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Resource Manual on IPPM in Rice World Education Philippines, Inc. The principal pathway for N2 to enter living organisms from the air is via certain types of bacteria. Many of these bacteria live in soils. Others, like Cyanobacteria, live in aquatic systems like rice paddies and inside the cells of specific kinds of algae for example, the blue-green algae. Cyanobacteria in a rice paddy may fix up to 100- 150 kilograms of N per hectare per year. Other bacteria, such as those of the genus Rhizobium, live inside the roots of leguminous plants, and still others live inside the leaf cells of plants, like Sesbania. Both Cyanobacteria and Rhizobia are so successful at fixing large amounts of nitrogen because each has access to large sources of energy. The Cyanobacteria live inside algae, so they are closely associated with algae. Through photosynthesis, the algae are able to offer carbohydrates (C) to bacteria, and in return, the bacteria fix nitrogen, some of which benefits the plant. In a similar manner the Rhizobium bacteria are associated with the roots of plants, receive food (carbon) from the plant roots, and pay back its host with nitrogen. The limited paths in which nitrogen finds its way into living systems explain why nitrogen, of all the soil nutrients, would most likely be lacking in soil. Yet all life requires N in large amounts; as a result, natural supplies of N are almost always limited and most plants become very good at competing for N supply. a. The N Cycle One of the mysteries of life is that although nitrogen is needed by all living things in fairly large amounts, and although our atmosphere is a huge reservoir of nitrogen, the paths by which nitrogen can enter ecosystems are very limited - the doorway opens through a few species of bacteria. As a result, nitrogen tends to be a limiting nutrient in the growth and development of many organisms, most especially plants. A limiting nutrient is the one (and by definition, there is only one at a time) nutrient that is in greatest demand and whose absence slows the entire process of growth. Nitrogen can take several forms in the soil, depending on how it combines with other atoms. In addition to the many forms in which nitrogen is found in living things, nitrogen can be found in inorganic forms in the form of nitrogen salts: ammonium [NH 4 +], nitrite [NO 2 -], and nitrate [NO 3 -]. Like any salt, all three forms of Nitrogen salts are highly water-soluble and as a result can be found most anywhere on the surface of the planet. The nitrogen salts are very actively cycled and recycled back and forth between inorganic forms and organic forms. The two main forms accessible to plant roots, and therefore the forms highlighted here are NH 4 +, and NO 3 -. 18
Resource Manual on IPPM in Rice World Education Philippines, Inc. Synthetic nitrogen is applied at a rate of about 40 million tons per year, worldwide. Deposition (entry into the soils) of nitrogen from pollution is roughly 100 million tons per year. This is mostly in the form of nitrous oxide (N 2 O, one of the greenhouse gases) and NH + 4 in dry deposition, and in the form of nitrate (NO - 3 ) and ammonia (NH 3 ) in wet deposition (rainfall). Nitrogen input to soils from pollution can be as much as 40 kilograms per hectare per year. Biofixation of N accounts for roughly 140 million tons per year worldwide. b. Too Little N If nitrogen in the soil is low, almost all plant functions are disturbed and the most direct result is that plant growth is stunted and plant color turns pale green to yellow. Four N atoms surround a single magnesium atom to form the core of the chlorophyll molecule; hence, limited N reduces the photosynthetic capacity of the plant. Nitrogen is highly mobile in plants, so when N is low it drifts from older leaves to newer leaves and the older leaves will turn light green, yellow or even pink. This is one good indicator of nitrogen deficiency, although people must be careful not to get this confused with one of several diseases or even soil drainage problems. 19
Page 1 and 2: Resource Manual on Integrated Produ
Page 3 and 4: Compiled and published by: World Ed
Page 5 and 6: ACKNOWLEDGM ENT World Education (IN
Page 7 and 8: 1. Leaves 48 2. Flower 49 3. Fruit
Page 9 and 10: Sources 203 Part XII: Farm Record K
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