Ann Perry at the USDA summarizes a study done by the USDA Agricultural Research Service the pros and cons of solid and liquid dairy manure as fertilizer. Here is a brief excerpt from that study…

Agricultural Research Service (ARS) scientists have found that solid dairy manure is better than commercial fertilizer in mitigating the amount of phosphorus that can accumulate in water percolating through the soil. But using liquid dairy manure can make it worse.

These findings could help farmers in the semiarid western United States protect local watersheds from agricultural pollutants. Idaho is now the second-largest milk producer in the western United States, and farmers there are using substantial amounts of dairy manure for fertilizing irrigated crop fields. Phosphorus can fuel the excessive growth of algae and other plant matter in freshwater ecosystems.

To read the entire summary, link to USDA ARS.