Today everyone knows that  and    are the chemical formulae for water and carbon dioxide molecules. The formula   means that each molecule of water consists of 2 atoms of the element Hydrogen and one atom of the element Oxygen. That is very simple chemistry. Chemistry represents each element with a Capital letter with or without a second lower case letter. More than 100 elements are known. Growers only need to know the names and symbols for 15 elements that are essential for plant growth plus just a few more common elements that occur in water or plants. Plants are unable to grow and complete their life cycle if they do not have access to all of these 15 essential elements. This table contains most of the chemistry growers need to know.

Essential Element	Symbol	Plant uptake form	Atomic weight
Hydrogen	H		1
Carbon	C		12
Oxygen	O	, ,	16
Nitrogen	N	&	14
Phosphorus	P		31
Potassium	K		39.1
Sulphur	S		32.1
Calcium	Ca		40.1
Magnesium	Mg		24.3
Iron	Fe		55.9
Manganese	Mn		54.9
Zinc	Zn		65.4
Copper	Cu		63.6
Boron	B		10.8
Molybdenum	Mo		95.9
Non Essential but Common elements
Silicon	Si	?	28.1
Sodium	Na		23
Chlorine	Cl		35.5

The greatest part of the plant dry weight of plants is in compounds of hydrogen, carbon and oxygen which the plants obtain from water and air using the energy of sunlight by photosynthesis. The other nutrients, known as the mineral nutrients are taken up from soil or nutrient solutions. The major or macro nutrients (N, P, K, S, Ca and Mg) are taken up in relatively large amounts, while only very small quantities of the micro-nutrients or trace elements are absorbed (Fe, Mn, Zn, Cu, B, and Mo).

Nutrients pass from the soil or nutrient solution into plants as ions, that is charged molecules dissolved in water. Anions are negatively charged, the important anions are nitrates ( , compounds of nitrogen and oxygen), phosphates ( ), sulphates ( ) and chlorides ( ). Important positively charged cations are ammonium ( ), potassium ( ), calcium ( ), magnesium ( ) and sodium ( ).

Nutrient solutions are made from soluble fertilisers (which are chemical salts) and or with acids or bases. Salts are solid compounds containing and  which separate into their respective cations and anions when dissolved in water. Acids are anion combined with a hydrogen ion ( ), while bases are cations combined with a hydroxyl ( ) ion. Potassium sulphate is a typical fertiliser salt with the formula , that is the compound consists of 2 potassium atoms, a sulphur atom and four oxygen atoms.  The mole or more usually spelt mol is the chemical unit for   atoms, ions or molecules. Atoms vary in weight and the table above lists their atomic weight. The atomic weight of an element is the weight of that element in grams that contains exactly the same number of atoms as there in 12 grams of Carbon (the carbon 12 isotope). 1 gram mol of carbon weighs 12 grams. Note that 1 mol of   contains 2 mols of K, 1 mol of S and 4 mols of oxygen. The weight of 1 gram mol of   is equal to the sum of its atomic weights (=2 x 39.1+32+4 x16) or 174.2 grams. Thus the concept of the mol can be used with atomic weights to allow the proper proportions of salts, acids or bases to be used with ordinary weight measurements for formulating nutrient solutions or for many other chemical calculations.