Plants consist of 80-95 % of water, depending on specie, less than 15-20 % percent consists of carbon (C), oxygen (O) and hydrogen (H). The remaining 1.5 % is made up of the macro and microelements. This small portion will be our focus.
All plants require macro- and microelements in specific ratios in order to grow properly. The soil provides most of these elements to plants in conventional agriculture. Due to the depletion of nutrients in the soil, growers have to replace the most important micronutrients through granular fertilizers. These fertilizers are designed to dissolve over a period of days. The soil provides the medium in which the fertilizer could dissolve and also provides a buffer that prevents the pH and electrical conductivity to vary to much (Through cation exchange capacity of the soil). In hydroponic systems there is no soil, except the nutrient solution and the inert growth medium that does not contain any nutrients. All nutrients must be supplied through the nutrient solution (with hydroponic fertilizers). The disadvantage of not having soil is that the nutrient solution does not provide any buffer to variations in pH and electrical conductivity. It is thus critical to manage the content of the nutrient solution every day. In soil it is necessary to analyse the nutrient content a few weeks before planting, calculating a fertilizer program and applying it on a weekly basis to the plants. There is no need for further analysis. If the plants do show some deficiency symptoms, it is easy corrected by applying some granular fertilizers. However, if there are toxicity symptoms there is nothing the grower could do. The soil provides the plants with a supply of water, a constant supply of nutrients, a supply of oxygen and it kept the plant upright.
Hydroponic fertilizers are not the same as those used in conventional agriculture. Well known granular fertilizers are L.A.N., Superphosphate (11.5%), 1:0:1(49), 3:2:3(32) and 2:3:5 (21). All these fertilizers cannot be used in hydroponic systems since they are not 100% water soluble. If used, a sediment layer accumulates at the bottom of the tank which is not soluble at all. This sludge will block irrigation systems and damage pumps. Hydroponic fertilizers are completely water soluble and do not leave any sediment at the bottom of the tank if mixed correctly.
There is a huge difference in calculating and applying the fertilizer requirements in hydroponic and conventional agriculture. In conventional agriculture the amounts that need to be applied are measured and expressed in weight per unit area such as 100 kg.ha-1. For instance, if nitrogen was needed a grower would apply 500 kg.ha-1 of 1:0:1(49). It is relatively easy. There are no other factors that need to be calculated. The pH can only be manipulated by applying either gypsum or lime. The only nutrients ever considered are nitrogen, phosphorus and potassium. Other elements such as calcium, magnesium and sulphur are hardly ever calculated and only in extreme cases when a soil analysis indicates that there might be some deficiencies. In some cases, as with tomatoes, the complex fertilizers such as 2:3:2(33) have added Zn since tomatoes have high Zn requirements. The total amount of hydroponic nutrients are calculated as a concentration, either parts per million (ppm) or milliequivalents (me). The exact concentration is measured and maintained by measuring the electrical conductivity of the nutrient solution. Further, all elements are calculated and the ratio between the elements are also taken into account. Thus the calculation of hydroponic nutrient solutions is a much more complicated task than the calculation of the fertilizer requirement in conventional agriculture.