Refilling nutrient solutions are only required when a closed (NFT) system is used. With open hydroponic systems the nutrient solution is not re-used so no refill solution is required. Very little information is available on the subject and methods should be developed according to system design. Each system will have unique requirements because of factors such as:

  • plant growth stage
  • variety
  • number of plants
  • total volume of water in the system and
  • climate

The nutrient solution in open hydroponic systems are made-up and applied in its entirety to the plants. Thereafter a new solution is made up. With closed hydroponic systems the situation is quite different. Plants continuously absorb nutrients from the re-circulating water, so the concentration of nutrients in the solution, change every minute.  At a certain critical stage the nutrient concentration needs to be refilled or replaced to its original concentration.  Note what has to be corrected: the CONCENTRATION. If you have a 5000L tank in an open hydroponic system, you know how much nutrients have to be added, but with closed hydroponic systems, you will still have some water left in the nutrient tanks. So the question is, how much fertilizers must be added? This process is quite often referred to as “topping up”.

The objective is to develop a refill solution that replenishes both nutrients and the water. It is important to note that in recirculating systems, imbalances in nutrient replenishment are cumulative. So it is important to understand the basic principles of nutrient replacement in closed circulating systems.

Most nutrient formulations last approximately 2-3 weeks depending on environmental and plant factors. During summer months, when plants are growing rapidly, a formulation might only last a week after which the composition should be changed. Using a fixed formula to replenish nutrients in a closed hydroponic system can cause the build-up of some nutrients in the solution and provide a false EC reading. There are three major factors that influence the rate of uptake of various elements. These factors are:

  • The crop type
  • Growth stage
  • Environmental conditions such as temperature, humidity and light intensity

There are many basic formulations available from various fertilizer companies that can be used as refill solutions, however, care should be taken that these solutions are not used without taking a leaf sample or water sample in order to prevent build-up of excess ions.

The solution composition and foliar analysis should be considered in developing a refill solution. The composition of the solution is determined by the nutrient requirements of the crop during the growth stage.  Always use milli-equivalents and not parts per million (ppm).  A starting point for a refilling nutrient solution is the ratio of the elements in the plant leaves, which can be determined from a reference book on plant analysis.

Foliar analysis is based on the nutrient concentration in leaf tissue. Average nutrient concentrations of whole plants are usually less than the concentrations in single leaves, so a refill solution based solely on leaf tissue concentration will over supply nutrients for stems, seeds, and fruits. Average concentrations in tomatoes are:

  • Nitrogen (N): 4.5-5.5 %
  • Phosphate (P): 0.6-1.0 %
  • Potassium (K): 4.0-5.5 %
  • Calcium (Ca): 1.5-2.5 %
  • Magnesium (Mg): 0.4-0.6 %
  • Sulphate (S): 0.3-1.5 %
  • Iron (Fe): 80-150 ppm
  • Manganese (Mn): 70-150 ppm
  • Boron (B): 35-60 ppm
  • Zinc (Zn): 30.45 ppm
  • Cupper (Cu): 4-6 ppm

Nutrient deficiencies can develop quickly in closed hydroponic systems since there is no growth medium to act as a buffer.

When developing a refill solution the growth stage of the plant should be taken into consideration. The three basic stages are:

  • Early vegetative growth, which is primarily composed of fast developing leaf tissue. A starter solution is used at this stage.
  • Vegetative growth stage, during which growth is composed of about equal amounts of stem and leaf tissue development. A vegetative refill solution is used during this period.
  • Reproductive growth stage, during which leaf growth is reduced and nutrients are mobilized into fruits (standard refill solution).