Combustion of fossil fuels by farm machinery is the second largest source of greenhouse gases from the agricultural sector. Intensive agriculture typically involves the following field operations:
1. Clearing residues from the previous crop
2. Killing weeds
3. Breaking up soil into fine particles to prepare a uniform seed bed and encourage microbial release of soil nutrients
4. Depositing seeds at the proper soil depth and distance between adjacent plants
5. Placing a band of fertilizer where it nurtures germinating seeds without exposing them to excessive concentrations
7. Fertilizing after crop establishment
8. Removing more weeds
9. Harvesting the crop
In developed countries, farm machinery performs most of these operations. The operations require numerous passes of a tractor and implements over a field, and each pass consumes between 1 liter and 34 liters of diesel fuel per hectare of crop. This translates to a fuel efficiency of 0.15 to 5.0 km L-1 (0.4 to 11.8 mpg).
Minimum tillage refers to agricultural management practices that leave large pieces of plant residues in a field and reduce turnover of top soil layers. With minimum tillage, farmers eliminate one or two field operations and combine other operations into a single pass of a tractor. This diminishes soil compaction and soil erosion and conserves fuel. For example, converting wheat production from standard multiple-pass practices to minimum tillage (from a moldboard plow to stubble mulch system) saves 8.2 liters of diesel per hectare, or 20% of total fuel use.  Minimum tillage, despite expectations to the contrary, has no significant effect on soil carbon sequestration. 
Several recent technical developments facilitate minimum tillage. One is the availability of herbicide-resistant crops that permit herbicide applications to kill weeds even after the crop has emerged. Another is precision agriculture, whereby global positioning sensors (GPS) steer tractors and align implements to within 2 cm of the same spot, permitting the combination of several operations into a single pass or into multiple passes onto a specific location. For example, a farmer, in one pass, may till a narrow strip of soil to establish a uniform seedbed for the crop, plant seed in this strip, and inject fertilizer; on a subsequent pass, the farmer may clear out weeds precisely around this strip or apply additional fertilizer within the strip.
 Downs, H. W. and R. W. Hansen (2007) Estimating Farm Fuel Requirements. Colorado State U. Extension, http://www.ext.colostate.edu/PUBS/FARMMGT/05006.html, accessed Sept. 25, 2007.
 Veenstra, J. J., W. R. Horwath, and J. P. Mitchell (2007) Tillage and cover cropping effects on aggregate-protected carbon in cotton and tomato. Soil Science Society of America Journal 71:362-371.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents