Nitrogen Sources for Hydroponics
Nitrogen is a fundamental element for healthy plants but in hydroponics, growers need to know the different nitrogen forms and sources, and how plants process them when used in nutrient solution.
Unlike other elements, nitrogen sources for hydroponics can be little confusing. Plants can take up both nitrate (NO3-) and ammonium (NH₄⁺) and these can be supplied via several different fertilizers and organic sources. Understanding how plants process nitrogen sources and the effect these can have on growth, development, physiological disorders, nutrient interactions, and solution pH are all aspects of hydroponic plant nutrition we should know a little about.
Why is Nitrogen so Important to Plants?
macroelement required in large quantities for uptake. Foliar nitrogen levels for most crops are in the three to six percent range depending on species and stage of growth.Nitrogen (N) is an essential component of amino acids in proteins and chlorophyll and, along with potassium, is a
Without nitrogen, growth will rapidly dwindle to a halt, and deficiency symptoms such as yellowing or purpling of the lower leaves will occur as nitrogen is mobilized from old to new foliage. Nitrogen-deficient plants are often quite distinctive with spindly, thin growth, limited root extension, fruit and flower drop is common, and yields severely reduced.
Excess nitrogen, however — or more specifically a high nitrogen-to-carbon ratio — can cause problems of its own, these include lush, soft foliage growth, reduced shelf life, and an increase in calcium disorders such as tip burn. Plants also have the ability to take up luxury amounts of nitrogen, well beyond their actual requirements.
Balancing nitrogen uptake and obtaining the best results from the nitrogen sources used in a hydroponic nutrient solution can vary from crop to crop and under different growing conditions, however, there is potential to manipulate this to improve crop growth and yields.
Nitrate and Ammonium in Hydroponics
Nitrogen can be supplied in two forms in hydroponic nutrient solutions: Nitrate (NO3-) is the dominantly used source, while smaller quantities of ammonium (NH₄⁺) can also be applied. Both are available for plant uptake and are supplied via fertilizer such as calcium nitrate, potassium nitrate, ammonium nitrate, ammonium sulphate and even, in much smaller quantities in the nitric acid used for pH control.
When nitrogen is absorbed as nitrate, inside the plant it becomes mobilized first to ammonia and is then incorporated into the amino acid glutamine. When the ammonium form of nitrogen is supplied, the ions are absorbed rapidly by plant roots, but they must not be absorbed more rapidly than they can be utilized in the plant tissues or a toxic reaction will occur.
Theoretically, ammonium-N should be the superior source of nitrogen because it could be used more efficiently in the plant than nitrate-N. If ammonium-N only were absorbed by root systems, the internal nitrate-to-ammonium conversion that occurs inside plant cells and requires a supply of energy would not be required.
However, in reality, if nitrogen is supplied only in the ammonium form, the toxic reaction of the accumulation of uncomplexed ammonium overrides any potential for this greater efficiency of assimilation. Despite this, there can be some benefits to supplying a small percentage of total nitrogen as ammonium, but overdoing this can cause some serious problems.
Ammonium-N, being a cation with a positive charge (NH₄⁺) also competes for the uptake of other cations such as potassium, magnesium, and calcium and an oversupply of the ammonium form of nitrogen can induce deficiencies in these elements. Tip burn in lettuce and many other leafy crops and blossom end rot of tomatoes and capsicum are common issues when the ratio of ammonium to nitrate in a nutrient solution is too high.
Ammonium toxicity itself often presents as yellowing or chlorosis between the veins of young leaves, followed by scattered brown, necrotic spots (depending on the species), and leaf edges may curl upwards or downwards, root tips may die back, and overall root growth is reduced; this type of root damage can also provide an entry point for root disease such as Pythium.
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Why Use the Ammonium Form of Nitrogen?
