Nutrients – Under and Over Use
By Dr. Lynette Morgan
Nutrients are the basis of any hydroponic system and since we need to meet all of the plants nutritional requirements, it’s important to know what you are supplying and what can go wrong. With any nutrient solution the two factors to keep in mind are firstly the composition of your nutrient – does it contain all of the elements required for plant growth in the correct ratios. And secondly, with your balanced and complete nutrient solution, what strength or ‘EC’ should it be running at for your particular crop, stage of growth and type of hydroponic system, and how do we measure this.
The nutrient solution – composition
Many growers prefer to buy a ‘pre-mixed’ nutrient solution which simply needs to be diluted (for liquid concentrates) or dissolved in water before use. Often these ‘pre-made’ nutrients come in 2, 3, 4 or even more ‘parts’ so a grower can change the ratio of the mineral elements to allow for either vegetative or fruiting growth or for different crops. There are many excellent brands of these pre-mixed nutrients on the market, however, many growers have come across major problems when they try to use some of the ‘indoor plant food’ or other nutrients which have been designed for plants growing in soil or a pre fertilized potting mix. Often these types of products are not suitable for hydroponics because they are not designed to be a ‘complete plant food’. It is always preferable to buy a nutrient mix which is sold especially for ‘hydroponic’ use, and is a ‘complete’ plant food. To be ‘complete’ a hydroponic nutrient needs to have the essential elements for plant growth these are:
- Nitrogen (N)
- Potassium (K)
- Phosphorus (P)
- Calcium (Ca)
- Magnesium (Mg)
- Sulphur (S)
- Iron (Fe)
- Manganese (Mn)
- Copper (Cu)
- Zinc (Zn)
- Molybdate (Mo)
- Boron (B)
- Chlorine (Cl)
The levels that these elements are present in your hydroponic nutrient tend to vary between brands, since there is no one single recommendation for concentrations. Many nutrients may also contain some of the ‘beneficial elements’ such as Nickel (Ni), Cobalt (Co), Silica (Si) or Selenium (Se). While these are not ‘essential’ (plants will still grow without them), they can be beneficial to many crops.
Whether you make your own nutrient solution from the different fertilizer salts, or buy a pre-made brand, problems can, an often do, arise with deficiencies of one of more of the nutrient elements. Common reasons for this are that (1) the nutrient strength may be too low, resulting in insufficient nutrients for the plants in general. (2) The nutrient formula you are using may not be completely balanced, and one (or more of the elements) may be deficient. (3) Occasionally, growers may unintentionally leave out one of the fertilizer salts or the wrong fertilizer was used when the nutrient formula was weighted out. And just to complicate matters further, even if your solution is well balanced, sometimes environmental and internal plant conditions prevent the uptake of certain nutrients and deficiency symptoms then result.
Signs of Deficiency
Each of the mineral elements required by the plant has its own set of ‘deficiency signs and symptoms’ and growers can learn to identify many of these. Many of the signs are similar in appearance, but others are very distinct and most good gardening and hydroponic books will detail what these signs are. Briefly the deficiently symptoms for each of the elements are listed below (these may vary slightly between different plant species and depending on how severe the deficiency is):
Nitrogen (N): Plants are short, leaves tend to be pale green-yellow in color, especially on the older foliage. On tomato plants, the undersides of the leaf and stems can develop a purple coloration.
Phosphorus (P): Plants are usually stunted, and a dark green color. Symptoms occur on the older leaves first and plant maturity is often delayed. Phosphorus deficiency in some plant species can be due to conditions being to cold for uptake of this element, rather than a lack of phosphorus in the nutrient solution.
Potassium (K): The older leaves become yellowed with scattered dark (brown or black) spots, followed by tissue death. Severe deficiency will stunt the plant and all foliage will become yellowed and curled. On lettuce the leaves may take on a yellowed, bronzed appearance starting on the older foliage.
Sulfur: Deficiency of sulfur is not common – there may be a yellowing of the leaves, first seen on the new growth.
Magnesium: Deficiency is common on tomato crops with the older leaves developing yellowed areas between the veins which stay green.
Calcium: Young leaves are affected before older leaves and become distorted, small in size with spotted or necrotic (dead) areas. Bud development is inhibited and root tips may die back. Tip burn on lettuce is a symptom of calcium deficiency but is also caused by other factors not associated with a solution deficiency. Blossom end rot of tomatoes is also caused by a deficiency of calcium within the fruit tissue (not necessary in the nutrient solution), and is more of a ‘calcium transport’ problem within the plant under certain environmental conditions.
