Introductory Agrometeorology/Agrometeorological Normals for Crops

Objectives of the chapter

• To understand agrometeorological normal in some crops

Agroclimatic Normal

Climatic normal means the degree of temperature, amount of rainfall, humidity, etc., which distinguish optimal conditions from abnormal, both because of excess and insufficiency.

Rice

Temperature and solar radiation influence rice yield by directly affecting the physiological processes involved in grain production and indirectly through the incidence of pest and diseases.

Temperature - The difference in yield is mainly due to temperature and solar radiation received during its growing season. It requires high temperature, ample water supply and high atmospheric humidity during growth period. Rice tolerates up to 40°C provided water is not limiting. A mean temperature of 22°C is required for entire growing period. If high temperature drops lower than 15°C during the growth phase, the rice yield is greatly reduced by formation of sterile spikelets. The period during which low temperature is most critical, is about 10–14 days before heading. Solar radiation - Low sunshine hours during the vegetative stage have slight ill effect on grain production, whereas the same situation during reproductive stage reduce the number and development of spikelets and thereby the yield. For getting grain yield of 5 t/ha, a solar radiation of 300 cal.cm₂/day is required. A combination of low daily mean temperature and high solar radiation during reproductive phase is good for getting higher yield. Rainfall - Rice requires high moisture and hence classified as hydrophytes. Rice requires a submerged condition from sprouting to milky stage. The water requirement is 125 cm. An average monthly rainfall of 200 mm is required to grow low land rice and 100 mm to grow upland rice successfully.

Wheat

Temperature - Optimum temperature for sowing is 15–20°C. At maturity, it requires 25°C. At harvest time, wheat requires high temperature of 30–35°C and bright sunny period of 9–10 hours. Moisture - One ha of wheat consumes about 2500–3000 tones of water. Water deficiency at the heading stage results in shriveled grains and low yield.

Maize

This crop is best suited for intermediate climates of the earth to which the bulk of its acreage is confined. Temperature - Maize requires a mean temperature of 24°C and a night temperature above 15°C. No maize cultivation is possible in areas where the mean summer temperature is below 19°C or where the average night temperature during the summer falls below 21°C. However, high night temperature also results in low yield. Moisture - Maize is adapted to humid climates and has high water requirements. It needs 75 cm of rainfall during its growth period. The average consumptive use of water by maize is estimated to range between 41 and 64 cm. From germination up to the earing stage, maize requires less water. However, at flowering, it requires more water and the requirement reduces towards maturity.

Cotton

It requires 4-5 months of uniformly high temperature (28–45oC) during its crop growth period. Mean air temperature of 21–29oC is required at vegetative period. The optimum air temperature for reproductive phase is 27–32oC; mean sunshine hours are 8-9 hrs/day; and mean RH is 70%. But at boll development and boll opening period (September to November), RH less than 70% and 8 hrs. of sunshine are ideal for good cotton production. The growth rate of cotton crop is increased at 25–30oC. Temperature below 15oC retards growth and reduces the square (bud) formation. Moisture - The minimum rainfall required for cotton is 500–650 mm. Heavy rainfall during early stage is undesirable. Dry autumn months are desirable for good quality produce. Excess rainfall at later stage may cause shedding of leaves, squares and bolls. It also stimulates top growth, delays maturity and changes colour of lint. High humidity favours many pests and diseases.

Sugarcane

Mean temperature for optimum germination is 30oC. Mean temperature for optimum growth is 35oC. At temperature less than 20oC, growth is reduced. Ideal climate is 4–5 months of hot period with temperature of 30–35oC followed by 6–8 weeks of cooler period for better maturity. Moisture .sugarcane requires about 60-70 cm rainfall

Potato

Potatoes produce a fibrous root system. As a result, potatoes are often unable to exploit nutrients and soil moisture at depth within a soil profile. While root growth occurs when soil temperatures are between 50 to 95˚F (10 to 35˚C), best, most active root development is at soil temperatures of between 59 and 68˚F (15 and 20˚C). Leaf (haulm) growth occurs at temperatures of between 44.6 to 86˚F (7 to 30˚C) , but optimal growth is at around 68 to 77˚F (20 to 25˚C). Optimum temperatures for stolon growth are similar. The potato tuber is an enlarged portion of the stolon. The initiation of this tuber is triggered by short day lengths (photoperiods), and involves growth hormones. The colder the soil temperature, the more rapid the initiation of tubers and the greater the number of tubers formed. The optimum soil temperature for tuber initiation is 59 to 68˚F (15 to 20˚C). Under these conditions, the potato plant will have short stolons and shoots. Longer day lengths delay tuber initiation and favor the growth of the stolon and shoot. High temperatures also reduce tuber formation. Late varieties seem to be more sensitive to long day lengths or high temperature conditions. Low nitrogen and high sucrose levels in the plant favour the formation of more tubers.Once formed, tubers grow rapidly, reaching a maximum rate of up to 1,249 lb/ac/day in temperate climates.

Soybean

Soybean is a tropical crop but its cultivation now extended to subtropical and temperate regions or climates. Climatic requirements of soybean are almost similar to that of maize. Minimum and maximum temperature for seed germination are 5 and 40°C respectively. Minimum temperature for growth is 10°C and maximum rate of development between planting and flowering occurs at 30°C. High night Temperature increases vegetative growth and no of branches. Low temperature delay flowering. Flowering is poor below 20°C and increases upto 32°C beyond which it increases flower abscission and pod setting. The yield reduces to half above 33°C. Soil temperature of about 30°C increases seed yield by more than 50% while 10°C decrease the yield by similar magnitude. Soybean is basically a short day plant. Day length influences the rate of development. In short day types , increased day length may result in the delay of flowering and taller plants with more nodes. Soyabean is often cultivated as kharif rainfed crop and as rabi crop on receding soil moisture after kharif crop. It can come up in areas with rainfall varying from 600 to 1000 mm. Soil moisture stress at flowering and pod development reduces the seed yield by about 50%. Excess soil moisture inhibits nitrogen fixation. Low atmospheric humidity significantly reduces the seed yield.