1. Introduction
   • Some basic information
2. Macro nutrients and trace elements
3. Composition
4. Deficiencies
5. Toxicity
6. Water quality
   • (Im)purity of fertilisers
7. Nitrogen (nitrate, urea, ammonium)
8. Phosphorus
9. Salinity, strength, concentration, EC, CF
10. Acidity (pH)
11. Stock solutions
12. Application and equipment
13. LIQUID FEED FORMULATIONS
    • The classical way
    • Standard formulation
    • Container-grown plants
    • Carnations
    • Chrysanthemum
      • Winter flowering cut flower crops
      • Spring flowering
      • Summer flowering
      • Autumn flowering
    • Cymbidium orchid
    • Roses
14. Liquid feed calculations

Introduction

Plant nutrition, liquid feeding, or sometimes called fertigation, is supplying feed simultaneously with water through watering equipment. Liquid feeding supplies optimum amounts of all nutrients, including trace elements to growing plants. Liquid feeding is essential with soil-less cultures (pumice, sand), and is an option for container grown plants (in potting mix) and for flower crops in greenhouses (in soil). It is then a supplement to base dressing and an alternative to topdressing fertilisers. Small plants require only small amounts of nutrients. It is not appropriate to mix large amounts of nutrients through the potting mix of young plants, because excess would be wasteful and could even be detrimental to small plants. Therefore supplementary feeding is given as the plants grow.

Generally liquid feeding is more accurate and easier to apply evenly than solid fertiliser. Especially when the crop is mature with leaves covering the soil surface or the container it can be hard to get an even spread of solid fertilisers. Reduced labour costs, improved growth rate and enhanced quality of plants are important advantages of liquid feeding. With careful nutrition management, profit per m² of capital invested in greenhouses increases.

Some basic information

‘ppm’ means ‘part per million’, and is a unit for concentration of salts in water. Ppm equals milligram per liter. So 1 ppm = 1 gram per million gram water = 1 g/m³ = 1 mg/l water.

EC and CF are measures for the strength (or concentration) of the liquid feed. It has to do with the total amount of nutrients. EC = electric conductivity; CF = conductivity factor. CF of a liquid is ten times bigger than the EC of that liquid. Units for EC are millimhos or (more used internationally: milli-Siemens per centimeter mS/cm) measured at a temperature of 25^°C. Normally EC ranges from 1 to 4 millimhos (or mS/cm), and thus CF ranges from 10 to 40.

pH expresses the acidity of a liquid. pH of 7 means neutral, lower than 7 means acid; higher than 7 means alkalic. The optimal pH for plant growth is 5 -7, or 5.5 - 6.5.

Macro nutrients and trace elements

Macro nutrients are those that are needed in great quantity. These are nitrogen (N), sulphorus (S), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Trace elements or micro elements are essential for plants, but only in miniature amounts. Only iron (Fe) is used in relatively large quantities although considered as a micro-element. Other micro elements are manganese (Mn), boron (B), copper (Cu), zinc (Zn) and molybdane (Mo). Trace element supply is essential for soil-less mixtures and is advised but mostly not critical for growing in potting mix. It can be either incorporated into the potting mix or applied via a liquid feed.

Composition

The composition of fertigation is the ratio of the different compounds in the liquid feed. The advised composition depends on the species, variety and stage of the plants' growth. To obtain the required composition, all nutrients used in liquid feeding must be weighed and measured accurately. Precise amounts of a precise composition may be applied when there is one crop per greenhouse. However, when a variety of plants are grown at different stages of growth, a standard feed suitable for the majority must be used. Fortunately crops tolerate a wide nutrient range, or in other words a wide range of crops can be grown together successfully on one nutrient solution (see ABC of NFT).

Deficiencies

Liquid feeding is often able to correct nutrient deficiencies (shortage of a nutrient) quickly. Deficiency of an element can cause very characteristic symptoms. Once it has been determined which element is deficient, the problem can be fixed with liquid feed. Calcium nitrate applied at 1.5g/litre maintains growth of seedlings when pricking out has to be delayed. Applied at the same rate, calcium nitrate also corrects yellowing of polyanthus, nemesia etc, when soil temperatures are low in winter.

Toxicity

Nutrient elements have different levels of toxicity in different plants. Some well-known toxicity problems are:

• Zinc toxicity can occur when new galvanised pipes are used. Water in new pipes may be 0.5 ppm, rising to 4 ppm zinc when water is static overnight. Condensation drips from new galvanised pipes can cause localised toxicity. Zinc level should not exceed 0.2 ppm.
• Sulphur is an essential trace element, but it can be an impurity in water supplies.
• Domestic water softeners replace calcium with sodium. They are not suitable for plants.

