Ancient Origins. Developed independently by geographically distant populations, evidence suggests that agriculture first appeared in Southwest Asia, in the Fertile Crescent area of Mesopotamia. Around 9,500 B.C., farmers first began to select and cultivate food plants with specific characteristics. Though there is evidence of earlier use of wild cereals, it wasn't until after 9,500 B.C. that the eight so-called founder crops of agriculture appear: first emmer  and einkorn wheat, then hulled barley, peas, lentils, bitter vetch, chick peas and flax.

By 7000 B.C., sowing and harvesting reached Egypt. By 6000 B.C., farming was entrenched on the banks of the Nile River. About this time, agriculture was developed independently in the Far East, with rice, rather than wheat, the primary crop. By 5000 B.C., Sumerians had developed core agricultural techniques including large scale intensive cultivation of land, mono-cropping, organized irrigation, and use of a specialized labour force.

Evidence suggests that Maize was first domesticated in the Americas around 3000-2700 B.C. The potato, the tomato, the pepper, squash, several varieties of bean, and several other plants were also developed in the New World, as was extensive terracing of steep hillsides in much of Andean South America.

Roman agriculture built on techniques pioneered by the Sumerians, with a specific emphasis on the cultivation of crops for trade and export.

Agriculture in the Middle Ages. During the Middle Ages, Muslim farmers in North Africa and the Near East developed and disseminated agricultural technologies including irrigation systems based on hydraulic and hydrostatic principles, the use of machines such as norias, and the use of water raising machines, dams, and reservoirs. Muslims also wrote location-specific Farming manuals, and were instrumental in the wider adoption of crops including sugar cane, rice, citrus fruit, apricots, cotton, artichokes, aubergines, and saffron. Muslims also brought lemons, oranges, cotton, almonds, figs and sub-tropical crops such as bananas to Spain.

Renaissance to Present Day. The invention of a three field system of crop rotation during the Middle Ages, and the importation of the Chinese-invented moldboard plow, vastly improved agricultural efficiency.

After 1492, a global exchange of previously local crops and livestock breeds occurred. Key crops involved in this exchange included the tomato, maize, potato, cocoa, tobacco, and coffee.

By the early 1800s, agricultural practices, particularly careful selection of hardy strains and cultivars, had so improved that yield per land unit was many times that seen in the Middle Ages. With the rapid rise of mechanization in the late 19th and 20th centuries, particularly in the form of the tractor, farming tasks could be done with a speed and on a scale previously impossible. These advances have led to efficiencies enabling certain modern farms in the United States, Argentina, Israel, Germany and a few other nations to output volumes of high quality produce per land unit at what may be the practical limit.

Crop improvement. Domestication of plants is done in order to increase yield, improve disease resistance and drought tolerance, ease harvest and to improve the taste and nutritional value and many other characteristics. Centuries of careful selection and breeding have had enormous effects on the characteristics of crop plants. Plant breeders use greenhouses and other techniques to get as many as three generations of plants per year so that they can make improvements all the more quickly.

Plant selection and breeding in the 1920s and '30s improved pasture (grasses and clover) in New Zealand. Extensive radiation mutagenesis efforts (i.e. primitive genetic engineering) during the 1950s produced the modern commercial varieties of grains such as wheat, corn and barley.

For example, average yields of corn (maize) in the USA have increased from around 2.5 tons per hectare (40 bushels per acre) in 1900 to about 9.4 t/ha (150 bushels per acre) in 2001. Similarly, worldwide average wheat yields have increased from less than 1 t/ha in 1900 to more than 2.5 t/ha in 1990. South American average wheat yields are around 2 t/ha, African under 1 t/ha, Egypt and Arabia up to 3.5 to 4 t/ha with irrigation. In contrast, the average wheat yield in countries such as France is over 8 t/ha. Variation in yields are due mainly to variation in climate, genetics, and the use or non-use of intensive farming techniques (use of fertilizers, chemical pest control, growth control to avoid lodging). [Conversion note: 1 bushel of wheat = 60 pounds (lb) ≈ 27.215 kg. 1 bushel of corn = 56 pounds ≈ 25.401 kg] In industrialized agriculture, crop "improvement" has often reduced nutritional and other qualities of food plants to serve the interests of producers. After mechanical tomato-harvesters were developed in the early 1960s, agricultural scientists bred tomatoes that were harder and less nutritious. In fact, a major longitudinal study of nutrient levels in numerous vegetables showed significant declines in the last 50 years; garden vegetables in the U.S. today contain on average 38 percent less vitamin B2 and 15 percent less vitamin C.

