Dissolved Solids in Water (TDS)

Bad water can cause big problems. Pure water is often not available to hydroponic growers. Almost all domestic water supplies contain certain "dissolved solids," minerals that cannot be filtered out in the way that particles can. Generally these conditions won't cause too much trouble. A simple pH adjustment will usually correct an imbalance caused by "hard" water.

However, there is a limit. In some areas the amount of total dissolved solids or of specific elements in the water supply can combine with elements in the nutrient solution resulting in nutrient lock-out. This may occur when well water is used to mix nutrient solution or where the municipal water supply is very hard. Water containing more than 50 parts per million (ppm) of calcium and magnesium (called "total hard- ness") can create serious problems. Other common elements that may be present in hard water include various carbonates, sulfur, sodium, iron and boron.

Your municipal water supplier can provide you with an analysis of your water supply. If you are using well water, there are many laboratories that can provide you with an analysis if you send them a sample. If the news is bad, it may be necessary to collect rainwater (a good idea wherever possible), install a reverse osmosis filtration system, deionization system, steam distillation system or use purified water (not mineral or "spring" water).

Dissolved solids (ppm) can be measured by using an instrument called a conductivity meter. Pure water will not conduct electricity. The higher the amount of dissolved solids the solution contains, the higher its conductivity will be. Thus, the conductivity meter can measure the electrical conductivity in the solution and interpret that measurement as ppm. Generally this method is the best available to the home grower to measure water quality before nutrients are added and to identify dissolved solids (ppm) after adding the nutrient mix.

It is critical that the nutrient solution not exceed the plant's tolerance for dissolved salts. That tolerance can range from extremely low for some plants such as orchids, to a very high for salt-tolerant crops such as barley. Unless you know the specific tolerance of a given crop, it is best to use a nutrient between 800 and 1,200 ppm.

When in doubt, remember that it is always better to apply too little nutrient than too much. The typical "dose response" curve of plants to variations in nutrient concentration shows three distinct and sharply defined zones: a "deficient zone" where there are insufficient nutrients for healthy plant growth; a "tolerant zone" in which sufficient nutrients are available; and a "toxic zone" where nutrient concentration is too high (too strong) for healthy plant growth.

A complicating factor in determining nutrient strength is that not all salts give equal electrical conductivity readings at specific concentrations. For example, monopotassium phosphate, a common salt used in the composition of plant nutrients, offers very poor conductivity and is practically invisible to conductivity meters. Nutrient solutions containing high monopotassium phosphate levels will appear to be much weaker than they actually are. It is important to be aware that this type of nutrient is stronger than it appears to be, based on your readings.

Always follow the manufacturer's recommendations for mixing nutrient, then measure the conductivity of the resulting solution. This will tell you what "indicated ppm" should be for that particular nutrient solution when mixed with your water supply, although "actual ppm" is probably higher.

As plants consume nutrients and water, the nutrient strength will change in the hydroponic reservoir. In hot, dry regions it is common for plants to transpire lots of water; if you measure the ppm you may find that it rises. It will be necessary to top off the reservoir with water and bring the indicated ppm down to a reasonable level. In cool, humid environments you may find that the ppm drops; this is because the plants are consuming nutrients and not transpiring lots of water. It will be necessary to top up the reservoir with nutrient solution in order to bring the indicated ppm up to its proper level.

A fast growing crop can consume huge amounts of nutrients. If you have a small reservoir it is important to change the solution frequently. Depletion of the solution will result in slow, spindly growth and sickly plants. A large reservoir in proportion to the total bio-mass will not have to be changed as often. Small plants, or naturally light feeders will deplete nutrients more slowly. Different types of plants have differing nutrient needs. The composition of nutrient solutions for all types of plants will contain the same elements as the list at the beginning of this article, however, the ratio of these elements can differ greatly. These variables can be striking when the nutrient needs of one type of plant are compared with the needs of another.

For example, orchids prefer a nutrient that is not only mild (low ppm) but also of a different NPK ratio in comparison to a high metabolism plant such as a fruit producing annual which must complete its entire life cycle within one growing season - from seed germination, through seedling, vegetative growth, flowering, fruit and seed production. Moreover, the fruiting annual which is going through this high-speed metamorphosis in less than one year will also have greatly differing nutrient needs during the various stages of its life cycle.

During rapid vegetative growth a plant can use lots of nitrogen, but a flowering or fruiting plant needs more phosphorus and magnesium. Hydroponic cultivation enables the grower to provide different diets for the crop at different times during the growth cycle. One of the great advantages of hydroponic over soil cultivation is the ability to manipulate nutrient concentrations for enhanced plant growth.

There are manmade nutrient formulations on the market that provide the same NPK combination throughout the plant's life cycle. The best of these are crop-specific formulations. Many manufacturers produce a particular product for orchids, another for tomatoes and perhaps another for indoor ornamentals.

These products will provide reasonable nutrition for the particular crop for which they are designed. However, since it is not possible to alter the NPK combinations during the various phases of growth, it is not possible to perform "nutrient manipulation" with general-purpose products. A multi-stage nutrient that permits adjustment of total ppm and NPK ratios will help you gain the full advantage from your hydroponic system.