For some crops a decreasing toxicity order has been established:

• Carnations: magnesium chloride - calcium chloride - sodium chloride - magnesium sulphate -sodium sulphate - sodium bicarbonate - calcium sulphate.
• Poinsettia: sodium bicarbonate - magnesium chloride - sodium chloride - calcium chloride - magnesium sulphate - calcium sulphate.

Water quality

The quality of water applied to plants is very important, and it is advised to test the water quality to determine purity, mineral content and pH, especially for soil-less cultures. Testing facilities are available through MAF and commercial laboratories. If necessary install a water treatment system using filters. Water treatment helps to prevent blocked nozzles and to allow efficient water distribution.

The most important contaminants of irrigation water are calcium, magnesium, carbonates, sulphates, sodium bicarbonate, sodium chloride, iron and boron. Where water supply contains large amounts of calcium, magnesium or sulphates, these should be deductd from the amounts added to obtain the required amount. Where water contains large amounts of calcium and magnesium carbonates, frequent irrigation increases pH of the mix appreciably. For instance in summer with frequent watering, a water alkalinity equivalent to 200 ppm calcium carbonate can raise pH by 1.0 to 1.5 units in 10 weeks. These carbonates can be neutralised by adding acids. If iron is present in the water it should not be deduced from the amount added, as this iron is not taken up by the plants.

Sodium bicarbonate is more toxic than sodium chloride. Bicarbonates in water can cause iron deficiency in plants: 200 ppm bicarbonates seldom causes toxicity, but 500 ppm bicarbonates is the maximum tolerated without water treatment. However, plants respond differently: 1000 ppm sodium bicarbonate in water is satisfactory for carnation and primula, but 500 ppm sodium bicarbonate is not suitable for tomato and chrysanthemum.

For general purposes, irrigation water should not contain more than: 100 ppm calcium, 50 ppm sodium, 25 ppm magnesium, 0.75 ppm boron, 60 ppm carbonates, 250 ppm sulphate, 70 ppm chloride. Water can be classified, considering the various aspects, as follows:

Table 1: Water classification

Fertilisers suitable for liquid feeding.

(Im)purity of fertilisers

Most fertilisers as they come from the factory contain impurities, depending on the quality or grade. The grades are: laboratory reagent (purest grade available), commercial grade (very pure), technical grade or agricultural chemical (impure, impurities can precipitate), analytical reagent (pure to impure).

All figures in the fertilisers table (above) are for the pure chemical, and lower grades may contain considerably less nutrients so check the grade (or purity) of the material purchased. The supplier should be able to confirm the composition of the grade being supplied.

Nitrogen (nitrate, urea, ammonium)

It is advisable to use nitrate sources of nitrogen, e.g. potassium nitrate, ammonium nitrate or calcium nitrate on soil-less mixtures. Urea is a suitable form of nitrogen for liquid feeding plants growing in soil or soil mixes. The pure compound is relatively cheap, non corrosive, non-toxic and very soluble. Used at the same rate, urea supplies three times the amount of nitrogen as calcium nitrate. Ammonium is another source of nitrogen. It affects the pH and the buffering capacity of the nutrient solution. Ammonium should supply only a small part (10-20%) of the nitrogen demand.

Phosphorus

Phosphorus levels may alter unpredictably in soil-less mixtures, sometimes dropping to nil, so routine applications are advised. Do not rely on the amounts present in the base dressings. Phosphorus may be applied as phosphoric acid in liquid feeds: 80 ml supplies 30 ppm phosphorus and neutralises 2 milli-equivalent bicarbonates/1000 litres. N.B. Phosphoric acid is corrosive. Polyammonium phosphate formulations are not suitable because of the interaction with other liquid feed chemicals.

Salinity, strength, concentration, EC, CF

The strength (or concentration) of the liquid feed has to do with the total amount of nutrients, in the feeding. It is often expressed as electric conductivity (EC) or conductivity factor (CF). CF is the most commonly used term in New Zealand and NZ concentration meters refer to CF. Overseas literature makes reference to other terms such as EC, PC, atmospheres and osmotic pressures.

Units for EC are millimhos or (more used internationally: milli-Siemens per centimeter mS/cm) measured at a temperature of 25°C. CF of a liquid is ten times bigger than the EC of that liquid. Normally EC ranges from about 1 to 4 millimhos (or mS/cm), and thus CF ranges from 10 to 40.

