Chapter 4: Manage hydroponic plants

Hydroponic farming represents a transformative approach in Kenyan horticulture, enabling efficient plant production without soil by using nutrient-rich water solutions. Managing hydroponic plants effectively requires specialized field management practices that ensure optimal growth, maximize yields, and maintain plant health. This chapter focuses on key aspects of hydroponic farm management, including gapping, fertilizer application, and pest and disease control, all tailored to the unique conditions of hydroponic systems within Kenya's horticultural sector.

4.1 Field management practices

Field management practices are the regular jobs that keep a growing hydroponic crop healthy between transplanting and harvest. In a hydroponic unit the "field" is the greenhouse, tunnel or net house with its benches, channels and tanks, and the work is done there every day, not once a season. Three practices decide much of the crop's success: filling the gaps left by plants that die, feeding the crop through the nutrient solution, and keeping pests and diseases below the level that causes loss. Because all the plants in a recirculating unit share one nutrient solution, a mistake in one practice reaches every plant, and good records are part of the work. Example: a tomato house that is checked each morning for gaps, solution readings and pests can correct a problem in a day, while the same problem found a week later may need half the crop replaced.

4.1.1 Gapping

Gapping is the replacement of plants that have died or failed to establish, so that every planting position in the system carries a healthy plant. Empty positions waste water, nutrients and bench space, they let light fall on the channels and tanks, where it grows algae, and the neighbouring plants grow unevenly. In hydroponics a gap is also a warning: a plant dies for a reason, such as root disease, a blocked dripper, a pump failure or damage during planting, and the cause must be found before the replacement is put in.

Purpose of gapping

• Uniform crop: a plant of the same age and size in every position gives an even harvest that meets the buyer's grade.
• Full use of the system: each position carries the nutrient, water and space that was paid for.
• Less algae: covered positions keep light off the water and the roots.
• Early warning: a pattern of gaps, for example at the end of a channel or beside one dripper, shows a fault in the system.

Timing and frequency of gapping

Gap in the first week or two after transplanting, while the spare seedlings are still the same size as the planted ones. A small group of spare seedlings is kept in the nursery tray or in a spare corner of the system for this purpose. After that a replacement plant lags too far behind the others, and a gap is better filled by moving a neighbour or leaving the space covered. Inspect the unit for gaps every morning at first and later at least twice a week, while the daily plant check continues.

Procedures for gapping

1. Walk the benches and mark every gap and every weak plant with a coloured peg or a label.
2. Find the cause: look at the roots of the dead plant, check the flow at that position and check the readings for the tank.
3. Remove the dead plant, its net pot and medium, carefully so as not to splash disease on the neighbouring plants, and put them in a closed bin.
4. Wash the net pot or container, or use a new one, and wipe the cutting tool with disinfectant.
5. Select a healthy spare seedling of the same age and size and transplant it into the gap, with the plug in contact with the film, solution or dripper.
6. Check the position the next day, and again after a week, and note the number replaced in the record book.

Challenges in gapping and how to deal with them

• Transplant shock: handle the seedling by a leaf or the plug, not the stem, and shade the replacement for a few days.
• Late gapping: a plant put in after the others is shaded by them, so replace early or leave the gap covered.
• Repeated gaps: the same position failing again means a fault in the system, not in the seedling, and the fault is corrected first.
• Shortage of spare seedlings: sow a few more seeds than the number of positions, so that there are spare seedlings of the same age.

4.1.2 Fertilizer application

Fertiliser application in a hydroponic system is the feeding of the crop through the nutrient solution, because the plants take all their water and mineral nutrients from it. The fertiliser used is a set of soluble salts that dissolve fully in water, and it is mixed to a recipe for the crop and the stage of growth. How the recipe is worked out is taught in Determining nutrient solution. Here the work of applying it, and of watching its effect on the crop, is described.

Nutrient management in hydroponics

The grower controls the strength of the solution through its EC (electrical conductivity), and its acidity through its pH. A solution that is too weak starves the plants and one that is too strong burns the roots or causes salt stress, so both are measured every day, at the same time. The pH is kept in the range 5.5 to 6.5 for most crops, because outside it some nutrients cannot be taken up even when they are present.

Types of fertiliser used

• Complete hydroponic mixes: ready-made powders or liquids sold for hydroponics, usually in two or three parts, containing all the major and minor nutrients.
• Single salts: individual soluble fertilisers such as calcium nitrate, potassium nitrate and magnesium sulphate, used by growers who mix their own recipe.
• Micronutrient mixes: a small amount of iron (usually as a chelate, which keeps it available), manganese, zinc, boron, copper and molybdenum.
• pH adjusters: a dilute acid to lower the pH and a dilute base to raise it.

