That farmers are a major contributor to the bay's woes isn't in any serious dispute, at least not among scientists. Farms represent about one-quarter of the land in the watershed but they are the single greatest source of nutrient and sediment pollution, according to the EPA, accounting for 42 percent of the nitrogen, 58 percent of the phosphorus and 58 percent of the sediment that winds up in the Chesapeake Bay. Poor irrigation practices, destructive tilling of soil, and excess fertilizer and pesticide use are all part of the problem. Even so, many farmers have made profound changes in how they run their operations, often voluntarily, to reduce water pollution. But that's all for naught if other polluters in the watershed — whether they are cities that lack proper stormwater controls or underperforming regional sewage treatment plants or farmers spreading manure on fields already saturated with phosphorus — don't do the same.