The CF or EC value of a liquid feed is calculated from the separate values of the various fertilisers plus conductivity value of the water supply. The CF equals the sum of the cations in milli-equivalents (and also equals the sum of anions). EC equals the CF divided by 10.

The relationships between the various terms are as follows: CF = % soluble salts x 66.66; CF of 100 equals 10 micromhos; CF of 10 equals 1 millimhos; EC of 1 equals 1 millimhos; 1 Atmosphere = 0.365 x EC

The recommended EC may vary according to the stage of growth, time of year, vigour of plants, amount of slow release fertiliser in the mix and size of plant in comparison with size of container. A low value is CF = 10 (EC =1), while a high value is 5 times as high or more. Dilute feeds are used when feed is given at every watering, while more concentrated feeds can be used if the substrate is leached now and then with water.

The strength is often varied according to the season. In summer, when the evapotranspiration rate is high, feeding and watering must be adjusted to avoid increased salinity between waterings. In summer much extra water is required with a little extra feed, thus feeding of a lower EC is used. Also seasonal adjustments to the composition can be made: under poor light conditions growth of flower crops is often weak and soft, but a higher ratio of potassium : nitrogen (K : N) may correct the problem. Also, as plants reach maturity they require more potassium and less nitrogen.

Acidity (pH)

Acidity of a liquid is expressed as pH. pH of 7 means neutral, lower than 7 means acid; higher than 7 means alkalic. The optimal pH for plant growth is 5 -7, or 5.5 - 6.5. If the pH is too high, diluted acid should be added (strength 10 - 25%). If the pH is too low, then a diluted base solution should be added (e.g. 10% KOH).

The pH has a tendency to increase if nitrogen is given as nitrate, and pH tends to decrease if nitrogen is given as ammonium. Therefore often 10-20% of teh nitrogen is given as ammonium.

Stock solutions

A stock solution is merely a concentrated solution. A relatively small amount of stock solution is added to a large volume of water to obtain the right concentration. Stock solutions can be made in advance and used later. Stock solutions can contain only one salt or a mixture of salts. However, some fertilisers cannot be mixed with others in high concentrations because salts would precipitate.

The substance present in the greatest quantity is calcium nitrate and another big one is magnesium sulphate. However, high concentrations of calcium nitrate and of magnesium sulphate will cause precipitation of calcium sulphate and thus these two should not be present in the same stock solution. Actually, calcium nitrate is incompatible with ammonium sulphate, potassium sulphate, ammonium phosphate, sulphuric acid, and sulphates of iron, zinc, copper, manganese and magnesium. Magnesium sulphate is incompatible with calcium nitrate and ammonium phosphate. Sulphates of iron, zinc, copper, manganese are incompatible with calcium nitrate, ammonium phosphate, phosphoric acid.

Other combinations of fertilisers are possible. Therefore mostly two stock solutions are made: ‘A’ containing calcium nitrate and iron chelate (iron colours the stock solution brown); and ‘B’ containing the potassium and magnesium fertilisers, as well as all micro elements.

Application and equipment

For flower crops grown in greenhouse soil, nutrients must be applied to the soil prior to planting the crop, and supplementary feeding is applied during later growth. It can be applied through a handheld hose, overhead watering or automatic irrigation system, e.g. trickle irrigation. It can also be given in a soil-less culture system or NFT system. If liquid feed is applied by hose or overhead irrigation it may be necessary to apply plain water immediately afterwards to avoid scorching of the leaves.

Various equipment is available for accurate liquid feed applications, so consult the manufacturer or agent for advice on correct installation. Some chemicals are corrosive, especially when concentrated in the stock solution, hence it is advisable to check that all parts of the feeding and watering system are resistant to corrosion. This check must include pipes, joints, washers and nozzles.

The concentration of the diluted liquid feed must be checked with a conductivity meter. There are flow-type electrodes fitted in pipes leading from the dilutor that can check the fertigation liquid during the operation.

Liquid feeding may be applied via capillary matting or in a sub-irrigation system. A problem of recycling is that Pythium and other disease may spread through the greenhouse. Also, recycling may result in increased salinity: 100 ppm of nitrogen in sub-irrigation can be equivalent to 300 ppm by normal liquid feeding.

