Most hydroponic crops only require a small root zone volume. However, as Lynette Morgan points out, the restriction methods used are just as important as the result they produce .
One of the most well-recognized advantages of hydroponics is the small root zone volume required by most crops. This means tidy, compact, yet highly efficient systems can be constructed indoors without the need for huge beds or deep containers of soil to contain the root systems of plants we want to grow. In the early days of the industry, one of the greatest advantages of hydroponics was touted as being that the regular supply of a well-balanced and complete nutrient solution meant there was always sufficient water and minerals for growth, thus a large root system would not be needed or produced by the plant. While this is partly true, rooting volumes required by plants under well-managed soilless culture are considerably smaller than those of plants in soil. Taking this to extremes with highly restricted root zones can cause growth problems and a loss in hydroponic productivity. On the other hand, mild root restriction can have positive effects on some plants if applied correctly and with knowledge on how this may affect plant physiology.
The more restrictive the hydroponic root zone volume, the more well-managed the production system needs to be to ensure plants are supplied with the water, nutrients and oxygen they need for optimal growth and production. However, even a constant supply of water and nutrients cannot overcome all the constraints posed by an overly small root volume, and for that reason, an important aspect of hydroponic system design is to use a suitable rooting volume for the type of plant to be grown. To complicate matters further, there are no exact recommendations for the optimal size of root zone volume for different hydroponic plants. Optimal root volume is dependent not only on the plant species and cultivar, but other system and environmental factors such as the frequency of nutrient application, and hence, replenishment of oxygen and nutrients, plus factors that influence the plants requirements for these.
Some studies have been carried out to determine the effect of root zone volume and restriction on yields of crops, such as hydroponic tomatoes and peppers. It has been reported that the highest yields with peppers were obtained with a perlite substrate volume of 4.38 to 4.75 gal. per plant (as compared to 0.87, 1.76, 2.37 and 8.71 gal. per plant); however, this response to substrate volume is also highly correlated to the conditions the experiment was run under, rates of nutrient application and substrate aeration.
Since all the nutrients, water and oxygen required by a plant are supplied via the nutrient solution, the root system does not need to rapidly grow and expand to forage for these resources in a large depth of soil. However, root growth in plants, even in hydroponics, is a continual one, so over time the root density increases as new roots are produced. The regeneration of new roots is in fact essential for normal plant development. Roots respond to gravity and to touch when they contact a solid surface, thus, in a restricted growing container, they tend to head downwards and form a mat in the lower regions of the growing substrate. Eventually, with continual root growth, the point will be reached where extreme root binding occurs and overall plant growth restricted even with the regular supply of a nutrient solution. In very restricted root volumes, this prevention of further new root growth has been shown to be a result of both root self-inhibition and also caused by growth limiting factors such as root exudates. The size of the optimal rooting volume in hydroponics, therefore, must allow for this continual root growth, but at the same time, nutrients, water and oxygen being regularly supplied can be considerably less than that required by a soil-grown crop.
Root Restriction and Hydroponics
With hydroponics, we tend to largely forget what is happening down in the root zone and often assume that since a well-balanced nutrient solution is being applied regularly, even if the roots have a very restricted volume for growth, they will be fine. In a perfectly designed hydroponic system, this may be the case; however, many systems, unbeknownst to the grower, can suffer from problems directly related to root zone restriction. The most common of these is the availability of oxygen used by plant roots in the process of respiration.
Plants under certain conditions have a very high requirement for oxygen within the root zone, particularly under the protected and warm conditions provided year-round with indoor gardens. A restricted root zone has a limited potential to hold oxygen and thus relies heavily on oxygen replenishment, be that via dissolved oxygen in the nutrient or oxygen percolation down into the root zone during irrigation. If root requirement for oxygen is greater than the replenishment rate in a restricted volume, than root function begins to slow, as does the uptake of water and nutrients. Eventually a lack of oxygen can cause root cell death increasing the risk of root diseases such as pythium. The more restricted the root zone volume, the greater the replenishment rate of oxygen must be. In hydroponics this can be achieved in a number of ways. First, some growing mediums contain larger pores than others and allow oxygen to disuse faster down into the root zone. Second, nutrient solutions carry dissolved oxygen so both increasing the dissolved oxygen content of the solution via aeration and making sure the root zone is not over saturated with water ensures more oxygen is available for root uptake.
