by Dr. Lynette Morgan
There is a range of minerals, compounds and organic based substances found in soils which we don’t often think of as being part of a ‘soilless system’. Since plants evolved in soil, they developed the ability to make use of a number of substances which, while not essential for crop growth, have the ability to enhance it and increase yields. Minerals such as silica, nickel, cobalt and selenium fall into this category – while not essential for plant growth they have the ability to enhance growth and development in soilless systems. Many of these trace elements are required in such tiny quantities that they often find their way into hydroponic systems as ‘contaminates’ in the water supply, as dust, or in the fertiliser salts we add. However, other soil-based substances are amounts, and therefore don’t find their way naturally into most hydroponic systems. One example of this are the humic and humic-derived acids which occur widely in mineral soils, peats and some natural waters. Humic acids are water soluble organic acids derived from organic matter which has decomposed fully, though not all organic humus contains useful humic acids.
All humic acids contain carbon, hydrogen, oxygen and nitrogen with small amounts of sulfur and phosphorus, and are from a large family of organic compounds which have similar characteristics. They are categorised more on the process by which they are extracted and isolated rather than by their chemical structure. The organic matter in soil is divided into that which is undecomposed matter and that which is fully decomposed, and termed ‘humus’. This humus can be further divided into soluble humic acids and insoluble ‘humin’. The soluble humic acids have three major divisions ‘humic acid’, ‘ulmic acid’ (also called hymatomelanic acid) and ‘fulvic acid’. Fulvic acid is a short chain molecule which is yellow in colour and soluble. In horticulture it is the humic, and particularly the fulvic acids which are most reactive and effective in stimulating plant growth.
What do Humates do?
The beneficial effect of adding the correct humic substance on plant growth has been known for a number of decades, but specific effects in hydroponic systems are still being investigated. It is known that application of humic substances to soils low in organic matter or in nutrient solutions have produced very significant responses (Rautham and Schnitzer, 1981). Humic compounds can be absorbed by the plant roots and transported to the shoots, thus enhancing the growth of the whole plant.
In the early 1970s the effect of humic preparations on root formation in cuttings was examined and this revealed that humic and fulvic acid possess properties that are extremely beneficial to initiation and growth of roots on geranium cuttings. Early on, it was shown that root formation of bean seedlings was stimulated and maximised by applications of fulvic acid solutions. In fact, one of the most widely accepted affects of humic acid application on root systems is the promotion of root development. In hydroponic tomato plants, humic acid application resulted in higher root fresh and dry weights and higher levels of certain mineral elements in the shoots and roots (iron content was especially pronounced) than those plants grown in solutions with no humic acid addition. This same effect has been found in crops of hydroponic wheat, cucumber and tobacco. A 500% increase in root length in wheat has been reported, along with an increase in lateral and secondary root length in tobacco plants. Humic acid as a seed treatment or substrate drench has been proven to be effective in increasing root weight in cucumber, squash, geranium, marigold and carrot. In a hydroponically grown cucumber crop, not only was root growth increased, but the uptake of nutrient elements (N, P, K, Cu, Fe and Zn) was increased and the number of flowers formed was also found to be greater in the humic acid (fulvic acid) treatment. Supplying hydroponically grown sugar beet plants with humic acid in the nutrient solution increased yields of above ground parts and roots by up to 117% and 22% respectively and also increased the Nitrogen, Calcium, iron and manganese contents of the foliage.
Humic acids as natural chelates
Many of the trace elements we use in horticulture are not very soluble at the pH ranges the crops are grown at. For this reason they are prone to becoming ‘unavailable’ for plant growth, despite being applied in an inorganic fertiliser form. For this reason, in hydroponics we supply trace elements such as iron in a chelated form using a synthetic chelation agent such as EDTA. Chelation occurs when certain large molecules form multiple bonds with the micro element – this protects the micro nutrient from reaction with other ions in solution. Humic acids have this important ability to be chelating agents – they are in fact excellent in this role as they are strong enough to protect the micronutrient, but weak enough to release the micro element to the plant when required. Fulvic acid is particularly good for this role of natural chelation as it has the ability to enter the plant and move throughout its tissue. In organic production systems where synthetic chelation agents such as EDTA cannot be used, addition of humic acid appears to be the ideal way of ensuring micro nutrients remain available to the plant through a more natural form of chelation.
