The rice plant belongs to the genus Oryza of Gramineae family. The genus Oryza has 24 species, of which 22 are wild and two species viz. Oryza sativa and Oryza glaberrima are cultivated. All the varieties found in Asia, America and Europe, belong to Oryza sativa and varieties found in West Africa belong to species Oryza glaberrima. Further, sativa rice varieties of the world are commonly grouped into three sub-species -viz. indica, japonica and javanica. Rice grown in India belongs to the indica. The varieties developed in Japan belong to japonica and javanica are cultivated mainly in Indonesia.

Rice is the most important crop of India and it occupies 23.3 per cent of gross cropped area of the country. Rice contributes 43 per cent of total food grain production and46 per cent of total cereal production. It continues to play vital role in the national food grain supply. It is the staple food of nearly half of the world population. It ranks third after wheat and maize in terms of worldwide production. Asia accounts for 90 per cent and 92 per cent of world's rice area and production respectively. Thus, rice production, consumption and trade are concentrated in Asia. One third of Asia's rice production is consumed in China and one fifth in India. Among the rice growing countries in the world, India has the largest area under rice crop (about 45 million ha.) and ranks second in production next to China. India and China together accounts for 56 per cent of the total production and about 50 per cent of world's area under rice during 1997-98. From production point of view, China ranks first in the world and accounts for 34.6 per cent of total production of world during 1997-98. India accounts for 21.5 per cent of total rice production of world during 1997-98. Other important rice producing countries and their respective share in the world production of rice during 1997-98 were : Indonesia 8.8 per cent, Bangla Desh 4.9%, Vietnam 4.6%, Mynammar 3.3%, Thailand 3.7%, Japan 2.2%, Brazil 1.6%, United States of America 1.4% and Russian federation 0.1%.

The productivity of rice in India is higher than Thailand, Pakistan, Bangladesh, Nepal and Brazil but much below than the productivity in Japan, China, Korea, U.S.A. and Indonesia. The rice productivity in India during 1999 - 2000 was 1986 Kg./ha., which is below the world average productivity of 2551 Kg./ha. during the same year.

Global paddy output in 1999-2000 reached 598 million tonnes, registering 2.6 per cent rise on the previous season. Rice is primarily a high-energy or high caloric food. The protein content is less than wheat. The protein content of milled rice is usually 6-17 per cent. The biological value of its protein is high. Rice contains low fat about 2.0 to 2.5 per cent. In milling process much of fat is lost. The calcium content in rice is generally low. B-group vitamins are found in rice grains as in wheat. Valuable protein, vitamins and minerals are lost in the milling process of rice. The embryo and aleurone layer of rice are removed during course of milling. However, much of the loss of nutrients can be avoided through parboiling process.

The by-products of rice milling are used for a variety of purposes. Rice bran is the most valuable by-product of rice milling industry. It is obtained from the outer layers of the brown rice. Generally, rice bran consists of pericarp, aleurone layer , germ and a part of endosperm. Bran removal amounts to 4 to 9 per cent of the weight of paddy milled and is abundant in oil. Raw rice bran contains about 18 to 20 per cent oil whereas parboiled rice bran contains about 22 to 25 per cent oil. The de-oiled bran, which is a rich source of protein (about 17%) and vitamins (Vitamins A and E ), is used as cattle feed and poultry feed. It is a good source of foreign exchange earnings. Rice hulls can be used in manufacture of insulation materials, cement and cardboard . It is also used as litter in poultry keeping. Rice straw is used as cattle feed and it is also used as litter during winter season.

The origin and history of rice probably dates back to the antiquity. As per archaeological evidences and by the many references, India and Burma should be regarded as the centre of origin of cultivated rice. It has probably been the staple food and the first cultivated crop in Asia. Rice has been cultivated in India since ancient times. In fact, rice was known in India before the present era as per the reports of the many investigators based on the study of Sanskrit and various other languages in South-Eastern Asia.

China is another ancient region of rice cultivation. In this connection, the investigators have reported that pottery from excavated has been found to carry imprints as well as rice husk. The period of that culture is estimated at 2000 B.C., but a greater antiquity has also been claimed. The region covering Burma, Thailand and Cambodia is well suited to rice cultivation. This region has also a large population of wild rices.

India lies between 8° and 35° N latitude, with a tropical and sub-tropical climate. Among the various weather elements, rainfall is the most single important factor, which determines the extent, growth and production of rice crop.