Methods of fertiliser application

1. Dissolve each part of the fertiliser in its own concentrated stock tank, and keep the calcium part separate from the part that contains sulphates and phosphates, because they form a solid when concentrated together.
2. Fill the working tank with clean water, add the stock solutions in the order stated by the supplier, stirring after each, and never add them together undiluted.
3. Measure the EC and pH, and correct them by adding more stock, more water, or acid or base. Calibrate the meters as the maker advises, so that the readings can be trusted.
4. Feed the crop from the working tank through the system, by pump, drippers, channels or wicks.
5. Increase or reduce the strength according to the stage: weaker for seedlings, stronger as the plants grow, and with more potassium when the crop is flowering and fruiting.
6. Top up with water daily and change the solution on the farm schedule, recording the volume and the readings each time.

Worked example: each stock tank is mixed at 100 times the strength of the working solution, so the working solution is 1 part of stock A, 1 part of stock B and 98 parts of water. A working tank of 500 litres needs 500 ÷ 100 = 5 litres of stock A and 5 litres of stock B, and 500 - 5 - 5 = 490 litres of water.

Common nutrient deficiencies and their management

A deficiency sign may also be caused by a pH outside the range, a damaged root or a very strong solution that prevents uptake, so the readings and the roots are checked before the recipe is changed.

4.1.3 Pest and disease control

Pest and disease control in hydroponics is the set of measures that prevent pests and diseases from entering the unit and limit their spread when they do. The closed, warm, humid conditions of a greenhouse suit many pests, and the shared solution can carry root diseases to every plant in a few days, so the emphasis is on prevention and early detection. The method used is integrated pest management, which combines several methods so that no single one, especially a chemical, is relied on alone.

4.1.3.1 Causal agent

A causal agent is the pest or pathogen (an organism or a virus) that causes pest damage or a disease. It is identified first, because the control measure depends on it.

• Insects: aphids, whiteflies and thrips suck sap and may spread virus diseases, and leaf miners tunnel inside the leaf.
• Mites: spider mites feed on the underside of the leaf and spin fine webbing.
• Fungi: powdery mildew and grey mould (Botrytis) attack leaves, stems and fruits, while Fusarium causes wilt and is spread through the roots.
• Oomycetes (water moulds): Pythium causes root rot and damping off, and spreads in water, so it is a main risk in float tanks and recirculating systems.
• Bacteria: some bacteria cause soft rot and wilting, and spread through water and on tools and hands.
• Viruses: spread by insects such as whitefly and thrips, and by hands and tools, and they cannot be cured, only prevented.
• Fungus gnats: small flies whose larvae feed on roots in damp media and may carry fungal spores.

4.1.3.2 Signs and symptoms

A sign is the visible presence of the causal agent itself, such as an insect, a web or a fungal growth. A symptom is the plant's reaction to the attack, such as yellowing, wilting or distortion. Both are used to make a diagnosis.

Integrated pest management

• Cultural methods: remove dead plants and debris, keep the greenhouse and benches clean, and avoid waterlogging and very high humidity by ventilating.
• Physical methods: fit insect-proof net on the vents and a double door at the entrance, hang yellow or blue sticky traps, and wash small pest colonies off with water.
• Biological methods: use natural enemies of pests, such as predatory mites for spider mites, parasitic wasps for whiteflies and aphids, and ladybirds for aphids.
• Chemical methods: use a pesticide only when the pest has passed the level the farm sets for action, choose a product registered for the crop, and follow its label for dose, protective clothing and the waiting period before harvest. Spray the leaves with care so that none runs into the nutrient solution.

Scouting and early detection

Scouting is a regular walk through the crop to look for pests and symptoms. Check the underside of leaves with a hand lens, count the insects on the sticky traps each week, and look at the roots of a few plants in each system. Record what is found and where, so that a trend is seen early. A diseased plant is removed and bagged, without shaking it, and the roots of its neighbours are checked.

Hygiene and prevention

• Wash hands and sanitise tools between benches, and use a footbath at the door.
• Use clean water, clean media and seedlings that were raised in a clean nursery.
• Keep new plants and visitors away from the crop until checked.
• Clean and disinfect the tanks, channels and pipes between crops.
• Keep the greenhouse and the area around it free of weeds, which shelter pests.

Chapter Summary

Managing hydroponic plants requires careful attention to field management practices, beginning with gapping to ensure uniform plant growth and optimal use of space. Proper fertilizer application is critical to supply essential nutrients directly to the plant roots, supporting healthy development in the soilless system. Effective pest and disease control involves identifying causal agents and recognizing their signs and symptoms early to prevent widespread damage. Harvesting hydroponic plants demands precision timing to maintain quality and maximize yield. Additionally, managing wastes generated during the hydroponic process is essential to maintain a clean environment and promote sustainability. Together, these practices contribute to successful hydroponic farm management by optimizing plant health, productivity, and environmental care.