Salinity and Hydroponics (Part 1)

At an Australian Hydroponics Association conference in Adelaide a few years ago, during one of the workshop sessions the following problem was posed:

“Joe” is growing lettuce using NFT. He needs to discard 8,000 litres of solution each time he makes a new batch. Old solution (now waste water) is replaced daily in the summer but only weekly in winter. Joe also has some spare land (around 2000m2) where he used to grow lettuce in the ground before he moved into hydroponics.

What should Joe do to overcome this problem?”

Lettuce plants are particularly sensitive to salinity, by this we mean the presence or accumulation of an excessive level of mineral salts including common salt (sodium chloride) and nutrient salts in the nutrient solution can be damaging to plant growth and development. Salinity damage will cause physiological stress and tissue necrosis if the stress is not quickly reduced to a safe operating level (and hence the reason Joe thought he needed to replace the nutrient solution).

Salinity can be defined in two ways. It may be the total amount of soluble salts in the nutrient solution (ie. the conductivity), or it may refer specifically to the level of sodium chloride in the solution.

Sodium chloride is not normally present in the chemicals used in the standard nutrient solution, except as a minor impurity. However, these impurities can only really be avoided by using highly purified chemicals to make the nutrient solutions. For many growers this would make the cost of nutrient preparation prohibitively expensive.

Therefore the question remains:

“Where do these contaminating salts come from and how can their accumulation in the nutrient solution be managed most effectively?”

Knowing the answer to this question will allow growers use their nutrients and water with increased efficiency.

All waters (except especially purified (eg distilled) water) have some dissolved salts in them. They are normally only considered saline if the salt concentrations are high enough to damage plants when used for irrigation. At one extreme we have seawater which is highly saline and does not support normal land based plant growth while at the other extreme we have distilled water. Most water sources contain much lower salt concentrations, however, many underground water sources are moderately saline and therefore undrinkable (non potable) or brackish. Some streams and rivers can also become saline if they are fed from a brackish source or exposed to prolonged evaporation on their route to the sea. Ideally water used for domestic or horticultural use should be low in salts but in reality most water used in hydroponics will also contain some salts unless treated to remove them. The dissolved salts found include both anions and cations as they always exist in pairs so the electrical charge on the ions is balanced and neutral.

Each water source has its own unique characteristics and composition. While the analyses may vary with the location, the ionic composition of useable water is similar across a range of sites as it must conform to some minimum standards to be potable. Typical analyses of tap water from various cities throughout the world are given in Table 2. The water analyses at the bottom of the table shows the extent of the range of salinity occurring on earth and contrasts markedly with the other water sources that can be used for hydroponics or human consumption.

In a nutrient solution used for hydroponics the most important source of contaminating chemicals is normally the water used to prepare the nutrient solution and the water added to replace the water transpired by the crop. If this water has not been purified to remove salts the concentration of salts in the nutrient solution will steadily increase. However, there could be other sources of contaminating ions, for example these might be added into the system as impurities present in the chemicals used to make up the “secret” ingredients added to the nutrient solution. Sodium silicate preparations may be a case in point where the alleged benefits from adding silica to the plant may be more than offset by the additional sodium ions added.

One of the best papers at the Adelaide conference was given by Dr. Ben Robinson who concluded by advising all hydroponic growers “not to meddle with the standard nutrient solutions”. The nutrient solutions may not be perfect but they are robust and have been compiled by experts often doing many experiments. While “we all know best” it is rather impertinent and courting financial disaster to meddle with the nutrient composition on the assumption that you know better than 40-50 years of accumulated knowledge by experts in plant nutrition. Certainly, at Massey University we do not meddle –we use a formulation developed over many years by hydroponics experts such as Alan Cooper or Hoagland.

There are major differences in the sensitive of crops to the sodium and chloride ions for example in tomatoes, common salt (sodium chloride) may actually be added to nutrient solutions as the cheapest chemical to increase conductivity (to 6.0), and thus provide improved fruit quality (with a trade off in reduced yield). Fruit yields decline and quality improves when 200 mg/l or more sodium chloride was used. However, lettuce is a lot more sensitive to damage by common salt, and increasing the concentration of salt would be a recipe for disaster as the crop is particularly sensitive to excess sodium chloride and the most desirable conductivity for the crop is is usually about 1.5.

There is no doubt however, that even with moderately saline water lettuces could be produced successfully using rock wool and a water to waste system, but such a strategy does not take into account the need to dispose of the leachate in an environmentally responsible and sustainable manner. The fewer the number of nutrient dumpings the easier it will be to deal with waste nutrient solution.

Recirculating systems such as NFT have much to commend them for hydroponic crop production including sensitive crops like lettuce, but essential components of the system are the quantity of water required for transpiration and the quality of the water used to replenish the system.

If a grower is producing 0.1hectares of NFT lettuce, then the amount of water transpired through the crop will be very similar to the evaporation from a raised pan evaporimeter. It is unlikely in Adelaide (even on the hottest day ) that the evaporation will exceed 15mm/day. Even if it does occasionally exceed 15 mm per day we will use this figure as a starting point for our case study.

If 1 mm of water evaporates from the raised pan evaporimeter this is equivalent to 1 litre of water being transpired per square meter of crop cover.

Therefore, on our hypothetical property, 0.1 hectares of mature lettuce plants have the potential to transpire water equivalent to 15mm of water per m2 of crop cover per day which is the same as 1,500 litres/day.

Assuming that the total volume of the nutrient solution in the entire system is 9,000 litres, this would mean that every day 1,500 litres of fresh water would need to be added to the system plus nutrients to compensate for plant use. The salinity of the solution (arising from the salt impurities in the water supply) will increase every day throughout the summer when adding water if it has not been demineralised.

