Properties of Water Soluble Fertilizers

From BASF: The Chemical Company Website

Providing the appropriate fertility levels is one of the most important aspects of producing greenhouse crops.  Many growers use water soluble fertilizers (WSFs) to deliver nutrients to their plants.  There are several factors to consider when using WSFs; some of them include the fertilizer’s formulation, the source of nutrients it is made of, its solubility, its basic or acidic properties, and price.

Water soluble fertilizers are available in various formulations which offer differing quantities of nitrogen, phosphorus, and potassium (NPK), but may also contain additional elements including micronutrients. Every formulation displays the NPK in a standardized format such as 17-5-17.   The first number always refers to the percent nitrogen, the second expresses the percent phosphorous in the oxide form (P2O5), and the last number always represents the percentage of the oxide form of potassium (K2O).  In the above example, 17 percent of this formulation nitrogen, 5 percent is phosphorous, and 17 percent is potassium. 

The exact formulation to use needs to be determined on a grower by grower basis by matching the nutritional requirements of the crops being grown with the type of irrigation water being used (water quality).  The main characteristic of water growers must take into consideration is alkalinity, which is essentially the bicarbonates and carbonates in the water.  Alkalinity greatly affects the pH of the growing mix over time, which influences nutrient availability and uptake. It is also important to know the soluble salts (EC) and quantities of other nutrients such as calcium and magnesium that the irrigation water contains.  Once these items are identified by submitting water samples to a laboratory for nutritional analysis, an appropriate fertilizer can be chosen.

Most WSFs have some effect on the pH of the growing mix after application. Each fertilizer is categorized by its acidity or basicity (it’s acidic or basic properties).  It is very important for growers to know and understand the affects acidic or basic fertilizers have on media pH and crop production.  The specific acidity or basicity of each WSF is located on the product’s bag and/or technical sheet. The measurement used (expressed as pounds of calcium carbonate per ton) represents the amount of calcium carbonate required to neutralize a ton of fertilizer.

Acidic fertilizers can be used to lower the media pH over time, while basic formulations gradually increase the media pH.  The higher the potential acidity value, the more of an effect it will have on lowering the media pH.  Conversely, the lower the potential acidity value, the less affect a fertilizer will have on lowering the media pH.   A similar relationship occurs with the potential basicity – the higher the basicity value the greater affect the fertilizer has at raising media pH.

Selecting a fertilizer is also dependent on the crops being grown and what type of growth a grower is trying to achieve.  Fertilizers containing high levels of nitrate nitrogen provide toned growth (compact growth of small leaves and short internodes), while fertilizers with high percentages of ammoniacal nitrogen promote soft, lush growth (large leaves and long internodes). 

Fertilizer Application Frequency and Rates

The most common method of applying WSFs is using a constant liquid fertilizer program which involves applying a dilute solution of fertilizer at each watering.  This type of fertilization method ensures that the nutrients in the soil are always available for plants to uptake with little fluctuation in their concentration over time.  Some growers apply fertilizer on an as needed basis or with scheduled applications such as once per week.  These applications consist of using a more concentrated fertilizer solution than what is applied with a constant liquid fertilizer program. Applying nutrients as needed often leads to variability as the nutrients fluctuate greatly between the applications-- ranging from very high concentrations to very low levels of nutrients available for plant uptake between each application.

The fertilizer rates of WSFs are usually expressed as parts per million (ppm) of nitrogen in the solution being applied.  Growers using constant liquid feed programs commonly use 50 to 150 ppm per irrigation (depending on the crop and container size being produced).  When fertilizers are applied intermittently, the application rates are higher and often range from 200 to 400 ppm.  It is generally recommended to apply low application rates on a regular basis, which is effective at maintaining plant health, without encouraging excessive growth. 

The amount of fertilizer to apply and application frequency varies depending on the plant species; some plants are light feeders, while others are considered heavy feeders.  The age of a plant also affects the fertility rate; young seedlings require less fertilizer than do older plants.  Additionally plants grown under high light levels will generally require high fertility levels than plants grown with lower light intensities.

It is important for growers to conduct routine soil tests to determine the current nutritional status of their crops.  When these tests indicate fertilization is required, a nutrient form, formulation, and rate can be determined.