Experimental Results – use of Fulvic Acid in Organic Hydroponics
There is considerable interest these days in organic hydroponic systems using naturally derived nutrients. While this system is certainly achievable, the nutrients which must be used for the system to be considered ‘organic’ often require processing by micro organisms before plants can take these up and therefore lack the synthetic chelating agents we commonly use in traditional hydroponic systems. Humic acids have been proven in previous studies to promote the conversion of a number of elements into a form available to plants and more importantly, acts as a natural chelation agent of many nutrient ions. For this reason, Fulvic acid, a water soluble derivative of humic acid, was used as an addition to a liquid organic nutrient to determine its effect on hydroponic crop yields.
In this trial, dwarf green bean seeds were sown into coconut fibre media contained in 12 litre plastic pots. The crop was supplied with an organic nutrient solution at a CF of 25 (EC of 2.5) and a pH of 7.6, to the point of 10% run off, once per day. The coconut fibre media held sufficient moisture for irrigation to only be applied once per day. Organic nutrient application once every 24 hours also assists with the microbial activity in the media which is required for nutrient conversion. Water was used to slightly flush the media once every eight days to prevent excessive salt build up. 20 mls of Fulvic Acid (‘Diamond Nectar’ from General Hydroponics – USA) per litre of nutrient was applied at each irrigation from the time of emergence of the second true leaf until crop harvest. The crop was grown under standard greenhouse conditions through the summer period and harvested after 12 weeks of growth. A total of 360 plants were grown in randomised blocks.
At the time of harvest, both bean weight and the weight of the plant tops (stems and leaves) was determined. The fulvic acid treatment results in a 36% increase in bean weight at harvest and a 36.5% increase in the above ground portion of the plants. This represents an overall increase in plant growth as well as in harvestable yield. The plants treated with fulvic acid flowered on average four days ahead of the control plots.
There are a number of ways in which the addition of fulvic acid to an organic based nutrient could enhance nutrient uptake and plant growth. The organic nutrient used was not ‘fully mineralised’ and would have required a certain degree of mineral conversion in the media before many of the nutrients became available for crop growth. Humic acids are known to not only promote microbial growth and development, but to also assist with the conversions of a number of elements into the plant available form, they also participate in the decomposition of rocks and minerals in the soil – so their effect on breaking down organic and inorganic substances is likely to be important in this experiment. The role of fulvic acid as a natural chelation agents is also likely to be vital in this type of system where no synthetic chelates can be added. Iron in particular, we would usually supply as a chelated compound, in this system, fulvic acid chelation of micro nutrients such as iron and manganese may be assisting with the availability of these minerals which are naturally low in many organic nutrient sources.
Apart from the effects on the nutrient and uptake of minerals, the addition of fulvic acid is also likely to have had a stimulatory effect on plant growth – as this has been proven in systems using inorganic nutrient sources. Humic and fulvic acids have been widely proven to increase the rate of development and length of root systems and to accelerate cell division – this effect would also have been contributing to the increase in foliage and bean yield obtained from this trial.
While the addition of humic and fulvic acids to both soil and soilless systems has in the past proved to be beneficial to crop growth and development, it also appears to have considerable advantages to ‘organic hydroponic’ systems where a complex range of micro organisms, organic compounds and substances need to co-exist for the system to be successful.
References and sources:
The fulvic acid (Diamond Nectar) used in this trial was supplied by General Hydroponics-USA.
‘Humic Acid Seed and Substrate Treatments Promote Seedling Root Development’ By J A Hartwigsen and M R Evans. In HortScience, Vol 35(7), December 2000 page 1231.
‘Effects of a Soil Fulvic acid on the Growth and Nutrient Content of Cucumber (Cucumis Sativus) Plants’. By B S Rauthan and M Schnitzer. In Plant and Soil Vol 63, 1981, page 491.
‘The effect of commercial humic acid on tomato plant growth and mineral nutrition’ By A F Genevini, P Zaccheo, P Zocchi. In Journal of Plant Nutrition 1998 Vol 21 no 3, page 561.
‘Study of the action of humic acid in sugar beet in hydroponic culture’ By C Sanchez, M Ortega, B Perez. In Anales de Edafologia y Agrobiologica 1972 Vol 31 3 / 4 page 319.
‘Mineral Magic – Humate Technology’ General Hydroponics – California, USA Dr Lynette Morgan is the Director of Research at SUNTEC International Hydroponic Consultants, based in Manawatu, New Zealand. www.suntec.co.nz/consultants.htm