Rice is indigenous to the humid area of tropics, sub-tropics and temperate regions. It has a wide physiological adaptability and is grown successfully from below the sea level to 2000 meters above the sea level. There are many varieties of rice which are cultivated with differential response to climatic factors, such as temperature, rainfall and day length. The soil types and different physiographic factors are also quite relevant in the cultivation of rice crop. The indica varieties of rice (Oryza sativa ) are grown mainly in the tropical countries. These varieties are photosensitive and maturity period is affected with the date of planting.

Rainfall is the most important weather element for successful cultivation of rice. The distribution of rainfall in different regions of the country is greatly influenced by the physical features of the terrain, the situation of the mountains and plateau. The regions experiencing very heavy rainfall in the country are given :-

1. Western Ghats (the western slopes and the coastal region.)

2. In the Assam region.

3. The sub-montane Himalayan region, Deccan plateau, Eastern Ghats with coastal plains and the vast Gangetic plains.

Generally south west monsoon from Arabian sea becomes active on the Malabar coast in the end of May or in the first week of June. At the same time, the south-west monsoon from the Bay of Bengal also strikes the hills of north-eastern India in the Assam region and in the Sub-Himalayan West Bengal. Gradually, south-west monsoon extends northwards and reaches western Rajasthan.

Rainfall is the most important weather factor to determine the paddy production because rainfall during the active phase of the initiation of panicle primordia is significantly beneficial. Thus, rainfall always gives beneficial effect even when this factor is taken jointly with other climatic elements, such as the mean temperature and sunshine. Therefore, rainfall is one of the most important climatic elements to determine the growth and yield of rice crop.

Temperature is an other climatic elements which has a favourable and in some cases unfavourable influence on the development, growth and yield of rice. Rice being a tropical and sub-tropical plant, requires a fairly high temperature, ranging from 20° to 40°C. The optimum temperature of 30°C during day time and 20°C during night time seems to be more favourable for the development and growth of rice crop. The low temperature affects the tillering rate. The period of tillering is prolonged due to low temperature but low temperature gives more tillers and more panicles than higher temperature. Low temperature depresses the internodal elongation and thereby induces the partial emergence of panicles. This phenomena further affects the rate of photosynthesis and also induces partial sterility. However, low temperature during the period of ripening, prolongs the ripening period and enables the plant to maintain green leaves. Such condition contributes to the accumulation of carbohydrates in the grains.

Sunlight is very essential for the development and growth of the plants. In fact, sunlight is the source of energy for plant life. The response to solar radiation is a varietal character. The yield of rice is influenced by the solar radiation particularly during the last 35 to 45 days of its ripening period. The effect of solar radiation is more profound where water, temperature and nitrogenous nutrients are not limiting factors.

Bright sunshine with low temperature during ripening period of the crop helps in the development of carbohydrates in the grains. Solar radiation is a limiting factor for upland rice, because upland rice is grown during rainy season. Therefore, low productivity in the case of upland rice is a problem in the tropics.

Rice is grown in India in different types of soils. These soils in which rice is grown are so extraordinarily varied that there is hardly any type of soil on which rice can not be cultivated with some degree of success. The classification of soils has been done depending upon the soil texture, colour of the soil etc. The rice soils of India has been classified into 17 types as mentioned below and these are depicted in the Map No.1.

The state-wise area under rice during 1999-2000 is given in Table 1. The area occupied under rice in West Bengal is the highest followed by Uttar Pradesh, Madhya Pradesh, Bihar, Orissa, Andhra Pradesh, Assam , Punjab , Tamil Nadu, Maharashtra and Karnataka. These states put together accounts for about 92 per cent of the total area under rice in the country. These states except Punjab also constitute the traditionally rice growing areas in the country. Punjab has emerged as rice growing state in the country during early seventies. The other states in the country have, however, limited area under rice crop.

The distribution of various kinds of rice soils in the country is discussed region-wise below :-

The Humid Western Himalayan Region

This region comprises of sub-montane soils, hills soils and tarai soils. The distribution of this region is in the states of Jammu and Kashmir, Himachal Pradesh and Kumaon and Garhwal division of Uttranchal. The rice soils which are found in Jammu and Kashmir have been formed from the alluvium brought by the major rivers Chenab, Ravi, Tawi and their tributaries. Such soil is found mostly in Jammu and Kathua districts.

The sub-montane soils are found in Anantnag , Baramulla and Srinagar districts. These soils are formed from the alluvium deposited in the valley floor by the Jhelum and the Indus rivers. They are silty loam to clay loam and are neutral to alkaline.

