Substrate and Hydroponic Systems

Substrate and hydroponic systems have been developed over recent years as a means of improving production consistency, and to ease crop management and harvesting, leading to higher crop yields and better quality. In addition, these systems help to isolate and minimize issues with disease, which is increasingly difficult to control in soil cultivated crops where soil disinfection is either not permissible or difficult.

Alternative growing systems used in substrate production include the use of inert rock wool, perlite, or volcanic/pumice stone and organic media such as peat, coir and coco peat. Growers also utilize nutrient film techniques (NFT), which are substrate free.

These closed systems are easy to disinfect and the pH and EC are more easily controlled. Drainage water is collected and re-cycled, helping to improve nutrient efficiency. The optimal pH range is 5.0–6.0. Below this, root tips can be harmed. Any excess ammonium in solution can alter pH to unacceptable levels. If the pH is lowered some nutrients are more available (e.g. manganese) and potentially toxic.

In contrast though, when the pH in the root zone rises above 7.0, nutrients such as P, Mn, Zn, Cu and Fe, become less available.

The amount of different nitrogen forms is important as ammonium-N alters the pH. Common practice is to use a maximum of 14-20ppm N-NH4 (1.0-1.5mmol NH4) - with the majority of nitrogen supplied as nitrate. Because of the smaller volume of substrate compared to soil grown crops, high levels of sodium and chloride can cause greater salinity issues in substrate systems. Maximum levels in water are < 1.5mmol Na/l (35ppm) and < 1.5mmol Cl/l (53ppm). In recirculation systems these maximum levels should not exceed the uptake by the crop.

Crops in a hydroponic system need a full package of nutrients. To reduce work and labor cost, nutrient solutions are prepared in a highly concentrated stock solution which is then diluted. The stock solution is generally a 10% solution (100x concentrated) and the final solution is diluted to 1.0–3.0g/l depending on the crop and local conditions.

Because of nutrient antagonism and interactions, it is not possible to mix all nutrients in one high concentration stock solution. For example, calcium containing products cannot be dissolved with phosphate or sulfate containing products. Therefore a two tank - A + B tank - system is used. The higher the EC, the higher the nutrient concentration in the solution.

Growers using hydroponic or substrate based systems also need to adjust the final EC of the nutrient solution according to radiation. On cloudy days, water uptake is lower and therefore EC needs to be higher to ensure sufficient nutrient uptake. On sunny days the water uptake is high and therefore the EC should be lowered.

It is important to test the substrate, water source and drip water on a regular basis – normally every week - and adjust it accordingly to ensure an optimal ratio of nutrients are available. Growers should also analyze the pH and EC of the substrate several times a week.

Refer to the Yara Fertigation Manual for details on how to manage a two tank system.