The effect of applying nutrients to the solution should be minimal as they are related to conductivity, of which salinity is an integral component. Any increase in salinity will reduce the additional application of nutrients, unless specific nutrients are applied in response to a demand signalled by the plants or a specific ion electrode in place of the normal conductivity meter.

Therefore, if the critical level of sodium in the nutrient solution for lettuce is 100 ppm, and the water entering the system contains 50 ppm, we will need to replace the solution every 6 days. However, if the sodium concentration was only 10 ppm, then the life of the nutrient solution would be extended to 60 days or slightly less if the grower wanted to be confident of not ever exceeding the 100 ppm threshold .

It is clearly very important, therefore, to use water with a very low sodium content, in order to reduce the frequency of dumping the solution.

Impurities in the basic chemicals

The cost of nutrients used in hydroponic systems is not particularly high, relative to the overall production costs. Therefore it makes good economic sense to ensure that any impurities (particularly sodium and chlorine) in the individual constituents are minimized, especially for the chemicals used in the large quantities such as calcium nitrate and potassium nitrate. Some inferior grades of fertilizer can contain up to 2.5% sodium chloride, but then are unlikely to be used in hydroponic nutrient solutions.

“Use of “secret ingredients”

Some growers are resistant or conservative to the introduction of new ideas, while others are often too keen to engage in some form of experimentation or ‘Russian Roulette” with their primary income source all in the name of innovation. For instance, some years ago in the nursery industry before soilless media was widely used, there were some growers that used to add some special soil to the soilless media components believing it contained secret ingredients that helped grow better plants. Growers used these amendments without fully appreciating what it did to the physical properties and media structure.

A number of hydroponic growers may be open to all sorts of ideas and opportunities that are poorly substantiated. Anyone who believes in using these without a properly verified set of trials for comparison does not have a very sound basis making management decisions or building a solid business.

There may be some advantage in using a silicon nutritional supplement for some crops, but in this case the potassium silicate salt would be a superior material to the sodium silicate. Secret recipes are more likely to cause problems rather than a more open view of knowledge and technique. Poor root growth is more likely to be caused by poor aeration rather than a shortage of a critical element.

There are no ‘magic bullets’ in hydroponics. Just good opportunities for the sound application of the basic principles of crop production in a system that has much more potential than is possible in field scale production in soil.

Fertilisers used to supply nutrients vary enormously in their impact on the total salinity of the nutrient solution. The fertiliser effect on salinity is made up of two components, the salt index and the quantity of each nutrient required in the final solution.

Refer to Table 3 for the salt index and to the graph (Fig 1) to see how the solution conductivity is effected by different nutrients.

Fig 1. Effect of nutrient concentration on conductivity of nutrient solution.

The slope of each line represents the salt index per unit of the nutrient. Note the nutrients were added to tap water (CF=2) at 15°C. Hydrated magnesium sulphate (MgSO4.7H2O) was used, all other salts were anhydrous. All nutrients used were normal hydroponic grade.

Some other units are also in common use eg mmhos/cm = mSiemens/cm (or in the USA dS/m)

Resistivity per cm is sometimes quoted, this is the reciprocal of mmhos/cm

To convert % dissolved solids to a CF reading multiply by 66.7

When the conductivity of a nutrient solution is quoted without units it is most likely to be in CF units.

Finally, we must emphasize that salinity effects will depend on the crop and if the water source is unsatisfactory then the best solution will be to overcome the water problem before it enters the system. Dumping the nutrient solution every day is not likely to be an efficient way of producing a crop hydroponically.

If salinity is a potential problem, then in the final analysis monitoring using a specific ion electrode for either sodium, chlorine or both might be the best solution, and dumping only when a critical level is reached. For general purposes irrigation water should not contain more than certain prescribed levels of nutrients and other contaminating ions. The recommended limits on contaminants in water used for hydroponics are: 100ppm calcium, 50ppm sodium, 25ppm magnesium, 0.75ppm boron, 60ppm carbonates, 250 ppm sulphates and 70ppm chloride. Good quality water should have a conductivity between 250 and 750 mmhos/cm at 25°C and dissolved salts between 175 and 525 ppm.

Water with a low EC of 0.1 to ).5 dS/cm will give the grower the largest number of options for managing the salinity content of nutrient solutions.

The key factor for hydroponics, clearly is to insure the water being used to replenish the nutrient solution has as low a salinity reading as possible. In an arid environment this is very difficult to achieve due to high rate of evaporation and the effect this has on increasing low levels of contaminants to much higher concentrations. Therefore every possible strategy to conserve water must be investigated and used where applicable.

In greenhouse production, the greenhouse roof can be an excellent surface for the collection of water from rainfall. (assuming the rain is clean and free of pollutants),which can then be stored in a large reservoir, and provide high quality water for the replenishment of the nutrient solution.

Another strategy would be to purify existing water supplies.

This will be the subject of the next article.

So what is the answer to Joe’s problem?

Our suggestion is that he should approach the solution in the following manner.

1) Build a reservoir adjoining the greenhouse, and use this as a “temporary” storage for rain water from the greenhouse roof.

2) Avoid using any secret magic potions, or if he still insists on using them, he should ensure that they have a low or zero sodium content.

3) Ensure that ALL the water he uses in the hydroponic system is initially treated with ozone (to remove pathogens and some minerals), and then filtered.

4) Use reverse flow osmosis on this water to remove the bulk of the mineral ions.

5) The spare land not used for the reservoir should be used for evaporation pans to ensure that the highly saline water from the reverse flow system and from the dumped hydroponic solutions does not damage the environment downstream.

6) If considered necessary an ion exchanger should be used after the reverse flow osmosis system. This may, however be an unnecessary extravagance.