Elements in soils & plants.
Of the seventeen essential plant nutrients, fourteen are acquired directly from the soil. These fourteen essential nutrients make up a miniscule 4% to 5% of plant matter, but without all of them a plant could not complete its lifecycle. Plant nutrients are separated into ‘macro-‘ and ‘micro-nutrients’ based on their relative abundance in plant tissue. Micronutrients are those found in concentrations less than 100 ppm in plant tissues. Natural soils typically have sufficient total amounts of each micronutrient, but conditions and element properties limit availability for root absorption and plant utilization. In some places the parent material (rock) that the soil developed from is deficient in some elements and fertilization is essential to meet crop demands.
Numerous factors limit nutrient availability in soil, including: soil moisture and aeration levels, texture (the relative amounts of sand, silt and clay), organic matter content, pH, temperature, abundance of other nutrients, heavy metal content, bicarbonate content, and biological activity. This myriad of factors limits plant growth, not because the nutrient isn’t present, but in a form unavailable to plants. In soilless and hydroponic systems several of the same factors limit nutrient availability.
Under artificial growing conditions it is largely nutrient imbalance, high concentrations of other nutrients in solution and high solution and media pH that negatively affect availability of the micronutrients iron, copper, manganese and zinc. In soilless systems plants are grown in an inert media and all nutrients are supplied dissolved in water. Assuming initial nutrient concentrations are correct, imbalances can occur when nutrients accumulate in the growing media and solutions in recirculating systems are not changed frequently enough (every 1 to 2 weeks). Nutrient solution pH and media pH will change over time as the balance of nutrients changes and roots excrete pH altering compounds.
Increasing pH corresponds with decreasing availability of iron, copper, manganese and zinc. This is partially why hydroponic nutrient solutions should be between 5.5 and 6.5 and optimum pH for garden soils is 6.2 to 6.7. This is the pH range with the highest availability for the entire range of essential plant nutrients. For example, above pH 4 each unit of increase in pH decreases iron solubility by a factor of about 1000. If the pH is too low availability of these metals may become so high that toxicity becomes a concern.
As growers we manage our gardens to match nutrient availability with crop demand, but some things are beyond our control. Outdoor growers are limited by the soil and climate nature provides and indoor growers balance the pros and cons when selecting a production system. Despite our best efforts to create ideal conditions, the nature of some metal nutrients precipitates the need for chelation.
Nature of metal elements.
It isn’t a coincidence that very reactive metals, which are easily immobilized in soils and nutrient solutions are also essential plant nutrients. Iron, copper, zinc and manganese are all multi-valence cations, meaning they have a strong positive charge, attracting them to counter anions having a negative charge. Oxygen and hydroxide ions (OH-) react strongly with these metals, forming insoluble precipitates. The abundance of hydroxide in solution is directly related to pH, where hydroxide concentrations increase tenfold with each pH unit increase. This is why maintaining solution pH in the optimal range is so important in hydroponics.
Other negatively charged nutrients, like sulphate (SO42-) and phosphate (PO43-) also react strongly with these metals, compounding the issue with reduced plant availability of multiple nutrients. While pH is key to nutrient solubility in hydroponic solutions, the abundance of positive and negatively charged nutrients is also a factor, as ions with opposing charges are more likely to come in contact. Growers pushing nutrient solution concentration (measured as EC) to boost yields risk fall out; the precipitation of nutrients into unavailable forms.
It is the reactive nature of metal nutrients that make them useful for plant growth. Inside the plant iron, copper, zinc and manganese are involved in energy transfer (redox reactions), and enzyme reactions facilitating the synthesis of proteins, DNA and other metabolites. Plants have evolved techniques for solubilizing soil nutrients for absorption and internal transport. In fact, scientists, growers and fertilizer formulators have learned from plants and animals the best strategies for delivering these nutrients using chelation.
Chelation in nature.
Chelation is found extensively in nature and is used by both plants and animals for the handling of reactive metals that are essential for biological activity. In animals, hemoglobin is an iron chelator, keeping this element isolated for transport through the blood stream. Plants produce chelators such as mugineic acid, oxalic acid, citric acid and others to chaperone micronutrients to their destination. To access insoluble soil nutrients, plants use numerous strategies, including root exudation of hydrogen ions and siderophores. The release of hydrogen ions (H+) facilitates different nutrient absorption strategies, but in this context, it lowers soil pH near the root to solubilize nutrients. Siderophores are chelators that when released cleave tightly bound nutrients from their anchors and escort them to the root surface for absorption. Plants species that are adapted to soils with a pH above seven, notably those in the grass family, such as barley, produce very effective iron solubilising siderophores. Conversely, those species that grow in acid soils with higher iron solubility will experience deficiency when grown in high pH soils. Production of siderophores is upregulated under conditions of nutrient deficiency and may not be produced at all when nutrients are readily available. Bacteria and fungi also produce siderophores to satisfy nutritional demands. Minerals chelated by microorganisms can be utilized by plants and vice versa.
As any internet search will reveal, chelate is a Greek word for ‘claw’. Chelates, also known as ligands, are soluble chemical compounds that envelop, or as the name suggests, wrap their claws around the metal ion to protect it from reacting with other molecules. Think of a chelate like an invisibility cloak. Chelated nutrients remain in solution so they can travel to and be absorbed by roots.
