Good find! And bravo to you for finding a farm field that wasn't already covered in crops a foot tall.
Usually a toothbrush and water for wheats (unless it is a 1909-S, 1909-S VDB, 1914-D, or 1931-S).
For IH's, 2c's, Large Cents, Colonial Coppers--first a wet cotton swab rolled over the coin gently, then perhaps use the peroxide method IF the patina looks solid. NEVER use olive oil on a coin, button, or relic.
For nickels, usually I toothbrush with water, then bring out the detail with the oil from my fingers rubbed LIGHTLY on only the highest points of the coin. I use this method for CN IH's and FE cents too--since the nickel in them will generally mean that they're reddish colored fresh out of the ground. The oil on the fingers on the highest points creates contrast so you can see the design on the coin. If the nickel's patina isn't reddish, I sometimes just use water without any brushing. Or even dry brush them. (You will be able to tell, with some experience, which cleaning method is best)
For silver--when you dig it--do not field clean it. If you rub it at all, or pick at the dirt, you will scratch the coin.
While I hunt, I place any dug silver coins (fragile relics too) in a medicine bottle (free at any Walgreens, etc.) that is filled with cotton balls. Then when I get home, I soak the silver coin for a few minutes in water, then use warm water (with the drain plugged!) to Blast the dirt from the coin. Then while the water is running, use a camel's hair Very Soft make up brush to tease the remaining dirt off the coin (while the water is running the whole time over the coin--the water will carry the loose dirt away so that you won't scratch the coin). When it is done, pat dry or let air dry.
Don in SJ has had excellent results cleaning individual silver coins by heating peroxide in the microwave and dropping the silver in. This is probably the Best way to remove the dirt without scratching the coin. (I like it better than my method listed above.)
For any clad coins (modern stuff), there are recipes for coin tumblers for them here in the Cleaning Section of the forum if you want to clean 'em and spend 'em.
Sounds like you found a SILVER "war nickel" there. Well done!
Best Wishes and HH,
Buckleboy