The hill soils are found in the districts of Almora, Chamoli, Pithoragarh, Uttar Kashi and Dehradun of Uttranchal. These soils are shallow with fragments of rock occurring according to the elevations and have been categorized as red loam, brown forest soil, meadow soil and podzolic soil.

The tarai soils are found from north-west to the extreme north-east as a narrow strip. These soils are always saturated because of sufficient rainfall and high ground water table. These soils have been formed from transported materials by different rivers originating from the Himalayas. The tarai soils are very productive and responding well to fertilizer application.

The soils occurring in Himachal Pradesh are formed from parent rocks such as sandstones, gray micaceous sandstones and shales of Sub-Himalayan region. These soils are loam to silty loam and medium to high in organic matter. Generally, these soils are poor in available nutrients.

The Humid Bengal-Assam Basin and Humid Eastern Himalayan Region

The altitude of rice growing areas in this region ranges from few metres in Sundarbans in West Bengal to about 1600 metres in Mizoram state and more than 2000 metres in Arunachal Pradesh. Generally, the crop is grown in this region on flat lands to facilitate the supply of water requirement. One of the most limiting factor in this region is the availability of water. The alluvial soils deposited by the rivers mostly occupy the major part of the wetland rice soils in this region. These soils are formed from the silt deposition by the numerous tributaries of the Ganges and the Brahmaputra rivers. The water-table is high and drainage is poor in the wetland rice soils.

Soils in the Assam valley are acidic and high in available phosphorus and potassium and moderate in organic matter and nitrogen. In the north-eastern mountainous upland areas of Assam, the soil is lateritic. In the upland areas drainage is good. The rice crop is better grown in acidic soils in the pH range of 5.5 to 6.5 . In the Gangetic delta, rice is successfully grown in saline soils of Sundarbans.

The Sub-Humid Sutlej-Ganga Alluvial Plains

In this region a single crop of rice is grown during May-June to September-October. The temperature during winter remains low. The major group of soils in this region are calcareous alluvial, riverine alluvial, saline-alkaline, red yellow loam, red sandy and mixed red and black. The alluvial soils of this region are formed from the materials brought and deposited by the great rivers originating from the Himalaya mountain. The alluvial soils are rich in potash and calcium but are deficient in organic matter, nitrogen and phosphorus. The alluvial soil can broadly be classified as -

(a) light-textured alluvial soils of the west and north-west

(b) intermediate-textured alluvial soils in the central region

(c) calcareous alluvial soils in the north-east.

Saline soils are found all along on the left side of the Ganga river in the districts of Meerut, Aligarh, Bulandshahar, Mainpuri, Etah, Kanpur, Fatehpur, Allahabad, Lucknow, Pratapgarh and Sultanpur. The soils are highly alkaline, indurated and have below hard pan which obstruct the downward movement of water. The soils of Allahabad, Varanasi, Ghazipur and Balia are black and fine-textured. The soils are formed from the alluvial deposits transported by the Yamuna river from central parts of the country.

Bihar state is divided by the Ganga river into two halves i.e. north and south. The texture of alluvium of north Bihar region is sandy loam to clay loam and pH is neutral to alkaline. Generally, lime content is high in alkaline soils. The texture of south Bihar in the districts of Patna, Gaya and Shahabad is gray to black and light loam to heavy clay. The lime content in this soil is generally less and pH of the soil is alkaline. The soils of Ranchi, Hazaribagh, Santhal Parganas, Singhbhum and Manbhum are red. These soil are acidic. These soils are rich in potassium and poor in phosphorus.

The Sub-Humid to Humid Eastern and South-Eastern Uplands

In eastern Madhya Pradesh, Orissa and Andhra Pradesh the rice soils are laterite, red-yellow loam, red sandy, mixed red and black, deltaic alluvium, deep and medium-deep black, red loam and coastal alluvial. In Chhattisgarh plains, Mahanadi basin and in the eastern region soils are red and yellow. These soils are mainly sandy but sometimes silty loam to silt-clay loam are also found. Black soils are found in the Narmada valley. The laterite soils of Orissa are found in the eastern Ghat region. The soils of Khurda are mainly laterites. The soils of Balasore are gravelly.

In Andhra Pradesh, particularly in the districts of Nellore, West Godavari, Krishna, Guntur and Ongole, the soils are red and black. The alluvial soils of the deltas are very deep and well drained. These soils are very fertile. The alluvium soils of Godavari is different from the alluvium soils of Krishna. The soils of coastal districts of Srikakulam, Visakhapatnam, East and West Godavari, Guntur, Krishna and Nellore are purely sandy. In some parts of Medak, Nellore and Visakhapatnam districts laterite soils are found. These soils are red but they differ from red soils. Such soils are found in heavy rainfall and high temperature areas. These soils are acidic, pH ranging from 4.0 to 5.0.