The term chelate and complex are often falsely used interchangeably. A complex is a molecule that forms a chemical bond with an element but does not have the same isolation properties as a chelate. In a complex part of the bonded nutrient is exposed to reactions with other molecules, making complexes less stable than chelates.
Scientists do not completely understand the fate of chelates. It is believed that natural chelators release the target nutrient at the surface of the organism for absorption, freeing the chelate to bind with another element. However, the fate of chelates is not that simple. There is evidence that plants and microorganisms also absorb the nutrient in chelated form. The nutrient is either translocated directly in chelated form or transferred to a different chelator for internal transport. How organisms interact with chelators depends on the chelate, the nutrient being chelated and the species using the chelate. The life expectancy of chelators in the soil is subject to numerous biological and chemical forces and all will eventually decompose, requiring continuous release of new chelates.
Human applications for chelators.
Chelating reactions were first discovered over 100 years ago. This discovery led to many applications for natural chelators and the development of numerous synthetic chelators. The first applications addressed human and animal health. Chelated mineral supplements have much higher efficacy compared to consumption of raw, insoluble minerals. Chelated iron was one of the earliest examples. Today many foods are fortified with chelated nutrients. It wasn’t until the 1950’s that iron chelates were first applied to plants to address nutrient deficiency.
Chelators are ideal for addressing deficiency of reactive nutrients, but they can also help remedy toxic concentrations. Metal chelation therapy is the use of chelators to flush accumulated heavy metal toxins from the body. Similarly, chelators are being used to increase plant absorption of heavy metals in phytoremediation projects. Nowadays chelators are used in many industrial processes and products we consume, including soaps and detergents. Being so pervasive, the implications of chelators, especially persistent synthetic chelators, is of uncertain environmental concern and the subject of much inquiry.
Synthetic and Natural Chelates.
Several synthetic chelators have been developed, but the most commonly used in plant nutrition are EDTA, DTPA and EDDHA. Each of these has a different pH stability range, making them suitable for different conditions. EDTA is a very effective and stable chelator in the pH range of hydroponic solutions and most garden soils. Above pH 6.5 EDTA starts to dissociate. For high pH and calcareous soils EDDHA is a better, though costlier option. Compared to natural chelators, synthetic chelates tend to be more stable and resistant to decomposition. Early research suggested that plants do not absorb synthetic chelates, but that the chelated element is released at the root surface, meaning that a single chelator application could have an enduring effect. However, this is not true. Plants do absorb synthetic chelators and their breakdown products. However, the majority of chelated micronutrients are not absorbed with their chelator as plant tissue concentrations of the chelated element do not equal chelator concentration. Soil biology breakdown synthetic chelators and under sterile conditions chelators will persist much longer in the growing media.
As previously mentioned, there are a number of natural chelators and several have become commercial products. In recent years amino acids chelates have become all the rage in agriculture. The use of amino acid chelated nutrients for human and animal health have been around since the 1960’s but use in agriculture is very recent.
Not all amino acid chelates on the market are the same. An effective chelator must be soluble, plant available and form strong bonds. Not all amino acids satisfy these criteria as they vary greatly in size and bonding strength. Small amino acids that can be easily absorbed by plant roots, such as glycine show higher efficacy. Since amino acids – protein building blocks – are derived from plants, used by plants and readily decomposed by soil microorganisms they can be an excellent, environmentally friendly option. It has also been found that plants can use amino acids as a nitrogen source, giving additional fertilizer value. There is very good evidence that amino acid chelates are as effective if not more so compared to synthetic chelates. However, there is a lack of good science-based data and many studies have shown conflicting results. The inconsistency is likely due to differences in the types of amino acids, growing conditions and plant species. Many amino chelated products are not actual chelates, but weaker complexes. More research is needed to identify the bounds of amino chelate efficacy.
Fulvic acids are another natural substance used to increase nutrient availability and absorption. Fulvic acids are more like complexes than chelates. As with amino acids, not all fulvic acids are the same. They are a group of organic acids classified by their weight and solubility. Fulvic acids are relatively stable products of decomposition. A finished compost pile will contain some fulvic acid, but the vast majority of products on the market are fractions of organic matter from ancient deposits called Leonardite. The fulvic component is isolated from the larger molecular weight humic acids. Low molecular weight fulvic acids stimulate nutrient cycling and increase availability. They help to keep nutrients in an available form, can be absorbed by plant roots, and are a food source for soil microbes that assist with the turn over of nutrients.
While micronutrients are only required in small amounts by plants, without them life could not exist. In garden soils micronutrient deficiencies are most often due to limited availability, not inadequate soil stocks. In hydroponics, supply is more often the cause, though conditions in the growing media and nutrient imbalances might be inducing deficiencies. Adding more of a nutrient may actually exacerbate a deficiency or induce others if the cause is not accurately diagnosed. When treating an actual deficiency of iron, zinc, copper or manganese choose a chelated form to maintain solubility and rapid plant absorption. Foliar applications of chelated micronutrients are the quickest way to remedy a deficiency. Deficiencies are already advanced and yield reduced by the time visual symptoms are expressed. Closely monitoring changes in nutrient solution and media pH and electrical conductivity will help avoid costly deficiencies before they happen.