Hydroponics BC

Horticulture and Plant Biotechnology in Scotland
GROWING HYDROPONIC CROPS USING PERLITE


a reprint of an article
by G. R. Dixon, Department of Horticulture
West of Scotland College
August 1988


The Clyde Valley of Scotland has been a thriving center of glasshouse tomato production for much of this century. Over the last 20 years, however, sharp rises in fuel and labor costs and increasingly fierce competition from imported produce have led to a serious decline in area of cropped glass.

Today, there is a new optimism among the Valley's tomato growers. Those who survived the difficult years have made radical changes to the old-fashioned, low-yielding, inefficient methods of cropping in border soil. They have exploited each technical advance arising from research and development, advances in crop nutrition, energy saving, computerized environment control, crop protection and plant breeding. But the greatest change of all has been their adoption of a new and revolutionary concept of crop culture: HYDROPONICS.

HYDROPONICS--- Hydroponic growing systems are based on a new generation of superb rooting media - inert, sterile, uniform materials such as the mineral perlite and rockwool. These products act merely as supports for a complete nutrient solution on which the plants depend entirely for their water, mineral nutrients and oxygen. In the purest form of hydroponics, there is a complete absence of solid substrate, the nutrient solution itself acting as the rooting medium.

Commercial growers can choose between three main hydroponic systems: one which uses a recirculating solution, the nutrient-film technique (NPT), and two 'static' systems, perlite culture and rockwool culture. The perlite culture system was uniquely devised and developed by a team of Scottish horticulturists. It is now the most widely used glasshouse crop production method in Scotland and it has the fastest growth of any hydroponic technique in the U.K. It is currently the subject of worldwide interest, particularly for use in arid zone regions.

Tomatoes growing hydroponically in Perlite

WHAT IS PERLITE?--- Perlite is a volcanic glass formed when larva cools very rapidly trapping small quantities (2-5%) w/w) of water. When the glass is crushed and heated to about 10000C, the trapped water vaporizes and puffs out the softened granules to form white mineral foam.


THE IDEAL ROOTING MEDIUM--- Expanded perlite is physically stable and chemically inert. The porous nature of the cellular granules ensures a product that is light to handle, holds large quantities of readily available moisture and has a strong capillary attraction for water. Since it is free draining, it is also well aerated.

Its neutral pH, negligible nutrient content and a complete freedom from pests, pathogens and weed seeds combine to make this an ideal rooting medium for hydroponic culture.


THE PERLITE CULTURE SYSTEM--- When used for commercial crop production, perlite is contained in 20 to 30 liter perforated bags designed to hold three tomato plants. A constant supply of aerated nutrient solution is maintained in the perlite by sitting the bags in a shallow reservoir of solution formed in the bottom of an outer polyethene gully by a series of polystyrene dams. Perlite's strong capillary attraction for water automatically draws up water from the reservoirs at the same rate as it is lost by evapotranspiration, irrespective of weather conditions or stage of crop growth.


PLANT NUTRITION PROBLEMS CAN BE SOLVED BY THE APPLICATION OF SCIENCE.--- Since perlite is devoid of available nutrients, the liquid feed applied to hydroponically grown plants is their only source of nutrition. This solution must therefore provide all the essential elements required for healthy growth and development. Moreover, each individual nutrient must be added at a rate which exactly matches its removal by the crop if deficiencies or toxicities are to be avoided. Thus, potassium may be added at a concentration of 400 mg/l, molybdenum at a mere 0.04 mg/l. Yet despite the 10,000-fold difference in concentration, molybdenum is just as important as potassium; without it there would be complete crop failure.


CONTROL OF EARLY-SEASON VIGOR--- A common problem associated with hydroponically grown tomatoes in Scotland is the excessively 'strong' growth that can occur in the poor light of winter months, encouraged by unlimited supplies of water and nutrients, especially nitrogen. Strong plants are difficult to train, and unsatisfactory flower set results in poor quality fruit and low early-season yields.

The usual remedy to curb excessive vigor is to restrict water availability (indirectly) by applying concentrated nutrient solutions. In Scotland, however, the problem has been cured by limiting nitrogen, the key nutrient influencing plant vigor.

Tomato plants will not make any growth if there is a complete absence of nitrogen in the rooting substrate; they will grow far too strongly if there is a high nitrogen concentration. It follows, therefore, that there must be an intermediate level that produces 'balanced' growth. Solutions containing 120 mg N/I are therefore applied to perlite-grown crops until the lower trusses have successfully set fruit and light levels have improved. Thereafter, higher nitrogen concentrations (200 mg N/l) are required to help swell the fruit at maximum rates. Scottish growers are thus recommended to use two solutions - one intermediate, the other high in nitrogen - to give them maximum control over the growth and development of their crops throughout the season.


SUBSTRATE pH--- One of the main disadvantages of hydroponic systems is their lack of pH buffering capacity. Since pH has such a strong influence on the rates of absorption and solubility of nutrients, it is essential to maintain it within optimum limits.

The two main factors determining the pH of a nutrient solution in which plants are growing are the form(s) of nitrogen present (nitrate and/or ammonium) and the hardness of the water being used.

When plants absorb nutrients from a solution prepared with an ammonium-nitrogen source, they reduce solution pH and increase it when grown in solutions made up with nitrate. Since most hydroponic fertilizers contain almost 1OO% nitrate-nitrogen, the alkaline drift that this induces must be neutralized, necessitating the use of expensive and potentially hazardous acids.

However, horticulturists in Scotland have recently shown that a safer and cheaper way of stabilizing solution pH can be achieved simply by choosing the appropriate ratio of ammonium to nitrate. With our soft water supplies, pH stability occurs in solutions containing 14% of the total nitrogen in ammonium form, 86% as nitrate.


OPPORTUNITIES FOR HORTICULTURISTS--- The development of hydroponic systems - and perlite culture in particular - has been a major factor in the fight-back to prosperity by Scotland's glasshouse tomato industry. Not only do perlite-grown crops far out-yield those grown in soil, less energy is required to produce them, the turn-around time between crops is shortened and soil-borne diseases are greatly reduced.

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