No matter what nutrient formula you use, it is important to start with a clean water source. It is so important, in fact, that commercial growers often test their raw water before the greenhouse is even built. If there are potential problems with the water supply, growers might opt to choose a new location instead of trying to correct problems.
Unfortunately, hobby growers rarely have the luxury of moving to a new location just to grow a great garden. For this reason, it's even more important for hobbyists to identify any potential problems with their source water and take the proper steps to obtain the cleanest water possible. If you have never sent a water sample out for a lab analysis, then now might be a good time to do so.
A basic greenhouse water analysis costs about $40. It quantifies all of the soluble plant nutrients in the raw water and alerts the grower to potential problems. In the hands of a trained consultant, a water analysis also shows a grower how to customize his or her nutrient recipes to optimize quality and yield.
Is Your Water Source Too Hard for Garden Plants?
Even if a water analysis shows you have hard water with relatively high levels of minerals, it can still be excellent source water for growing great plants. Hard water typically has high levels of calcium carbonate and magnesium carbonate. Calcium and magnesium are both essential elements for plant growth. So, as long as no other elements are in the water at toxic levels, the water is suitable for horticultural use. In fact, there may be trace amounts of beneficial elements in hard water that are missing from more pure water sources.
For best results, use a hard water nutrient formula when using water that is high in calcium and magnesium carbonate. To compensate for excess minerals, hard water nutrient formulas are usually lower in calcium and magnesium, with reduced levels of sulfates, so a hard water nutrient formula complements the minerals already present in the raw water.
A laboratory water analysis will also show the alkalinity of the water. The higher the alkalinity, the greater the buffering capacity of the water against changes in pH. Since hard water typically resists changes in pH, a good hard water nutrient formula will usually be more acidic than standard nutrients. It will also have a higher ammonium to nitrate ratio. When a plant takes up a positively charged ammonium ion, the roots exude a hydrogen ion in exchange, helping to neutralize the excess bicarbonates in the hard water.
Read More: How to Understand a Water Quality Report
One problem with hard water is the formation of limescale. When phosphoric acid is added to adjust the pH, the bicarbonates are burned off as carbon dioxide and water, but the extra phosphorus may react with the calcium to form calcium phosphate. Calcium phosphate is what your bones are made of, and it is 95% water-insoluble. So the calcium phosphate often falls out of the nutrient solution as limescale. Once limescale is formed, both the calcium and phosphorus become unavailable to the plant.
Limescale can be easily prevented by adding a small amount of an amino acid blend to your hard water. The amino acids help chelate the calcium ions in the raw water. The word "chela" means claw, and the amino acids attach to the calcium ions like a claw, preventing them from reacting with phosphoric acid, and the calcium remains soluble. Some amino acids also stimulate root cells to open up calcium ion channels, allowing the calcium to be taken up by the plant thousands of times faster than simple osmosis. The result becomes stronger plants with stronger cell walls and improved resistance to temperature stress and disease.
However, not all hard water sources are good for plants. If the water is excessively hard, or if a water analysis shows toxic levels of trace elements such as sodium or boron, the water should be filtered or processed.
Is Your Water Source Too Soft for Garden Plants?
Some growers simplify water quality management by using only filtered water. Reverse osmosis (RO) filters remove all mineral ions from the source water, starting with a clean slate. Therefore, if you use an RO filter, all of the essential elements will come from your nutrient solution—not from the source water—creating the perfect mineral balance for plant growth. RO water is also great for topping off your reservoir between nutrient changes, replacing the water lost to evaporation without adding any unwanted minerals.
There are, however, a few downsides to RO water. It is so pure that all of the bicarbonates are removed from the source water. As a result, RO water offers no buffer to changes in pH. During rapid vegetative growth, when plants are taking up proportionally more nitrates, the pH can quickly spike upward.
During heavy fruiting and flowering, a time when plants take more potassium ions, the pH can then crash.
Extreme swings in pH can adversely affect plant growth. For example, if the pH spikes above 6.5, iron starts to become unavailable to the plants, and as the pH continues to rise, all of the essential metal ions become unavailable. As nutrient deficiencies develop, plant growth stalls and the plant's natural resistance to environmental stress is greatly hindered.
To know for sure if RO water is right for you, send a sample of your raw water to a lab for analysis. If the analysis shows that even one element is approaching toxic levels, an RO filter is definitely recommended. RO is also recommended for any application where a water softener is used. A water softener removes the calcium and magnesium ions from the hard water, but replaces them with sodium ions, often at toxic levels. If sodium levels exceed 50 ppm, the plants may not be able to reach their true genetic potential.
The best solution is to blend RO water with tap water to create a clean water source with just the right amount of pH buffering capacity. Not only will you improve the source water for your plants, but you will also extend the life of your filter. For example, if a water analysis shows that you can get by nicely with a 50:50 mix of RO and tap water, you could double the life of your filter, while improving the water quality to your plants!