Growing 101: Nutrient Solution Management

Plants need access to nutrients for the various metabolic processes used for growth and development. In a natural setting, these nutrients are supplied by nearby decomposing plants, animal waste and existing mineral content in the soil. In a home garden, decomposing material is generally relegated to the compost pile, so once plants absorb the available nutrients from the growing media, the media will become barren unless the nutrients are replaced.

For the hydroponic grower, nutrient solutions are the most common method of replenishing plants once the initial growing medium’s supplies have diminished. Hydroponic mediums generally don’t have any nutrient value at all and are entirely dependent on nutrition solutions from the start. Nutrient solutions are created by mixing measured amounts of nutrients (often nitrogen, phosphorus and potassium) into water (the solvent).

It is common practice to combine nutrients and water in a nutrient solution. Hydrophilic (water-loving) materials like salts tend to dissolve easily in water because water molecules have a positive charge on the side with the two hydrogen atoms and a negative charge on the oxygen side. These water molecules attach easily to many other molecules. If local tap water is questionable, reverse osmosis filters or other filtering systems can be used to clarify and clean the water before use.

Salts are often used as nutrient sources because they also have a positive and negative side but are generally held together with a weaker ionic bond. Water molecules attach themselves to either side of the salt molecule and split them apart. This is why many chemical hydroponic solutions make use of nutrient salts dissolved in water. Organic nutrients are often less miscible and should be shaken to create a suspension before use.

Nutrient solutions also require a suitable amount of available oxygen. The level of freely available oxygen in water is known as its dissolved oxygen (DO) content. Air stones are used to increase DO. The amount of air water holds depends on temperature. The warmer the water, the less air it will hold and the more important additional aeration becomes. If the amount of DO drops too low, water will go stale and create conditions for anaerobic (air-fearing) bacteria to develop, which creates a foul odor.

Concentrated Hydroponic Nutrients

There are several nutrient companies that make concentrated nutrient solutions, which are added to water according to the manufacturer’s directions. These directions often include a week-by-week schedule suitable for many flowering plants. Usually these schedules are designed to supply additional nitrogen during vigorous growth and additional phosphorus during the flowering and fruiting stages.

Never mix full-strength concentrates together before adding to the water, as this can have detrimental effects on the finished solution. When mixing a nutrient solution that has more than one concentrate added to it, make sure to add the concentrates individually to the solvent (water) and stir at least briefly between.

Application Rates for Hydroponic Nutrients

For a nutrient solution to be useful to plants, it must come into contact with or within close proximity to the root system. In a recirculating system, the nutrient solution is replaced periodically both to rebalance the mix of available nutrients and to remove any leftover salts in the solution. To help wash away undesirable salt buildup in a grow medium, a practice known as flushing involves applying clean water to the medium for the purpose of dissolving and removing any salts the medium might be holding on to.

Monitoring and Metering Nutrients in the Garden

After applying, you’ll want to monitor your nutrient levels closely through the use of meters. Meters are best at estimating nutrient levels when salt-based fertilizers are used since salts have a large impact on conductivity. Pure water does not conduct electricity, but once conductive materials such as salts are introduced, a nutrient solution will. If a little conductive material is added, little conductivity results. As more conductive material is added, the solution conducts electricity better.

Total dissolved solids (TDS) meters can be used to measure the electrical conductivity (EC) of a nutrient solution. The information taken can be displayed as an EC reading or as a parts per million (ppm) reading. The EC reading is easy to compare against other EC readings since it is simply a measure of how conductive a material is.

Ppm meters use the same principle with the results displayed slightly differently. There isn’t an exact conversion between EC and ppm, which often leads to confusion. Depending on which of the three most popular ppm standards are used, the conversion factor is 0.5, 0.64 or 0.7. While this might lead some to believe the EC scale should be adopted and the ppm scale abandoned, it isn’t that simple.

When calculating nutrient solutions from scratch, it is handy to measure how many milligrams of material is added to a liter of water, which is literally parts per million: one milligram of sugar in one liter of water is one part sugar per million parts of water. One part per 100 is used so often in similar calculations that it is generally abbreviated to a percentage.

Regardless of which scale you use, the information gathered with a meter, coupled with the specific nutritional needs of the plant, can be helpful in maintaining a suitable nutrient solution. EC and ppm meters only display the total amount of electrically conductive contamination present, not specifically what the contamination is. Also, remember that these meters do not provide specific nutrient values.

Final Thoughts

Nutrient solutions can be as simple as following directions on a bottle, or as complicated as mixing a chemical cocktail like a mad scientist, but they serve an important part of keeping your plants healthy and productive.