This lecture deals with the components of climate and how they effect crop production systems.

II. Learning Objectives
 

• Understand the role of temperature in crop production.
• Understand the role of precipitation in crop production.
• Understand the role of light in crop production.
• Understand the relationship of climate to soil structure and characteristics.
• Be able to explain the five major climatic zones, their locations and characteristics.

Temperature usually defies major climatic zones.  Temperature is modified by altitude and latitude.  Minimum and maximum temperatures provide information on temperature/plant compatibility.  Precipitation is seldom optimum for production of farm crops.  Distribution can be as important as total quantity.  There are two aspects of light that are important.  First, the effect of clouds in decreasing light intensity and second, the effect of latitude on angle of incidence and difference in day length.  Finally, there is a relationship between soil characteristics and climate.

There are five major climatic zones: poloar/subpolar, cold-temperate boreal, cool-temperate, warm-temperate (subtropical), and tropical.  The last three contain the majority of tillable land on the planet, and when divided on basis of adequacy of precipitation for plant use during available growing season, there are nine climatic zones studied in this class.  There are: cool-temperate (humid) (semi-arid) and (arid), warm-temperate - (Humid), (semi-arid) and (arid), and tropical (humid), (semi-arid) and (arid).

IV. Climate Components

1. Temperature

• Major climatic zones usually defined by temperature differences
• Tropic zone - near equator 25 degrees C for all months
• Altitude produces temperature zonation
• Mean annual temperature decreases 6oC/1000 m of elevation (3.5oF/1000 ft.)
• From rubber, Cacao and bananas to rice, citrus and sugar cane
• At elevations of 1200-1800 m - temp. Ranges resemble those at 35-45 degrees latitude (temperate zone)
• Latitude and altitude effect length of growing season
• Dates of spring planting in U. S. retarded 4 days/l degree of latitude, 5 degrees of longitude, or 100 m elevation
• Regions having short growing seasons less than 100 days restricted to grazing, forages, and some cereal grains
• Minimum and maximum temperatures provide information on temperature/plant compatibility

• Natural precipitation is seldom optimum for the production of farm crops
• Distribution week to week may be as important as the total quantity
• Frequency usually expressed as no. Of precipitation days/month or year.
• Thornthwaite (1948) concept of PET
• P (precipitation) to evapotranspiration to calculate monthly moisture index IM

Formula IM = 100 ( P / PET )   -1
 
• Values vary from 100 for most humid to 0-20 for sub-humid to -100 for arid zone
• Troll - based his system on numbers of humid and arid months/year
• Better suited to tropical conditions

• Two aspects of light important
• Effects of clouds in decreasing light intensity
• Effect of latitude on angle of incidence and difference in day length
• Rice yields in monsoon crop 25% less
• Sugar cane in Hawaii - heavy rainfall causes 40% as much intensity as other parts of islands - 146 T/ha U. S. 300 T/ha
• Light intensity less in humid than in arid climates
• Length of day - June 21 (12 hr at equator) (15 hours at 40o) (19 hours at 60o) 24 hours at Pole)
• Plants respond to different length of day and night
• Dividing line between short and long day plants is 12-14 hr.
• Long day; potato, timothy, sweet clover
• Short day; corn, sorghum, hemp
• Day neutral; cotton
• Through selective breeding, plants have adapted to different day lengths
• Present emphasis on wheat and rice - increase variety adaptation
• Light is scattered or absorbed by moisture, dust, smoke, and gases in air
• Light intensity usually increases with elevation

• Soil characteristics are related to broad differences in climate
• In area of similar climate, soil becomes an important factor in determining farm practices
• A detailed soil map necessary base for farming system

1. Five major climatic zones:

• Polar and subpolar
• Cold temperate boreal
• Cool temperate
• Warm temperate subtropical
• Tropical

• 1. Results in nine zones:

• Polar and subpolar zone.
• Vicinity of the two poles and at high elevation
• 560 million ha (4.2% of earth?s surface)
• None is classed as potentially arable

• Cold-temperate boreal zone
• 50^o N in North America, 55^o N in Europe and Asia
• Zone is not important in southern hemisphere
• Native vegetative primarily coniferous forests
• 1968 million ha (15% of earth?s surface)
• Except for ocean effect area - season too short for production of cultivated crops
•  1. 52 million ha (1.7% of earth?s surface classified as potentially arable
• 2. Crop production limited to some forage crops and cool season food crops