LIQUID FEED FORMULATIONS

There are many, many recommendations and recipes available for liquid feed (‘fertigation’) to flower crops and other ornamentals. As mentioned before, most crops tolerate a wide range of nutrition, or in other words one nutrition can be used successfully for many crops. We start with a classical recipe, previously used for NFT (nutrient film technique). In this formulation all fertilisers are diluted to an ideal strenght, and added to water in different dilution rates. This formulation shows the optimal levels of nutrition, from which variations can be made. Alternatively, instead of 9 stock solutions, only two stock solutions can be made up, containing all fertilisers (see above under stock solutions).

After that we list the composition in terms of main elements of some other recipes for some specific crops. For more detailed information we have to refer to other publications (later on HortNet).

The classical way

Starting solution

Dissolve each chemical separately in 1 litre water. Store in screw-topped labelled plastic containers. For every 1000 litre of water in fertigation tank add the volume of stock solution listed under dilution rate (or work in ml per litre). Maintain the CF between 20 and 30.

Topping-up solution

If CF rises above 30, top with mains water. If CF drops below 20, top up with solution below:

Maintain pH between 5.5 and 7.0. Add phosphoric acid to reduce pH and to supply phosphorus, or add potassium phosphate to supply phosphorus without reducing pH.

Standard formulation

Standard liquid feed suitable for most plants as a constant feed with every watering. An example of a standard feed is 100 ppm K, 100 ppm N, 15 ppm Mg, 15 ppm P.

Omit urea to provide a K : N ratio of 3 : 1 (or 100 ppm K and 31 ppm N). This is useful to improve quality or to harden the growth. Phosphoric acid can be applied alone once a week at 40 ppm P using 10.5 ml (or 17 grams)/1000 litres diluted feed.

Container-grown plants

If feeding only once a week, dilute at 1:200, increasing to 1:100 when plants are growing strongly (Hartmann & Kester, 1975 and Lamb, Kelly & Bowbrick, 1975)

Pot plants

Carnations

Liquid feed formulations recommended: constant feed of 190 ppm of N and 156 ppm of K at every watering, based on 300g urea (46% N) plus 400g nitrate of potash ( 1 3'X, N 39'96 K ) in 1000 litres water (Glasshouse Carnations, MAF, Christchurch).

Constant feed of 200 ppm of N and 200 ppm K2O (156 ppm K) based on 8.00 kg ammonium nitrate and 8.75 kg nitrate of potash in 100 litres of water. Diluted 1:200. Borax added if soil level below 2 ppm (Guernsey Hort. Adv. Service).

Constant feed of:

Constant feed:

Start liquid feeding as soon as plants are established at 1:400 dilution. Gradually increase to 1:200 during the following month.

Chrysanthemum

Three liquid feed formulations are recommended:

Winter flowering cut flower crops

200 ppm N : 200 ppm K at every watering using potassium and ammonium nitrate. Use a 1 N : 2 K base dressing.

Spring flowering

3 N : 2 K as soon as light conditions improve, e.g. 225 ppm N : 150 ppm K.

Summer flowering

2 N : 1 K liquid feeding. Base dressing of: 1.7 - 3.4 kg urea formaldehyde + 6.8 kg superphosphate + 3.4 kg potassium nitrate per 100 m².

Autumn flowering

Reduce nitrogen to 200 ppm N : 200 ppm K. Start feeding as soon as crop established, 5 days after planting. Liquid feed 2N : 1 K during long-day vegetative growth. More K than N during short days. Stop feeding when buds show colour (Framptons 1971). Liquid feeding at every watering from 2 weeks after potting until buds show colour: 200 ppm N + 15 ppm P + 150 ppm K + 15 ppm Mg

Cymbidium orchid

Apply once a fortnight. Increase ammonium nitrate to 300g for non-flowering young stock. Reduce or omit ammonium nitrate and increase potassium nitrate to 200g to promote flower spike production.

Stock solution:

Roses

200 ppm N and 200 ppm K. Add magnesium if required. Trace element mixture twice a year. Iron chelate three or four times a year (Rose Bulletin, MAF, Christchurch, Ball 1975)

Liquid feed calculations

To obtain a specified concentration of nutrient after dilution requires the following information:

1. dilution,
2. % element in fertiliser.
3. required concentration.

Weight of fertiliser required in stock solution in kg/10 litres is:

Example: At a dilution of 200 : 1 how much potassium nitrate is required to give a K concentration of 300 ppm?

To find concentration (in ppm) of diluted feed:

Information;

1. Dilution,
2. % element in fertiliser,
3. Weight of fertiliser used in stock solution.

Concentration of feed once diluted will be:

Example: What ppm K will be obtained from a stock solution containing 1.2 kg/10 litres KNO₃ at a dilution of 1 in100?