Maximizing Hydroponic Nutrient Absorption
Are your plants getting the most out of the nutrients you are giving them? If not, it might be time to assess the factors Lynette Morgan points out to ensure your plants are as strong and healthy as can be.
What goes on in the root system is often overlooked, unable to be observed or easily measured, and as a result maximizing nutrient uptake is something many growers don’t give much consideration to. However, nutrient uptake drives growth and development and is influenced by a whole range of factors, and not just those in the root zone itself. While nutrient uptake in soil and hydroponics follows similar processes, what differs is the root zone volume and overall environment. In soil, roots are largely free to expand and explore a potentially unlimited volume, whereas in hydroponics, root zones are severely restricted, and water and nutrients are delivered and replenished either continuously or at frequent intervals. This puts considerable pressure on management of the limited root zone and low buffering capacity in hydroponics to provide the ideal conditions required for maximum nutrient uptake.
The Process of Nutrient Ion Uptake
Nutrients can reach the root system by either diffusing through the substrate due to a concentration gradient, by being passively carried via the nutrient solution directly to the root surface, or by roots growing towards them.
As the root system takes up nutrient ions, the concentration of a particular element starts to decrease at the root-absorbing surface. This creates a concentration gradient between the root system and the root environment, which in turn causes mineral elements to diffuse towards the root surface. In hydroponics we can speed up this process by regularly bringing nutrients to the root surface via frequent irrigation or use of solution culture systems where the nutrient is continually passed over the root system. There is, however, a trade off in substrate-based systems; increasing the rate of irrigation frequency to replenish nutrient levels around the roots should not oversaturate the medium as this lowers oxygen that is required for root health and rapid nutrient uptake. In hydroponics, despite nutrients often being directly delivered to the root surface via irrigation, roots still expand through the substrate to access more of the moisture and nutrient elements contained in the medium, particularly in the base of growing containers where nutrient solution drains or is held in reserve.
Nutrient ions are absorbed by the roots mainly in regions containing the root hairs; however, some older root regions can also take up nutrients. Once nutrient ions have been absorbed by the roots, they can either move between cells carried in the water flow, or by diffusion across the extracellular cell-wall space, or by crossing the cytoplasm. Ions are taken up by root cells by either active or passive transport. Passive transport is down electrochemical potential gradients while active transport requires an energy input.
Once inside the plant, mineral elements and the water they are carried in are transported within the vascular system of the xylem and phloem vessels. Long distance transport around the plant from roots to shoots takes place largely in the xylem vessels, however, elements can be transferred from the xylem to the phloem by an active process to allow distribution of these around different tissues. Transport of mineral elements in the xylem is driven by both root pressure and transpirational water loss from the foliage of the plant. During the day when stomata are open and transpiration is occurring, this drives the xylem carrying nutrient ions from root to shoot. At night this process is controlled via root pressure. Therefore, any environmental factors that affect root pressure and transpiration rates in turn affect nutrient transport in the xylem vessels. These factors are wide ranging and include humidity, light levels, temperature, moisture levels, and CO2.
Root Size and Health
The size of the root system determines the root surface area available for nutrient ion uptake, and any factors such as Pythium or other root pathogens, which damage or overly restrict root outgrowth, will limit nutrient uptake. In these cases, mineral nutrient deficiencies are often seen on the leaves due to lack of functioning root cells, meaning the plant is unable to take up sufficient nutrients despite being regularly supplied with a fully balanced nutrient solution. For high rates of nutrient absorption, the root system requires not only a regular supply of essential nutrient ions, but also sufficient moisture and oxygen for healthy root functioning. Moisture is supplied via the nutrient solution and sufficient oxygen for root respiration is maintained via aerated pore space in substrates and dissolved oxygen in the nutrient solution and that which diffuses into water surrounding the roots. A well-aerated substrate or attention to maintaining dissolved oxygen levels in solution culture, combined with avoidance of over irrigation and saturation of the root zone, can ensure sufficient oxygen is available for root health and maximum ion absorption.
Light, Temp, and Humidity
The duration and intensity of light levels also drive nutrient uptake in hydroponics. Light controls the opening of stomata on the leaf surface and this determines the rate of transpiration which affects passive uptake, translocation, and distribution of mineral ions.