• The cool-temperate zones
• Divided in two zones IIIa and IIIb
• 22% of total land area (29% of potential arable land)
• Band between 35-55 degrees N
• Includes important agricultural regions of Europe, Asia, North American, USSR, and smaller areas of S. America and Australia
• Divided into humid areas and semi-arid and arid areas

• Humid cool-temperate zones
• Growing season 4-8 months by oceans)
• Precipitation makes crop production possible throughout growing season
• 691 million hectares - major portion now cropped
• Multiple cropping feasible by growing cool season crop and warm season crop in sequence

• Semi-arid cool temperate zones
• Soils developed under steppe and desert shrub vegetation
• 214 million hectares, USSR, North Western North America
• Salt accumulation common farming hazard
• N inadequate, available P often low
• Seasonally dry during part to all of growing season
• Moisture restricts choice of crops, limits land use to grazing, irrigation agriculture
• Wheat extensively planted

• Warm-temperate zones
• 21% of world?s land area.  18% potentially arable
• Between 20 degrees and 35 degrees N and S
• Temperature permit vegetative growth throughout entire year
• Frost-free period usually exceeds 200 day; winters are mild
• Region divided into two zones, humid and semi-arid
 
• Humid warm-temperate zone
• 255 million hectares, most now cultivated
• Precipitation is adequate for crop production for six or more months of the year
• Cool season or winter crop/warm season summer crop
• Intensive multiple cropping is possible in many locations
 
• Semi-arid and arid-warm temperate zones
• 300 million hectares, 10% of world?s total
• Great desert of Africa, Asia, Australia, North America, Middle East, and South America
• Wheat and grain sorghum
• Irrigation required for full crop production

• The tropical zones
• Nearly ? of world's potentially arable land
• 5 billion hectares, 38% of earth?s land area
• Three tropical zones contain ? of potentially arable land not developed for cropping
• Unfortunately, human population concentrated in Asia, undeveloped land largely in Africa and Latin America
• Zone between 20 degrees N and 20 degrees S
• Temperatures show little or no seasonal variation
• Agriculture dictated by moisture and soil characteristics

• The humid tropical zone
• Narrow belt near equator, excess of 1500 mm of rainfall/year in which 7+ months classified as humid
• Distinctive dry season up to five months
• 1 billion hectares, less than 1/2 cultivated
• Good water supply, 12 month growing season, great potential for increased food production

• Semi-arid tropical zone
• Wide belt across parts of South Asia, India, Africa, Northern Argentina, Northeast Brazil, and Central Mexico
• 530 million hectares of potentially arable land, 17% of world's total
• Precipitation ranges from 500-1500 mm
• Humid months range from 2-7
• 95% of precipitation during rainy season
• Managing soils and crops to make use of rainfall major focus of farming systems
• Low state of fertility requires soil building practices, careful fertilizer use

• The arid tropical zone
• Small in total area, 30 million hectares, 1% of earth's surface
• Production requires irrigation
• Limited precipitation, can support l crop under most favorable conditions
• In India alone, this zone covers 15% with 20 million people and 23 million head of cattle

    Temperature and moisture are critical for plant growth and determining crop plant selection and cropping system.  Plants differ in mean temperature for growth and development.  Corn does best with mean temperatures of 22-26oC where as sorghum stands temperatures in excess of 300 C.  The occurrence of drought is critical to crop growth.  Crops vary with their ability to withstand drought.  Irrigation increases the available alternatives for crop selection and cropping patterns.

    Temperatures and rainfall shape soil systems.  Cropping systems must be planned to conform to special soil properties.  Age of soil determines soil fertility and water-holding capacity.  Amounts of precipitation effects soil pH in the incidence of acidity and aluminum.  Soils with good quantities of available water in the root zone help plants avoid or endure periods of drought.

VI. Self Assessment
 

• How does temperature define climatic zones?  What is the impact of elevation and latitude on temperature?  Explain.
• What two aspects of light effects light intensity the most?  Explain how each effects crop plant growth.
• What are the five major climatic zones?  Which three have most of the world?s agriculture?  What is the impact of precipitation in further dividing these three into nine zones?
• Discuss the impact of moisture on plant growth.  What is the relationship between precipitation and soil properties?  Give examples to illustrate the relationship between climate, soil characteristics, and cropping systems.