During the 1980s, soilless cultures expanded enormously. Since then several kinds of substrates, both biodegradable and non biodegradable, have been developed. In this research the physicochemical characteristics of one of the most commonly used chemical inert substrates "rockwool" is investigated. The flow patterns occurring in rockwool slabs under both laboratory and practical situations were investigated, using a pigmented nutrient solution. As the 2 rockwool slabs became more saturated, flow patterns were spread more horizontally. The horizontal and vertical EC and pH gradients present in rockwool slabs under practical situations are represented in figures for a tomato crop. Both horizontal and vertical EC gradients with a magnitude of 4 mS/cm are present. Below the plants the pH in the slabs is lower, while there is almost no pH gradient in the vertical direction. The distribution of ions in rockwool was studied under laboratory circumstances as a function of the moisture content of the slab, the fertigation time, the fertigation volume and the vertical position in the rockwool slab. The moisture content of the slab, the fertigation volume and the vertical position in the rockwool slab affect the distribution of ions in the slab.

The pH of a nutrient solution is a property that is inherent to its composition. Changing the pH of a nutrient solution affects its composition, elemental speciation and bioavailability. The term 'speciation" indicates the distribution of elements among their various chemical and physical forms like: free ions, soluble complexes, chelates, ion pairs, solid and gaseous phases, and different oxidation states. For a standard nutrient solution elemental speciation is calculated for a pH range from 2 to 9. For each cation the formation of precipitates, ion pairs, complexes, and chelates is illustrated as a function of pH. The calculation of elemental speciation in nutrient solutions is an indispensable tool in the design, the analysis and the interpretation of experiments with the mineral composition of nutrient solutions in plant nutritional research. 861

In hydroponic plant nutritional research, nutrient solutions can be considered as aqueous solutions of inorganic ions. In this aqueous solution the ions are submitted to the laws of aquatic inorganic chemistry. This means that the ions are involved in the dynamic equilibria between complexation, dissociation and precipitation reactions. These chemical reactions seriously impact elemental speciation and bioavailability. As a result, plant roots experience a different nutritional composition. Ions withdrawn from the nutrient solution due to precipitation reactions, change the nutritional composition and are not available for uptake by plant roots. Like complexes, precipitates can buffer a nutrient solution, exchanging nutrients as these decrease by plant uptake. This research looks into the precipitation reactions that occur in hydroponic nutrient solutions. In the concentration range of nutrient solutions no precipitates are formed involving potassium, nitrate, ammonium or sulphate, while Ca 2+ and Mg 2+ form mainly at a higher pH precipitates with HPO 4 2-. Preparing nutrient solutions with tap-water, CaCO 3 is likely to precipitate. A good knowledge of the chemical reactions occurring in nutrient solutions is the first prerequisite in hydroponic plant nutritional research.

In the past, a large number of standard nutrient solutions has been devised. To investigate if there is an essential difference between these standard solutions, the mineral composition of 12 standard nutrient solutions formulated between 1865 and 1994 are compared with each other. Half of these standard solutions contains ammonium in a millimolar range.The effect of elemental speciation of the micronutrients in the 12 standard solutions on bioavailability is compared. The macronutrient composition is represented in trilinear co-ordinates, making a clear comparison of the proportions of both the cations and the anions possible. Also the pH and the total amount of ions present are compared.

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