Low light can become a significant factor in limiting the rate of nutrient uptake and transportation within the plant. Not only does high light stimulate plant nutrient demand and absorption, it also produces the energy required for root respiration and active uptake. For these reasons hydroponic growers in high light climates or under summer light levels adjust their nutrient formulations to allow for higher rates of absorption of certain elements. Many crops benefit from adjusting nutrient ratios and EC under differing light conditions and running summer or winter formations is standard practice under commercial hydroponic production.
Root zone temperature also influences mineral uptake via its effects on root respiration rate, root pressure, and root growth. Increasing root zone temperature increases root respiration and produces more energy for root outgrowth and active ion uptake, up to a point when temperatures become too high and damage plant cells. Increasing root temperatures to optimal levels of 75°F/24oC has been shown to increase the amount of P, K, Mg, Ca, Fe, and Mn in hydroponic tomatoes. One of the most common temperature related problems for some plants is the effect of sub optimal root zone conditions restricting the uptake of iron and inducing iron chlorosis. This type of iron chlorosis usually resolves as temperatures warm up under spring cropping.
Humidity levels, like temperature and light, influence the rate of transpiration, subsequent nutrient uptake, and translocation through the plant. High humidity reduces the vapor pressure deficit of the air surrounding the stomata and retards transpiration, which, in turn, restricts nutrient uptake and transport from roots to shoots. Calcium uptake and translocation out to leaf tips and young developing tips is particularly restricted by a lack of transpiration. Disorders such as blossom end rot and tipburn are often the result. Transpiration can be promoted with the use of good rates of ventilation and air movement across the leaf surface. Horizontal air flow fans (HAF) correctly positioned in a hydroponic crop can assist with the removal of the stale boundary layer of air that forms close to the leaf surface, removing excess humidity while stimulating photosynthesis and transpiration. This in turn promotes nutrient translocation and uptake of ions from the root system.
EC and pH
Electrical Conductivity (EC) and pH are more widely known factors that influence nutrient uptake in hydroponics. pH affects the availability of many nutrient ions for uptake. Most hydroponic systems have a tendency for pH to naturally increase over time and this can reduce the availability of some elements if left uncontrolled. pH in hydroponic systems is usually recommended to be kept within the 5.6–6.2 range which is optimal for most elements. EC levels are also important. Not only should the concentration of each element be sufficient to meet the demands of the plant and not become deficient in the root zone, EC levels that are too high can induce salinity damage and restrict uptake by the root system.
Some studies have shown that growth promoting rhizobacteria may provide a direct boost to plant growth by providing crops with fixed nitrogen, phytohormones, iron that has been sequestered by bacterial siderophores, and soluble phosphate. However, it’s likely the majority of root associated bacteria that play a beneficial role in hydroponic plant growth do so by producing the plant hormone auxin as indole-3-acetic acid (IAA). Studies have shown that inoculating hydroponic systems and different plant species with such bacteria lead to increased root growth and enhanced formation of lateral roots and root hairs, which may be at least partially attributed to bacterial IAA. This results in an enhanced tolerance to plant stress and improved ability to take up water and nutrients.
While beneficial bacterial can assist with nutrient uptake, they don’t necessarily need to be artificially introduced to a system. While a new hydroponic system may start off with very little in the way of microbial life, as soon as moisture and an organic carbon source (plants) are present, inoculation naturally begins. Micro flora develop rapidly after planting a crop in a hydroponic system and consume plant exudates, compounds in the nutrient solution, and dead plant materials with the composition of micro species affected by environmental factors and the source of nutrients. In most hydroponic systems the species of beneficial resident micro flora most commonly found are Bacillus spp. Gliocladium spp, Trichoderma spp, and Pseudomonas spp.
Nutrition absorption in hydroponics is driven and promoted by many factors that are under our control — optimizing nutrient formulations, EC and pH levels, root zone temperatures, oxygen, and moisture levels around the roots all play a role in maximizing nutrient uptake. The aerial plant environment also plays a role with humidity, temperature, light, and transpiration all stimulating root mineral uptake when controlled at the correct levels for the crop being grown.
Maintaining a healthy root system with use of beneficial bacteria is a newer area of nutrient uptake promotion that has shown considerable promise for hydroponic growers and is also worth investigating.