Hydroponic farming: How shade-net and plastic-covered greenhouses affect growth


By Digital team | 4 July 2018
farming; hydroponic; pests
Photo: Wilma den Hartigh

Hydroponic farming can be defined as a crop production system in which plants are grown in an artificial medium rather than in natural soil. All the nutrients are dissolved in the irrigation water and regularly supplied to the plants.

Today, hydroponic systems are used in areas where more usual field agricultural practices are not possible. They do, however, have their own advantages and disadvantages.

ADVANTAGES

  • Hydroponic systems enable the producer to create an environment that is optimal for plant growth.
  • All the nutrients can be applied, at the correct ratios, throughout the plants’ growing season.
  • Crop rotation is not necessary as no soil is used.
  • The growth medium used can be sterilised or discarded at the end of the season.
  • Weed control is limited, or even eliminated.
  • The harvested crop is free of soil particles.
  • Hydroponic systems in a controlled environment can result in higher yields than for plants grown in the open field.
  • Hydroponic systems uses less labour per unit area than in open-field production.

DISADVANTAGES

  • The initial capital outlay is fairly high.
  • The producer needs to have some degree of competence in plant science.
  • Soluble fertilisers are more expensive than those used for open-field production.
  • Hydroponics requires a very high level of management.
  • Not all vegetable crops can be grown profitably under hydroponics.

TEMPERATURE & DIFFERENT STRUCTURES

Temperature plays an important role in the growth of vegetable crops. All vegetables, therefore, can be separated broadly into two groups: warm season and cool season crops.

For example, tomatoes are a warm season crop, even though they are grown over a wide range of latitudes, and lettuce is a cool season crop.

Also read: Hydroponic farming: Shaping your tomato plants

Classification by the adaptation to temperature is perhaps the most useful crop grouping, as the distinct characteristics of a specific group have management implications.

Crops may also be classified according to their sensitivity to various environmental factors including water pH, salinity and tolerance to specific nutrient concentration levels. Such information is essentially useful in management decisions and in diagnosing problems in plant growth.

The production of lettuce in hydroponic systems and under a controlled environment makes for a clean, uniform and superior-quality crop throughout the year. Lettuce has a shallow root system, and lends itself to the gravel film technique hydroponic system.

Also read:
Growing lettuce in a closed system
Hydroponic farming: More on growing leafy vegetables

In hydroponics, the average temperatures for growing tomatoes range between 10⁰C and 30⁰C. Temperatures outside this range will result in poor or reduced growth and reduction in yield.

It is very critical to choose the correct structure for the right location. In South Africa, temperatures higher than 35⁰C (even as high as 50⁰C) coupled with high humidity are common in a non-cooling plastic tunnel. Opening the doors and flaps in a non-temperature controlled tunnel is not enough to cool down the tunnel structure.

High humidity restricts transpiration, leading to an increase of temperature inside plant tissue. If transpiration is restricted, water and mineral uptake by the roots is also limited, thus reducing plant growth.

The purpose of greenhouses is to create an internal climate more favourable to the growth of plants than the environment outside.

In South Africa, we typically try to increase low temperatures during winter and decrease high temperatures during summer, decrease the diurnal variation in daily temperature, decrease the amount of UV light and keep rain, wind and birds out. Different types of greenhouses (e.g. tunnels, glass/fibrehouses) are being used, and shade-net structure and plastic tunnels are popular.

A comparison was made between the 3 kinds of structures to supply valuable information for farmers.

A non-cooling tunnel (i.e. with 2 doors and 2 flaps), a 40% black and white shade-net structure and a cooling tunnel (i.e. with pad and fan system) gave 78%, 80% and 92% of marketable yield of fresh tomatoes, respectively. They can be used in areas where frost occurs.

As tunnels can get very hot, crops such as sweet peppers and tomatoes perform poorly in a tunnel structure without cooling. The loss (because they are unmarketable) is largely the result of physiological problems such as blossom end-rot, sunburn, cracking, and so on.

Climatic conditions affect the choice of infrastructure type to be used for vegetable crops. For example, in frost-free areas farmers can grow vegetables all year round inside shade-net structures.

Farmers producing in areas that experience frost would need tunnels to protect their crops against frost. Irrespective of weather/climatic conditions, cucumbers should be grown in a plastic, glass or fibre-glass greenhouse to protect the fruits from wind damage. The advantage of using structures covered with this material is that it makes winter heating possible, unlike shade-net structures.

ADVANTAGES OF SHADE-NET STRUCTURES

  • Using shade-netting is growing in popularity mainly because it is the cheapest system available.
  • They are relatively easy to construct.
  • Shade-netting allows for wind movement and thus air for ventilation.
  • A well-constructed shade-net structure offers protection against animals, birds, extremes of sun, hail and heavy rain as well as large insects.
  • Shade-netting cuts out up to 90% of the sun’s harmful UV rays.
  • It has a life span of approximately eight years, if fastened properly to the structure.

DISADVANTAGES OF SHADE-NET STRUCTURES

  • It is impossible to control the temperature and humidity.
  • It is impossible to keep out small insect pests.

ADVANTAGES OF PLASTIC-COVERED GREENHOUSES

  • As the house can be properly sealed, the temperature and humidity inside can be manipulated to suit the needs of the crop.
  • The crop cannot be damaged by wind, birds, heavy rain and hail.
  • Production can take place throughout the year in a temperature-controlled tunnel.

DISADVANTAGES OF PLASTIC-COVERED GREENHOUSES

  • Covering plastics have a limited lifespan and have to be replaced every three to five years.
  • The cost of plastic houses is considerably higher than for shade-net structures.
  • The construction of single and multi-span houses requires careful planning and expertise.
  • The temperature inside plastic tunnels rises very quickly.
  • Temperatures higher than 40⁰C are not uncommon in South Africa.
  • Special care should be given to air ventilation.

Also read:
Hydroponic farming: The basics – popular systems

  • This article was written by Martin Maboko and Silence Chiloane and first appeared in Farming SA.