Page 72 - Introduction to Agriculture by: Aqleem Abbas

Introduction to Agriculture Notes prepared by: Aqleem Abbas

Nutrients

Through photosynthesis, green plants manufacture their own organic food, using carbon dioxide and oxygen as raw materials.
The nutrients usually supplied to plants by soil are almost entirely mineral salts. Plant physiologists have discovered that plants
require carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, magnesium, sulfur, calcium, iron, manganese, boron, zinc,
copper, and probably molybdenum. Carbon, hydrogen, and oxygen are obtained in large quantities from water and air, but the
remaining elements are ordinarily supplied as salts by the soil. The relative amount of each of these elements required for normal
growth is different in each plant, but all plants require relatively large proportions of nitrogen, phosphorus, potassium,
magnesium, sulfur, and calcium. Iron, manganese, boron, zinc, copper, and molybdenum are supplied in minute quantities, and
are called micronutrients or trace elements. The specific salts used to supply these elements may be varied at the discretion of the
grower; a typical solution of primary minerals is composed of distilled water containing potassium nitrate, KNO 3 , calcium nitrate,
Ca(NO 3 ) 2 , potassium acid phosphate, KH 2 PO 4 , and magnesium sulfate, MgSO 4. In solution, the salts dissociate into ions;
+
-
potassium nitrate, for example, is available to plants as the ions K and NO 3 . A solution of micronutrient salts is added to the
solution of primary elements to complete the nutrient solution. A small amount of fungicide is usually added to prevent the
growth of mold.

Hydroponic culture method

Several culture techniques are used. The most practical commercial method is subirrigation, in which plants are grown in trays
filled with gravel, cinders, or other coarse materials, and periodically flooded with nutrient solution. The solution is allowed to
drain off after each flooding and may be reused as long as sufficient minerals remain in it. The water-culture method is used
widely for botanical experimentation. A common type of water culture consists of glazed porcelain jars filled with solution; the
plants are placed in beds of glass wool or similar material that are supported at the surface of the solution. Roots of the plants
penetrate the beds and remain in the solution. The least exact method, commonly called the slop method, is the easiest to operate.
Coarse, clean sand is used in place of soil, and nutrient solution is poured on the sand in approximately equal amounts at regular
intervals. A refinement of this practice is the drip method, in which a steady, slow feed of nutrient is maintained. Excess nutrient
solution is allowed to drain off in both slop and drip methods.

Hydroponic culture methods are being used successfully to produce plants out of season in greenhouses and to produce plants in
areas where either the soil or the climate is not suitable for the crop grown. During World War II, for example, several U.S. Army
units successfully produced vegetables hydroponically at various over-seas bases. In the 1960s hydroponic farming developed on
a commercial scale in the arid regions of the United States, particularly in Arizona, where research was also undertaken at state
universities. In other arid regions, such as the Persian Gulf and the Arab oil-producing states, hydroponic farming of tomatoes
and cucumbers is under way; these countries are also researching an additional group of crops that may be grown by this method,
as they have limited arable land.