There are many different plant nutrients on the hydroponics market today. Their function is to provide the optimum mix of Nitrogen, Phosphorous, Potassium, Calcium and various other trace elements, in order to sustain growth, improve yields and allow the plant to achieve its potential. The plants requirements will vary to some extent as it develops. Concentrations and plant food components may also vary with differing growing mediums. The food is absorbed through the plants roots and transported to the leaves, where it is converted into the sugars that the plant needs for energy.
The most important thing to remember about plant nutrition is that the NPK, (Nitrogen, Phosphorous, Potassium) Calcium and trace element ratios are correct. There can be a wide variation of ingredients in the various mixes for sale.
Because the plant will take whatever it requires from the elements available and leave the rest, the balance will alter as unused elements build up in the solution. If left unchecked this will result in a toxic build up of salts and a subsequent drop off in growth followed eventually by the death of your, well loved and nurtured, plants. This same result will occur if the water content is not replaced and the mixture strength increases. If the plant transpires 50% of the water from the supply tank, the concentration of elements within the solution will become dangerously high.
The concentration of salts in the feeding solution is measured using an Electrical Conductivity (EC) meter. The EC meter measures the strength of the solution in parts per million. This means that in a 1000 PPM solution there are 1,000 units of dissolved salts to every 1,000,000 units of water. The meter measures the total salt concentration in solution and does not discriminate between Potassium salts say and Calcium salts. It cannot tell the difference between a good and a bad mix, only their relative strengths.
The EC meter works by measuring the speed at which electrons travel between probes immersed in a solution. In distilled water, the electrons cannot find any impurities to use as footholds to cross the water and so the meter returns a 0 reading in mMho or mS (these are units used to measure electrical conductivity). As food is added to the water, the concentration of impurities in the form of salts increases and the electrons can find more footholds, and so cross the water faster. Thus the meter reading rises. Of course this is a very simplified explanation, but it should serve to give you an idea of the basics. One other important thing to remember is that as in all things chemical temperature plays an important part. The higher the temperature, the faster the electrons move and the higher the EC reading. This means that that in order to accurately assess your mixture