While nitrate may be the dominant and safer form of nitrogen to supply via hydroponic nutrient solution, there can be circumstances when a small percentage of the ammonium form has a benefit. Firstly, a safe level of ammonium is generally regarded to be no more than 10-15 percent of total nitrogen, however, this varies somewhat between crop species and growing conditions.
Under conditions of high light and rapid growth, the addition of even small amounts of ammonium-N has the potential to cause growth reductions and physiological disorders.
However, under low light and reduced growth rates, such as commonly occurs in greenhouses in winter, a certain percentage of ammonium-N has been shown to be beneficial for the growth of crops.
Providing some of the nitrogen as ammonium-N under slower growing conditions can accelerate the uptake and utilization of nitrogen, thus boosting growth. But care needs to be taken not to overdo this as too much ammonium-N can restrict calcium, potassium, and magnesium uptake which in turn restricts growth.
Ammonium-N is often incorporated into hydroponic nutrient products and formulations specifically to produce a pH buffering effect. In hydroponic systems pH tends to gradually increase as the plants take up nutrient ions, requiring acidification for pH control. Ammonium-N reduces the rate of pH increase, giving a greater buffering capacity to pH change in the nutrient solution and slowing or preventing the natural rise in pH that occurs over time. The uptake of ammonium by roots can result in a lowering of the pH in the nutrient solution as plants release positive hydrogen ions to balance the change in the root zone. For systems which may be using a hard water source that has a high alkalinity and thus requires a greater volume of acid to keep the pH down and within range, this pH buffering effect of ammonium-N can be particularly useful.
Nitrogen Fertilizer Sources
In traditional hydroponics, nitrogen sources include the fertilizer calcium nitrate, potassium nitrate, ammonium nitrate, ammonium sulphate, and ammonium phosphate, however, with the increased interest in organic hydroponics, aquaponics, and various natural supplements such as amino acids, nitrogen can be added via several different compounds.
What is important to understand is that organic sources of nitrogen supply the exact same nitrogen ions for plant uptake as chemical fertilizer. Whether the nitrogen has originated from an organic material, or amino acid solution, or from a chemical fertilizer, the plant roots all take up the nitrogen in the same format as plant-usable nutrient ions, nitrate, or ammonium. The only difference is the organic material, solution, or additive must be first broken down by microbial action to release nutrient ions into a form plants can take up. This form is exactly the same as the nitrogen forms supplied by fertilizer salts.
With organic nitrogen sources having this requirement for microbial break down, or ‘mineralization,’ before they become available for plant uptake, organic nutrients or supplements need some specific conditions for use as nitrogen sources. These must either be processed by microbial action before being used in the hydroponic system, so the nitrogen sources are readily available for immediate uptake, or undergo mineralization after being added to the system. Use of biodigester systems, composting, and vermiculture (worm farm) systems are all forms of pre-processing organic nitrogen sources via ammonification and nitrification by microbial action.
Small additions of organic nitrogen sources such as amino acid supplements can be successfully used in many hydroponic systems, as microbes naturally exist in soilless systems which will break these compounds down and release the nitrogen for plant uptake. This process, however, is somewhat slower than using traditional fertilizer salts which immediately dissociate when dissolved into water to give plant-available nitrogen for rapid root uptake. So, correcting a nitrogen deficiency in a hydroponic crop is much more speedy and accurate when using traditional hydroponic fertilizer than if organic materials are to being used.
Understanding how the different forms of nitrogen are utilized inside the plant and how this can be used to a good advantage is one of the many skills hydroponic growers can use to their benefit. While an excess of ammonium nitrogen can potentially be quite damaging to plant growth, a certain level — applied under the right conditions — can work to boost growth and help buffer changes in pH in the nutrient solution. Recognizing the signs and symptoms of the overuse of ammonium nitrogen is another important lesson as problems such as leaf damage, tip burn, and blossom end rot caused by ammonium toxicity are not always correctly identified. Experimenting with nitrate-to-ammonium ratios for different crops and under different growing conditions is one way of maximizing the potential benefits of these nitrogen sources.
Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.