Summary: PD prevalence rates appear higher in areas with acidic soils. A possible route is Aluminium -> Grass -> Cow -> Milk
In the past few months I've started a couple of threads suggesting an association between PD and:
- serum metal levels ;
- the fungus histoplasma capsulatum 
In this post I present evidence that links together these two theories by suggesting that soil type is the common feature for both of them.
While writing this post I came across a thread started by Ron in 2009, "Chemical from Soil Bacteria Shows Potential Neuron Toxicity" . Ron reported work done at the University of Alabama which implicated the bacterium streptomyces, which "is frequently found in dirt".
As with most of my posts the approach will be mainly epidemiological. We start with maps showing spatial variations in the prevalence of Parkinson's:
- US, Willis et al.
- England, my mapping of relative prevalence inferred using NHS prescription data .
The US PD data has a similar distribution to that of soils with pH less than 6. An exception is that the two distributions diverge south of the Great Lakes. See the map by BONAP , and compare with .
The British Geological Survey produce a map of topsoil pH values , compare with . The PD data from England shows an apparent relationship, albeit far from perfect, between high levels of PD and low topsoil pH values (in Dartmoor, the Welsh borders, the Lake District and the Scottish Borders) and low levels of PD and high topsoil pH (in the area from London north to the Humber). On the other hand, the similarity between the two factors is not seen in the zone from Manchester to Liverpool.
So, we have an apparent association between PD and acidic soil, let's go on to look at possible causal mechanisms.
Faltmarsch et al.  report aluminium concentrations of up to 50 times higher in the milk of cows fed on vegetation grown in acid sulphate soils. They go on to discuss the impact this could have on health, particularly Alzheimer's and Parkinson's.
Chen et al.  report "The meta-analysis of all prospective studies confirmed a moderately elevated risk of Parkinson’s disease among individuals with high dairy consumption: the RRs between extreme intake categories were 1.6 (95 percent CI: 1.3–2.0) for men and women combined, 1.8 for men (95 percent CI: 1.4–2.4), and 1.3 for women (95 percent CI: 0.8–2.1)." This does not seem a large impact, but aluminium could be a confounding factor. If the problem wasn't the milk per se, but the aluminium in the milk then the relative risk for people drinking milk from non-acidic areas would be lower, while people drinking milk from acidic areas would have a higher relative risk.
How does histoplasma capsulatum fit into the picture. In the thread  we gave links to maps showing the distribution of the the associated disease histoplasmosis, which closely followed that of PD in the US. We add here that histoplasma is a "Soil-based fungus that thrives in moist, acidic soil w/ a high nitrogen content". 
A difficuly with the hypothesis that histoplasma is a major cause of PD is that it is rare in the UK, yet PD prevalence levels are similar. This suggests that if fungi do play a role in PD, it is across a number of related species. It would be worth investigating other acid soil liking fungi, such as Blastomyces.
 "Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries"
Willis A., Evanoff B., Lian M., Criswell S., Racette B.
Neuroepidemiology. 2010 April; 34(3): 143–151.
Published online 2010 January 15. doi: 10.1159/000275491
 "Envionmental risks of metals mobilised from acid sulphate soils in Finland: a literature review"
R. Faltmarsch, M. Astrom, K. Vuori
Boreal Environment Research 13:444-456, 2008
 "Dairy products and risk of Parkinson’s disease"
Honglei Chen,1 Eilis O’Reilly,2 Marjorie L. McCullough,3 Carmen Rodriguez,3 Michael A. Schwarzschild,4 Eugenia E. Calle,3 Michael J. Thun,3 and Alberto Ascherio2,5
Am J Epidemiol. 2007 May 1; 165(9): 998–1006
 John Hopkins POC-IT Guide: Histoplasmosis