A traditional soil-grown tomato may average more than 52 gal. of rooting volume per plant, with almost unlimited access to soil depth to forage for water and nutrients. This can represent a considerable energy investment in root growth by the plant in search of resources for growth. In hydroponic water, nutrients and dissolved oxygen are delivered to the root system on a regular, or in the case of solution culture, continual basis, thus roots do not normally need to grow to excessive lengths in search of these. This should represent a better efficiency for plant growth in hydroponics-plants need to put less resources into growing large root systems, thus more energy can be diverted into the top of the plant. While this may seem to be simply a case of fewer roots and more shoot, flower, fruit or seed growth, there are other factors that complicate this situation. Plants co-ordinate their root and shoot growth by signaling with plant hormones produced in different organs. An overly restricted root system can signal via the production of hormones to the top of the plant and control shoot growth and other developmental processes. Shoots can also signal to roots via the plant hormone auxin produced in the top of the plant and transported down to the roots. Thus, the size, health and stress of the roots affect the shoots of the plant, and vice versa. This means even in hydroponics, an overly restricted root zone will restrict the above ground plant growth due to this root/shoot co-ordination and communication even if water, oxygen and nutrients are optimal and continually supplied.
Advantages of Root Restriction
The positive effects of root restriction for container-grown plants have long been known, with the art of bonsai being the most extreme example. When roots are severely restricted within the growing container for long periods of time with limited nutrients and root pruning practices, the entire plant becomes stunted and dwarfed. However, the balance must be maintained between keeping plant growth highly restricted and compact but at the same time healthy and alive. In hydroponic, seedling and ornamental plant and fruit crop production, milder root restriction practices are used for certain plants to help increase produce quality and productivity. Vegetable seedlings grown with some root restriction usually result in a shorter, hardier transplants that are better able to survive the stress of the planting out and establishment process.
Root restriction in fruiting crops such as apples and grape vines has been found to restrict vegetative growth while improving the quality of the fruit in terms of soluble sugars and other parameters. There is evidence with some hydroponic crops that root restriction in the seedling stage helps hold back excessive vegetative growth in the young plant, leading to earlier flowering, more compact plants and an advantageous vegetative vs. reproductive balance. Root restriction in these cases may take the form of careful selection of the size of the seedling rooting container, or, as is more common, holding the seedlings for longer in their propagation cubes or containers so that root restriction begins to occur before planting out.
Other studies have shown that root restriction can improve the nutritional value of hydroponically grown vegetables. This may be via a stress response similar to when crops are grown under deficient irrigation or with a high EC, or most likely a combination of internal plant processes triggered by compounds produced by the restricted root system. One study found that edible chrysanthemum, pak-choi, endive and lettuce hydroponically cultured in a deep flow system in restricted root zone tubes resulted in plants with an increased percentage of dry matter, C:N ratio, ascorbic acid (vitamin C) and anthocyanin contents. However, increasing root restriction also retarded growth, so a compromise restricted rooting volume needed to be established, one that produced sufficient foliage growth, but also an improvement in nutritional value. The optimized rooting volume, however, varied for each different species. Thus, it is difficult to make generalized recommendations for the ideal root volume for hydroponic systems.
In many hydroponic systems, individual plants may be grown in their own separate container or slab of substrate and some plants are often grown side by side allowing roots from separate plants to intermingle. Some studies have found that plants produce more root mass when sharing rooting space with a neighbor, as compared to plants growing alone. It is thought that this allows plants to enhance their competitive ability for nutrients, but that root overgrowth may occur in this situation at the expense of reproductive growth. These findings may be species specific; it appears that the roots of some plant species can sense the roots of neighboring plants and respond to them accordingly. Further studies in this area may eventually help us determine how plants grown side by side may be influencing the growth of each other in hydroponic systems.
Our objective as hydroponic growers is to provide sufficient root volume for each species so that roots are not overly restricted, yet at the same time make soilless systems efficient and manageable by not providing very large and unnecessary root containment zones. Since root restriction can at times have advantages, with hydroponics we have the tools and technology to manage the root system volume precisely.