If the world were to become vegetarian or vegan is a nice pipe dream. The reason is because there is no way for the whole world to be fed on plants alone. One of the most devastating problems that we are facing but hear little about is Salinization.
Stop now if you are set in your ways and refuse to let science or data sway you.
Data is taken from the wiki for easy access. http://en.wikipedia.org/wiki/Soil_salinity
And the focus being farming I pasted below the irrigation data.
Salinity due to irrigation
Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water. This water in excess of plant needs is called the leaching fraction. Salination from irrigation water is also greatly increased by poor drainage and use of saline water for irrigating agricultural crops.
Salinity in urban areas often results from the combination of irrigation and groundwater processes. Irrigation is also now common in cities (gardens and recreation areas).
Consequences of salinity
The consequences of salinity are
- detrimental effects on plant growth and yield
- damage to infrastructure (roads, bricks, corrosion of pipes and cables)
- reduction of water quality for users, sedimentation problems
- soil erosion ultimately, when crops are too strongly affected by the amounts of salts.
Salinity is an important land degradation problem. Soil salinity can be reduced by leaching soluble salts out of soil with excess irrigation water. Soil salinity control involves watertable control and flushing in combination with tile drainage or another form of subsurface drainage. A comprehensive treatment of soil salinity is available from the United Nations Food and Agriculture Organization.
High levels of soil salinity can be tolerated if salt-tolerant plants are grown. Sensitive crops lose their vigor already in slightly saline soils, most crops are negatively affected by (moderately) saline soils, and only salinity resistant crops thrive in severely saline soils. The University of Wyoming  and the Government of Alberta  report data on the salt tolerance of plants.
From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived.
|Region||Area (106 ha)|
|Near and Middle East||53.1|
|Asia and Far East||19.5|