We've recently had some discussion touching on the amount of horse manure deposited in a day in London prior to automobiles. In researching the origins and growth of the fascism of Ecology I've come across the following details-- to wrap up my end of this question, as it were.

A one thousand pound horse produces approximately fifty pounds of manure per day or about ten tons per year. In addition, from six to ten gallons of urine is produced which when soaked up by bedding can constitute another fifty pounds daily. Therefore, four horses in stalls can produce 160,000 pounds of manure and wet bedding per year. That is a mountain of manure by anyone's standards.
http://www.horsekeeping.com/horse_management/manure_management.htm

Penn State College of Agricultural Sciences, Dairy and Animal Sciences dept. offers the standard measurement of:

"An average 1,000 pound horse produces nine tons of manure per year. That is approximately fifty pounds per day."
http://www.das.psu.edu/frequently-asked-questions/how-much-manure-does-a-horse-produce

For some vastly different figures at a steep discount, and more information:

Stephen Davies, "The Great Horse-Manure Crisis of 1894," September 2004 • Volume: 54 • Issue: 9; rpt. Foundation for Economic Education. April 28, 2010

A horse will on average produce between 15 and 35 pounds of manure per day. Consequently, the streets of nineteenth-century cities were covered by horse manure. This in turn attracted huge numbers of flies, and the dried and ground-up manure was blown everywhere. In New York in 1900, the population of 100,000 horses produced 2.5 million pounds of horse manure per day, which all had to be swept up and disposed of. (See Edwin G. Burrows and Mike Wallace, Gotham: A History of New York City to 1898 [New York: Oxford University Press, 1999])

[....]

The larger and richer that cities became, the more horses they needed to function. The more horses, the more manure. Writing in the Times of London in 1894, one writer estimated that in 50 years every street in London would be buried under nine feet of manure. Moreover, all these horses had to be stabled, which used up ever-larger areas of increasingly valuable land. And as the number of horses grew, ever-more land had to be devoted to producing hay to feed them (rather than producing food for people), and this had to be brought into cities and distributed—by horse-drawn vehicles. It seemed that urban civilization was doomed.

[....]

*See Joel Tarr and Clay McShane, “The Centrality of the Horse to the Nineteenth Century American City,” in Raymond Mohl, ed., The Making of Urban America (New York: SR Publishers, 1997), pp. 105–30. See also Ralph Turvey, “Work Horses in Victorian London” at www.turvey.demon.co.uk.
http://www.thefreemanonline.org/columns/our-economic-past-the-great-horse-manure-crisis-of-1894/

This is interesting in that in 1792 when William Moorecroft opened the first veterinary surgery at 224 Oxford St, London, there were 150,000 horses in London alone, (over a quarrter million by the 1820s.)
See Paul Johnson, The Birth of the Modern. London: Phoenix Giant; rpt. 1996; p. 713.

I'm a huge fan of trains, planes, and automobiles.

For fuller discussion on this specific topic: http://nodhimmitude.blogspot.com/2010/04/modernity-and-horsefthers.html

And for even more on this fascinating topic:

http://nodhimmitude.blogspot.com/2010/04/power-to-people.html