I want to increase the micronutrient content of my hydroponic tomato crop but without using seaweed extracts. Should I use seawater at a dosage of between 1-5 ml per liter? Is this a good idea considering seawater contains a high amount of iodine which can cause an overdose problem in human health? How else can I use micronutrient elements to benefit my plants? – Jennifer M.

Hydroponic nutrient solutions already contain all the essential micro elements required for growth: these being iron, manganese, zinc, copper, boron, and molybdenum. What you are referring to are ‘potentially beneficial micro elements’, which are not essential for plant growth, but may give some advantage, with a few having human health benefits as well. These would include elements such as aluminum, cobalt, titanium, silicon, selenium, sodium, iodine, and some lesser investigated ones such as silver, chromium, rubidium, tin, vanadium, and tungsten.

Hydroponic growers have long been concerned that the use of nutrient solutions means many of these other elements, which are commonly found in soils, are not available to their crops and that the composition of the food produced hydroponically may not be as nutritionally diverse as soil-grown plants. However, this is not the case, since these beneficial elements are taken up in such tiny quantities, hydroponic plants tend to end up with the same composition as those grown in soil despite growers not deliberately adding these elements to the nutrient solution. The reason for this is that often water supplies (particularly well water or city water) contain tiny amounts of many of these beneficial elements that are available for plant uptake. Furthermore, trace amounts of beneficial elements are present in growing substrates as dust in the environment and as impurities in the chemical fertilizer or organic compounds used to make up hydroponic nutrient products and solutions.

Seawater is not a great option for boosting beneficial elements in a hydroponic nutrient solution. There is the potential for sodium and chloride to accumulate in a recirculating system despite tomatoes being fairly salt tolerant. A more sophisticated approach is to select some of the booster supplements sold on the hydroponic market, such as those based on concentrated vermicast extract, organic or mineral clay supplements (there are also silicon-based liquid boosters and even some hydroponic nutrient lines which have certain beneficial elements added).

Since many of these beneficial elements need only be present in minute quantities (parts per billion) using a professionally prepared supplement is a good place to start as it ensures no additional and unwanted elements, microbes such as pathogenic bacteria, high levels of carbon or other issues are likely to occur. As well as beneficial elements, research has found that healthy microbial populations within the nutrient solution and root zone can interact with some of these beneficial elements to boost mineral uptake and plant growth even further, so it’s worth considering using both microbial and mineral supplements in combination.

Best of luck with your growing,
Dr. Lynette Morgan