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’s EC you must record the PPM as mMho (mS) at a specific temperature.
As the PPM reading is a conversion from an electrical reading and as each addition of a different salt will alter the electrical properties, in order to obtain an accurate EC reading you will have to use a reference solution of a known value. Because the EC meter you are using will not necessarily have been calibrated for the mix used by the people who prepared your reference solution, these values can be quite inaccurate. In view of this, any reference solution that does not show the EC value in mS, or give you the conversion ratio that was used, is of no use for nutrient evaluation purposes. It is important to note that if the nutrient EC reaches 3,000 PPM (or the meter reads over 4.0mS) your plants will begin to show signs of nutrient deficiency even though they will have an excess. The reasons for this are quite complex, but basically it is because the chemicals dissolved in the solution are competing for the available water and the stronger ones are blocking out some of the weaker ones. This leads to the roots having to work harder to absorb the nutrients. By working harder they have to expend more energy at the expense of growth. If at this time the temperature rises and the water level drops, due to evaporation, your plants will, very probably, die.
Probably the most important factor that will affect your plant growth in relation to nutrient uptake is pH. Different types of plant prefer different pH values and it is important to ascertain which the optimum for the species you are growing is. The medium in which you are growing will affect the cation exchange capacity of the plant. This is the ability of the medium to hold nutrients on call for the plant roots to use. Normal soil has a high cation exchange rate (CEC) of between 100 and 200 equivalent units. A number of growing mediums and of course water cultures have a CEC of 0. This means that once a nutrient has passed the roots it cannot be taken up by the plant, and neither will it have any buffering effect.
The nutrients, the gasses, the trace elements, the water and the growing medium all have differing electrical charges and are all exchanging positive and negative charges around the roots of the plant. This ionic battle enables the roots to absorb the nutrients it needs to sustain the plant. If the pH is incorrect it stops the particle exchange. This is because the shapes and sizes of the charged particles will be different from the spaces available within the plant root tissue. The pH can be looked at a bit like a Yale lock and key. If all is correct the lock opens if the plant pH and the surrounding pH differ then the lock cannot open.
Different plants need different nutrients at differing stages of their growth. These nutrients have different charges and so in order to get the greatest nutrient uptake the pH must be closely monitored. If in doubt about the requirements of your plant try asking the manufacturer of your nutrients for help. After all he made the mix in the first place and so should know all there is to know about it. If your plants are not thriving look at the pH as the primary cause and try to work out which of the nutrients is not being absorbed and why.