Hydroponics Systems: Nutrient Solution Programs and Recipes

In hydroponic systems, most plant nutrients are supplied through nutrient solutions. Basic approaches to creating nutrient solutions are fertilizer programs, recipes, and complete fertilizers.

Fertilizer Programs

Fertilizer programs consist of a complete fertilizer supplemented with macronutrients. They are offered by some hydroponic and/or fertilizer companies and vary by hydroponic crop. An example of a fertilizer program is one offered by Hydro-Gardens for hydroponic tomatoes.

In this program, growers purchase Hydro-Gardens Chem-Gro tomato formula. It has an analysis of 4-18-38 and also contains magnesium and micronutrients. To make a nutrient solution, it is supplemented with calcium nitrate and magnesium sulfate, depending on the variety and/or stage of plant growth.

Figure 2. Directions for use from the back of the Hydro-Gardens Chem-Gro tomato formula bag. Photo: Elsa Sánchez, Penn State

Advantages to Fertilizer Programs

• Programs like these are easy to use.
• Minimal ordering of fertilizers is needed (only 3 in the Hydro-Gardens example).
• Making nutrients solutions requires very little or no mathematical calculations.

Disadvantages to Fertilizer Programs

• Fertilizer programs do not allow for easy adjustments of individual nutrients. For example, if the foliar analysis indicates that more phosphorus is needed. Solely using a fertilizer program does not allow for just phosphorus to be easily added.
• Another drawback is that fertilizer programs do not allow growers to account for nutrients already in the water source. For example, if a water source has 30 ppm of potassium, there is no way to adjust the amount of potassium applied in the fertilizer program.
• Fertilizer programs can cost more than using recipes to make nutrient solutions.

Nutrient Solution Recipes

Recipes are also available to make nutrient solutions. Recipes contain an amount of each nutrient to add to the nutrient solution. They are specific to a crop and are available in a variety of sources including through university Extension services, online, and through trade magazines. An example is the Modified Sonoveld’s solution for herbs seen below (Mattson and Peters, Insidegrower).

Modified Sonneveld Solution Recipe

• Nitrogen–150 ppm
• Phosphorus–31 ppm
• Potassium–210 ppm
• Calcium–90 ppm
• Magnesium–24 ppm
• Iron–1 ppm
• Manganese–0.25 ppm
• Zinc–0.13 ppm
• Copper–0.023 ppm
• Boron–0.16 ppm
• Molybdenum–0.024 ppm

It is up to growers to decide which fertilizers to use to make a nutrient solution from a recipe. Fertilizers commonly used include:

Growers calculate how much fertilizer to put in the nutrient solution based on the amount of a nutrient in the fertilizer and the amount called for in the recipe.

Hydroponics Systems: Calculating Nutrient Solution Concentrations Using the Two Basic Equations

Advantages to Nutrient Solution Recipes

• Nutrient solution recipes allow for adjusting fertilizers based on nutrients contained in water sources. For example, if a grower is using a water source that has 30 ppm potassium and making the Modified Sonneveld’s solution for herbs which calls for 210 ppm potassium. They would need to add 180 ppm potassium (210 ppm – 30 ppm= 180 ppm) to the water to have the amount of potassium called for in this recipe.
• Recipes allow for easily adjusting nutrients. If a foliar analysis report indicates that a plant is deficient in iron. It is easy to add more iron to the nutrient solution.
• Because recipes allow for easy adjustments, fertilizers can be used more efficiently than in fertilizer programs.
• Using recipes can be less costly than using fertilizer programs.

Disadvantages to Nutrient Solution Recipes

• It is necessary to calculate how much fertilizer to add to the nutrient solution. (Link to pubs on doing calculations). Some people may be intimidated by the math involved. However, calculations require uncomplicated math skills based on multiplication and division.
• A high precision scale is required to measure micronutrients because of the small amounts needed.