Generally, black soils are found in long stretches. Sometimes black soils are also found in isolated pockets along with the red soils. Drainage in black soils is poor and the content of soluble salt is high. These soils contain sufficient lime and pH ranges from 7.0 to 8.5 . These soils are also deficient in phosphorus and low in organic matter and nitrogen.

The Arid Western Plains

The soils of this region are alluvial, red-yellow and medium-deep black. These soils are found in Haryana, Rajasthan, Gujarat and the Union Territory of Dadra and Nagar Haveli.

The districts of Karnal, Ambala, Jind and Sirsa in Haryana state are having mainly alluvial and saline-alkali soils in low-lying areas. The rice is grown mainly in these districts of Haryana and in such soils.

Red-yellow soils are mainly found in Banaswara district and alluvial soils are found in Ganganagar, Alwar, Bharatpur, Jaipur and Udaipur districts of Rajasthan. Generally, these districts are growing rice in Rajasthan.

The soils of Sabarkanta and Ahmedabad districts of Gujarat are riverine alluvium. Rice is mainly grown in these two districts. Medium black soils are found in the district of Rajkot, Junagadh and Jhalwar. Rice is also cultivated in deep-black and coastal alluvium soils of Broach, Varodara and Kheda districts of Gujarat state.

The Semi-Arid Lava Plateau and Central Highlands

This region comprising of Maharashtra, Western and Central Madhya Pradesh and Goa, Daman and Diu Union Territories, having alluvium coastal alluvium, mixed red and black soils. The Ratnagiri district, parts of Kolhapur and south of Kolaba are having laterite soils. This is a high-rainfall zone and rice is cultivated in laterite soils. Non-laterite soils are found in high rainfall zone of Thana district, part of Kolaba and some areas of Nasik district in Maharashtra.

In Western Madhya Pradesh, alluvial soils are found which are formed due to the alluvium deposited by the Chambal river. These soils are neutral to slightly alkaline, nitrogen and phosphorus are low in these soils. However, potassium is medium to high.

The Humid to Semi-Arid Western Ghats and Karnataka Plateau

Karnataka, Tamil Nadu, Kerala and Union Territory of Pondicherry and Lakshadweep Islands are coming in this region. The soils of this region are laterite, red sandy or gravelly, red loamy, deltaic and coastal alluvium.

Karnataka state is having varied type of climate, ranging from humid to sub-humid. Laterite and lateritic soils are found in the coastal districts of north and south Kanara, parts of Belgaum, Shimoga, Chickmagalore, Hassan and Coorg. The black soils are found in the districts of Belgaum, Dharwar, Bidar, Raichur and parts of Bellary. These soils are calcareous, enriched with bases and alkaline. The districts of Mysore and parts of Hassan are having red soils where as some parts of shimoga and Chickmagalore districts are having red loam soils. These soils are well-drained and light-textured. Black and red soils are found in the districts of Belgaum, Bijapur, Dharwar, Raichur, Bellary and Chitradurga. Generally, upland areas are having red soils and lowland areas are having black soils. The districts of Tiruchirapalli and Thanjavur are the main rice growing areas in Tamil Nadu besides Kanyakumari, Changulput, South Arcot, eastern parts of Ramnathapuram and Tirunelveli. The Cauveri delta is called 'rice-bowl'. Black soils are found in all the districts except the Nilgiris district. The major portion of the Cauveri alluvium is in the Thanjavur district. This soil is poor in nitrogen, phosphorus and organic matter but is rich in potassium and calcium.

The major portion of the soils in Kerala is acidic. In the acidic soils of Alleppy, Quilon, Kottayam and Ernakulam districts of Kerala, rice is mainly grown, of which "Kuttanad" the rice-bowl of Kerala covers major rice areas. Alleppey and Kottayam districts are having a large isolated patches of peat soils (Kari soils). These are deep, black and heavy clayey soils, with poor aeration and bad drainage. These soils are having low content of available plant nutrients. They are generally inundated with saline water resulting in highly acidic.

The south of Kerala and the Malabar coast zones are having red and lateritic soils. These soils are also acidic in reaction. The soils which are found in the plains are deeper and possessing sufficient amount of nitrogen and organic matter, but they are deficient in phosphorus, potassium and lime.