The objective is to develop a recipe for a refill solution that replenishes both nutrients and the water. Plants have evolved to tolerate large nutrient imbalances in the root-zone, but in recirculating hydroponic systems, imbalances in nutrient replenishment are cumulative. It is thus important to understand the principles for nutrient replacement, especially when the solution is continuously recycled over the life cycle of a crop.
Traditional nutrient solution recipes, such as Hoagland solution, can be used as refill solution if they are diluted to about 1/3 strength so that the electrical conductivity is kept constant. Hoagland solution, however, was originally developed for tomatoes and is not always appropriate as refill solution for other types of plants.
Two factors must be considered in developing a refill solution:
1. SOLUTION COMPOSITION The composition of the solution (the ratio of nutrients) should be determined by the desired concentrations of each element in the plant. A starting point for refill solution composition is the ratio of the elements in the plant leaves, which can be determined from a reference book on Plant Analysis Interpretation. I am familiar with four books that list the optimum concentrations of nutrients in plant tissue (and there are probably other books):
• Plant Analysis: An interpretation Manual. 1986. D. Reuter & J. Robinson, (eds). Inkata Press, Melbourne.
• Plant Analysis Handbook. 1991. J. Benton Jones, B. Wolf, H. Mills. Micro-Macro Publishing, Inc. Athens, GA.
• Plant Analysis. 1987. P. Martin-Prevel and J. Gagnard. Lavoisier Publishing Inc. New York.
• Diagnostic Criteria for Plants and Soils. 1966. Homer Chapman. Univ. of Calif., Riverside, CA.
Each of these books is organized differently and each has strengths and weaknesses. I recommend collecting the information from all of them for a particular crop and comparing the recommendations for the optimum range of nutrient concentrations.
Foliar analysis is based on the nutrient concentration in leaf tissue because leaves conduct the most photosynthesis and thus have the highest enzyme levels in plants. Average nutrient concentrations of whole plants are usually less than the concentrations in leaves, so a refill solution based solely on leaf tissue concentration will over supply nutrients for stems, seeds, and fruits. We have made many measurements of nutrient concentrations in different parts of wheat plants. Table 3 shows the that the concentrations of most elements are much higher in leaves than in other plant parts.
Young plants easily develop nutrient deficiencies but rarely develop nutrient toxicities so we use a relatively concentrated initial starter solution. A refill solution with adequate nutrients for early vegetative leaf growth is usually too concentrated when plants are developing stems and leaves so we alter the composition of the refill solution with the growth stage of the plant to prevent nutrient accumulation in the solution. The life cycle can be divided into 3 stages:
• Early vegetative growth, which is primarily composed of leaf tissue (starter solution).
• Late vegetative growth, during which growth is composed of about equal amounts of stem and leaf tissue (vegetative refill solution).
• Reproductive growth, during which leaf growth is minimal and nutrients are mobilized into seeds or fruits (seed refill solution).
Root growth primarily occurs during early vegetative growth and is much less significant during late vegetative growth. Root growth decreases and even stops during reproductive growth.
The rationale underlying the differences between Hoagland's solution and Utah Wheat solution are not obvious so a discussion of differences is useful.
NITROGEN: When nitric acid is used for pH control, about half of the nitrogen is supplied in the pH control solution. Nitrogen in the refill solution can thus be less than in Hoagland's solution. Ammonium nitrate (NH4NO3) can be added to the pH control solution if necessary to obtain even higher levels of N in the plants, but ammonium reduces the uptake of other cations so it should only be used if necessary.
POTASSIUM: The supply of K is more constant with a low level in the starter solution and a more concentrated refill solution.
CALCIUM: Grasses have a lower requirement for calcium than dicots.
MAGNESIUM and SULFUR (MgSO4): We have not found that 1 mM is necessary.
IRON (Fe): The use of modern chelating agents means that iron can be maintained in solution and much lower levels can be maintained.
BORON: Grasses have much a lower requirement for boron than dicots.
ZINC and COPPER: These elements are ubiquitous contaminants. Hoagland and Arnon in the 1940's and 50's probably got most of these elements from contamination of the solution. Modern plastics, especially PVC pipe, greatly reduce copper and zinc contamination.
SILICON: A beneficial element. See section on silicon in this paper.
2. Solution Concentration The concentration of ions in the refill solution is determined by the ratio of transpiration to growth. Transpiration determines the rate of water removal; growth determines the rate of nutrient removal. A good estimate of the transpiration to growth ratio for hydroponically grown crops is 300 to 400 kg (Liters) of water transpired per kg of dry mass of plant growth. The exact ratio depends on the humidity of the air; low humidity increases transpiration but does not increase growth. Elevated CO2 closes stomates and increases photosynthesis so the transpiration to growth ratio can decrease to about 200 to 1.
A knowledge of these ratios is useful in determining the approximate concentration of the refill solution. For example, 1/4 strength Hoagland's solution is about right for plants grown in ambient CO2, but 1/3 strength Hoagland's solution may be required for plants grown in elevated CO2. Total ion concentration can be maintained by controlling solution electrical conductivity. If the conductivity increases, the refill solution should be made more dilute, but the composition should be kept the same. The electrical conductivity does not change rapidly so it is usually necessary to monitor it only a few times each week. We have successfully used this approach in long-term studies (months) without discarding any solution. This procedure can eliminate the need to monitor nutrient solution concentrations in the solution.
NUTRIENT RECOVERY IN PLANT TISSUE As mentioned earlier, the mass balance approach to nutrient management assumes that all of the nutrients are either in the solution or in the plant. Surprisingly few detailed mass balance studies to test this assumption have been conducted, however, studies in our laboratory and studies by Dr. Wade Berry at UCLA clearly indicate that the recovery of several elements is less than 100%, while recovery of some micronutrients is much greater than 100%. Table 5 indicates the average recoveries of elements from solution in six replicate 23-day studies. These recoveries are typical of recirculating hydroponic systems. Because recovery of macronutrients is 50 to 85%, additional macronutrients should be added to the refill solution. Reduced amounts of some micronutrients may be warranted when the contamination is reproducible.
TABLE 5. Average recoveries of the essential nutrients in plant tissue at the end of six replicate 22 day studies with wheat. The recovery of all of the macronutrients, and iron and boron was 50 to 85% of that added to the nutrient solution (minus what was left in solution at the end of the trial). The recovery of Mn, Zn, Cu, and Mo was greater than 100% because of contamination of the hydroponic solution from elements in the plastics or the magnetic drive pumps. Many different types of plastics were used to build this system and many plastics use zinc and copper as emulsifiers in manufacturing. These recoveries are typical in recirculating hydroponic systems.
DEVELOPING AN APPROPRIATE REFILL SOLUTION
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