Iron: Deficiency shows as a distinct yellowing between the leaf veins which stay green, on the new growth and younger leaves (this distinguishes it from magnesium deficiency which shows first on the older leaves). On crops such as tomatoes, iron deficiency may show when conditions are to cold for uptake, rather than be caused be an actual deficiency in solution.
Chlorine: deficiency shows as wilted leaves which then become yellowed and necrotic, eventually turning a bronze color.
Roots become stunted and thickened near the tips.
Manganese: Initially, an interveinal yellowing on the younger or older leaves, depending on the plant types. Brown, dry areas may develop and leaves may drop.
Boron: Plant size is usually reduced, the growing point may die back. Root tips often become swollen and discolored. Leaves eventually become thickened, brittle, and may be curled with yellow spotting.
Zinc: Short plants with a reduction in internode length and leaf size. Leaf edges may be distorted or puckered, Yellowing between the leaf veins may also develop.
Copper: Deficiency is rare, but young leaves may become dark green and twisted or misshapen, often with brown, dry spots.
Molybdenum: Older leaves develop interveinal yellowing, progressing to the younger leaves. Leaf edges may develop scorching or cupping of the leaves.
Solution strength – under and over use, measurement
Provided the nutrient you are using is complete and balanced, the concentration or strength of the solution has major effects on plant growth and development. This is why it is essential to be able to measure solution concentration, using a meaningful unit of measure. Many growers will still be working in ppm, using TDS meters, however there is now an industry move to standardize the unit of solution measurement to EC (electrical conductivity) which is a more accurate and meaningful way to monitor your nutrient. All a TDS or ppm meter actually does is to measure the EC of the solution, then use an approximate conversion figure to convert this to PPM. The problem arises is that this conversion figure is never very accurate, as different nutrient solutions with different compositions of nutrient elements will have different PPM values so using one conversion figure can be extremely inaccurate. What the plants root system is actually responding to is the EC (or osmotic concentration) of the nutrient so this is what we should measure. There are a number of different EC (sometimes called CF) meters, and the ‘water resistant’ pen type meters are commonly used by growers. Depending on where in the world you are, the units expressed on your meter may be different, however it is easy to convert between the different units of EC.
The most commonly used units are either Microsiemens/cm (EC) or conductivity factor (CF) (depending on which country you are in). Other units used or often expressed in crop recommendations are: Millimhos, micromhos, or millisiemens (mS). The conversion between all of these units is:
1 millisiemen (EC) equals 1 millimhos, equals 1,000 microsiemens, equals 1,000 millimhos, equals 10 CF.
It is simply a matter of shifting the decimal place to convert between the different units.
Running the correct EC for your particular crop and system is important. Some crops such as lettuce and other greens prefer a much lower EC than fruiting crops such as tomatoes, and each crop has its own ideal EC range for optimum growth. When the EC is being run to high for a particular plant, this will show as visible symptoms within the crop. A high EC, effectively puts the plants under ‘water stress’ since the plant cells begin to lose water, back into the more concentrated nutrient solution surrounding the roots. As a result the first sign of nutrient ‘overuse’ is plant wilting, even when supplied with sufficient nutrient solution. If the high EC conditions re not too severe, the plants will adjust to these conditions and you may see growth which is ‘hard’ in appearance – often a darker green then usually, with shorter plants and smaller leaves. When the EC is being run to low, the opposite occurs – greater amounts of water are taken up, growth will be soft and floppy and often a lighter green in appearance.
Fruit will have less flavour and the quality of the whole crop – in terms of dry matter, shelf life, firmness and colour will be reduced. Since other factors affect EC also, such as water uptake from the solution, concentrating the nutrients during warm periods, or nutrient uptake, dropping the EC under a different environmental conditions it is vital that the EC is measured, monitored and adjusted on a regular basis.
By focusing on the two most important solution factors – nutrient balance and nutrient concentration, the hydroponic solution will give maximum growth and yields. When things do go wrong, being able to correctly identify a deficiency symptom before it begins to severely effect your plants is also important, so as always, closely watching what your crop is doing is a growers best line of defence against solution problems.