertilizer Should You Choose?

Which fertilizer you choose for your commercial greenhouse will depend on the tools you have, the money you want to invest, the time you have to invest, and how targeted you need your fertilizer program to be. 

You’ll also need to consider what kind of amendments you need to make to your soil based on soil testing. For example, if phosphorus is the primary nutrient your plants need, liquid fertilizer will be a more efficient means of delivery. 

If your focus is on time efficiency and you already have a constant liquid feed system in place, water-soluble fertilizer is your obvious choice. Suppose you aren’t equipped with the proper tools and aren’t ready to invest in a constant liquid feed system. In that case, you might consider using a slow-release granular fertilizer with foliar applications of liquid fertilizer if needed. 

Think through the costs and benefits for your greenhouse and plants. Contact us or your local AG extension with questions specific to your plants or region.