Snook-
Yes Farmers who raise corn and wheat use a lot of nitrogen...But Nitrogen rarely runs off due to the volatile nature of its chemical compound. THere are 3 most common forms of nitrogen that are used in modern farm practices. 1. Anhydrous AMmonia, 2. Urea Ammonium Nitrate (UAN or 28% solution), and 3. Urea. Anhydrous ammonia is injected into the ground in nitrite form which is unavailable for plant nutrient uptake. It goes through a conversion process into a nitrate form which is then plant available. This product is only used in corn production and is the most succeptible to leaching of the 3 sources of nitrogen. It forms chemical bonds with organic matter in the soil which generally hold it in place. As I said in my earlier post heavy spring rains have the best chance to leach nitrogen, but most early preplant applications of ammonia have a stabilizer added to it which helps hold it in place. I have done studies on this product and it retains 75 to 90 plus percent of the applied nitrogen-You will always have some leaching due to soil type differences. For example sandy groud typically has lower organic matter and thus cannot hold as much ammonia in place versus clay soil. Most farmers know this and adjust their nitrogen plans by the type of soil they are working with. Both UAN and Urea are already in nitrate form when they hit the ground. THese two have more of a chane to leach. However there are also stabilizer products that are used with each of them. Also both of these products will actually volatilize into the air if left on top of the ground quicker than they will leach into a water system. The same holds with heavy spring rains, but most of the nitrogen used in corn production is applied in mid to late april all the way into june, depending on planting timing. Wheat fertilizer is applied from late february till mid april. These wheat applications have the best chance to end up in lake erie. THat being said though nitrogen cost per acre to a farmer is approaching $100/acre, so we try to make the applications as timely as possible to prevent loss.
The biggest difference between phosphourous and nitrogen is how phosphourus bonds to soil particles and nitrogen bonds to organic matter. When the maumee river turns brown that is soil sediment which probably has some phosphourus tied to it. As I said earlier thogh farmers have cut way back on phosphourus applications and most apply as they are planting any more. There are also laws in place regarding application of manure...but there is very little livestock in NW ohio and SE michigan anymore compared to 30 40 years ago.

To answer your questions regarding farm drainage, yes farmers put tile in the ground to help draw moisture out of their fields. THese tile are roughly 3 feet below ground, depending on the slope of the field. A 1/2 inch raid more than likely will not make it to the tile as the ground soaks it up especially if the ground is fairly dry. A hard 2 inch rain will actually have more surface runoff as well if it comes down hard and fast. These rains too have little to do with fertilizere runoff as the water doesn't go down in the soil profile. A rain like we are having now is the kind that tends to get into creeks as the ground becomes saturated. Water that moves through tile ends up in creeks and streams that eventually end up somewhere most likely lake erie. Tile help dry up fields quicker so that they can plant earlier. Once planting is over tile also help get the rain off as the crop is growing. Heavy rains can really damage young crops as their root systems become stunted.

I probably made this as clear as mud...but I hope I helped clear the picture a little more. Let me know if you have any more questions.
troy