Very recently, genetic engineering has begun to be employed in some parts of the world to speed up the selection and breeding process. The most widely used modification is a herbicide resistance gene that allows plants to tolerate exposure to glyphosate, which is used to control weeds in the crop. A less frequently used but more controversial modification causes the plant to produce a toxin to reduce damage from insects.There are specialty producers who raise less common types of livestock or plants.Aquaculture, the farming of fish, shrimp, and algae, is closely associated with agriculture.Apiculture, the culture of bees, traditionally for honey-increasingly for crop pollination.

Environmental problems. Agriculture may often cause environmental problems because it changes natural environments and produces harmful by-products. Some of the negative effects are:

-Surplus of nitrogen and phosphorus in rivers and lakes 

-Detrimental effects of herbicides, fungicides, insecticides, and other biocides

-Conversion of natural ecosystems of all types into arable land 

-Consolidation of diverse biomass into a few species

-Soil erosion

-Depletion of minerals in the soil

-Particulate matter, including ammonia and ammonium off-gasing from animal waste contributing to air pollution

-Weeds - feral plants and animals

-Odor from agricultural waste

-Soil salination

Agriculture is cited as a significant adverse impact to biodiversity in many nations' Biodiversity Action Plans, due to reduction of forests and other habitats when new lands are converted to farming. Some critics also include agriculture as a cause of current global climate change.

Policy. Agricultural policy focuses on the goals and methods of agricultural production. At the policy level, common goals of agriculture include:

-Food safety: Ensuring that the food supply is free of contamination.

-Food security: Ensuring that the food supply meets the population's needs.

-Food quality: Ensuring that the food supply is of a consistent and known quality.

-Conservation

-Environmental impact

-Economic stability

Agronomy is a branch of agricultural science that deals with the study of crops and the soils in which they grow. Agronomists work to develop methods that will improve the use of soil and increase the production of food and fiber crops. They conduct research in crop rotation, irrigation and drainage, plant breeding, soil classification, soil fertility, weed control, and other areas.

Selective Breeding

Agronomy involves selective breeding of plants to produce the best crops under various conditions. Plant breeding has increased crop yields and has improved the nutritional value of several crops, including corn and wheat. It also has led to the development of new types of plants. For example, a hybrid grain called triticale was produced by crossbreeding rye and wheat. Triticale contains more usable protein than does either rye or wheat.

Agronomy and Soil

Main article: Agricultural soil science

Agronomists study ways to make soils more productive. They classify soils and test them to determine whether they contain substances vital to plant growth. Such nutritional substances include compounds of nitrogen, phosphorus, and potassium. If certain soil is deficient in these substances, fertilizers may provide them. Agronomists investigate the movement of nutrients through the soil, and the amount of nutrients absorbed by a plant's roots. Agronomists also examine the development of the roots and their relation to the soil.

Soil Preservation

In addition, agronomists develop methods to preserve the soil and to decrease the effects of erosion by wind and water. For example, a technique called contour plowing may be used to prevent soil erosion and conserve rainfall. Researchers in agronomy also seek ways to use the soil more effectively in solving other problems. Such problems include the disposal of human and animal wastes; water pollution; and the build-up in the soil of chemicals called pesticides, which are used to kill insects and other pests.

Employment of Agronomists

Most agronomists are consultants, researchers, or teachers. Many work for agricultural experiment stations, federal or state government agencies, industrial firms, or universities. Agronomists also serve in such international organizations as the Agency for International Development and the Food and Agriculture Organization of the United Nations.