Hydroponics Systems: Using the Two Basic Equations to Calculate a Nutrient Solution Recipe

Complete Soluble Fertilizer Approach

Some growers use a complete soluble fertilizer including micronutrients to provide nutrients to their hydroponics crops. Using this approach, nutrients are usually applied based on the nitrogen needs of the crop. For hydroponic lettuce, this might be 100 to 150 ppm nitrogen.

Figure 3. (Left) 20-10-20 fertilizer used to supply nutrients to a hydroponic lettuce crop. (Right) A metal blade connected to a motor (yellow arrow) is being used to keep fertilizer in solution. Photos: Elsa Sánchez, Penn State

Advantages to Using a Complete Fertilizer Only

• Of the three approaches to making nutrient solutions, this is the simplest.
• Only one fertilizer needs to be purchased.
• Only one fertilizer injector is needed for the nutrient solution.
• Complete fertilizers are widely commercially available.

Disadvantages to Using a Complete Fertilizer Only

• Using only a complete fertilizer may not provide the proper balance of nutrients to plants.
• This approach may not provide adequate amounts of nutrients not in the complete fertilizer. For example, magnesium and calcium may not be included.

How to Choose Fertilizers

Regardless of your approach to making nutrient solutions, you will need to select fertilizers. Solubility, cost, market availability, and sources are all items to consider.

Solubility is the amount of a substance that will dissolve in a given amount of solvent, for hydroponics systems the solvent is irrigation water. Solubility is a quantitative term usually expressed as g/100 ml (grams of fertilizer/100 ml irrigation water) and varies with each fertilizer. It is determined by adding fertilizer to water until even with shaking or stirring the fertilizer does not dissolve. Solubility differs depending on water temperature and fertilizers have higher solubilities in hot water than cold water. Tables are available online listing solubilities of various fertilizers. Below are solubilities for some fertilizers commonly used in hydroponics.

*Source – Resh, Hydroponic Food Production, 7th edition

Other considerations are cost and market availability. Costs vary by fertilizer and some are easier to find than others. Use greenhouse-grade fertilizers. They have higher purity and greater solubility than standard-grade fertilizers.

Using nitrate or ammonium fertilizer affects plant growth. Ammonium is used immediately by the plant and is not stored. It can be used in the summer when you want the plant to grow quickly. It can also cause excessive vegetative growth, especially in poor light, and result in ammonium toxicity. Ammonium toxicity symptoms are yellowing and the death of growing points. Nitrate is reduced to ammonia before it is metabolized by the plant. Additionally, it can be stored in plant vacuoles. Compared to ammonium, it will hold vegetative plant growth back. It can be used in winter or cold weather or anytime that slower plant growth is occurring or desired. Some nutrients, including iron, manganese, and zinc, can be easily made unavailable to plants. Chelates are organic molecules that surround metals making them readily available for plant uptake even when pH changes.

Nutrient Solution Concentration and Fertilizer Compatibility

Nutrient solutions can be supplied to plants either at normal strength or concentrated. Common concentrations are 50-, 100- or 200-times normal strength depending on injector capabilities and the solubilities of the fertilizers used. When nutrient solutions are concentrated some nutrients can form precipitates that can clog drip lines.

To avoid this, use two stock tanks for concentrated nutrient solutions. A general guideline for deciding which nutrients to put in which tank is that calcium should be in a different tank than phosphates and sulfates. Using the Hygro-Gardens fertilizer program as an example, putting the concentrated calcium nitrate solution in one tank and the concentrated Chem-Gro 4-18-38 and magnesium sulfate in a second tank would follow this guideline.

Keeping Nutrients in Solution

Some fertilizers may be difficult to keep in solution due to solubility issues. Agitation can be used to improve solubility. For large tanks, a propeller-type implement connected to a motor can be used. Another option is a circulating pump. For smaller tanks, an air pump can be used. Keep in mind that fertilizers are salts and can corrode some materials. Use materials, such as stainless steel, that will not easily corrode for agitation.