The ice can either be used in block form or broken down into small pieces and used a little at a time. Another method is to put the dry ice into a Styrofoam container with holes in it to slow down the rate of evaporation. Since carbon dioxide is heavier than air it must be placed above the plants in order for it to drift down and surround the plants. In hydroponics plants are grown in a solution consisting of water and nutrients required for a particular plant, or within a system that uses a substrate or growing mix medium. There are several hydroponics techniques and systems that exist for producing plant crops. Hydroponics systems include the nutrient film technique, aeroponics, and the aeration technique. If you are a beginning hydroponic gardener you may want to use the pre-mixed nutrient solutions and observe their effects before you begin to make your own solutions. The basic nutrients needed in any hydroponic solution are potassium phosphate, potassium nitrate, calcium nitrate and magnesium sulfate. Natural enemies will feast on the spider mites until there aren’t enough for the predator to survive, then they will die off or leave in search of more food. Releasing predator insects into your hydroponics garden infestations of spider mites can be eliminated or, they can be avoided. There’s no need to worry with predator insects, because most will not harm your crops. The protein and enzyme molecules in a plant are structured in very specific shapes in order to catalyze a chemical reaction to build the plant cells. The need reactant molecules to fit precisely into their gaps so all of the negative and positive charges have to line up exactly. Plants are able to change their own cellular pH level to stop or help speed up a certain enzyme reaction. The temperature control units may also come in handy once you decide how many plants you want to grow in your space. The more plants you have the hotter your greenhouse will usually be. A temperature control unit can be set to turn fans or blowers on and off at pre-determined intervals. They can also be set up to open and close air ducts in your greenhouse or grow room.