Pat wrote:
..................................
I don't have any cow poop, but do have lots of horse poop. I presume that
will do...
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I was going to keep out of this one, since I live in the frozen
north and shouldn't know anything about watermelons. (It IS still
frozen. The lake is still frozen, the ground is still frozen and
mostly covered with snow.)

The ONLY way I am able to grow melons here is with fresh horse
manure. It has to be fresh because I need the heat it produces. I
get out there and dig a trench close to two feet wide and at least a
foot deep. Maybe closer to 18 inches deep. I go get a pickup truck
load of horse manure and fill the trench to the top. I then take the
soil from the trench and pile it on top, so that it is at least 6
inches deep down the center. Last of all I stretch a sheet of black
plastic over the whole thing.
I make a series of small slits in the plastic where I plug in pre
started melon plants. I have sometimes had leftover plants that I
put in another location without all the preparation. The difference
is like day and night (at least!). I get big sprawling plants and
good melons from the prepared beds. The other plants make a vine
about 5 feet long with one green, golf ball sized, melon when the
frost comes.
Now, in Missouri, there is little need to do all of that (unless you
want ripe melons in early June). You might want to try parts of it
though. Maybe you would want to try a test by just digging a hole
and filling it with fresh horse manure, cover with soil and plant
some seeds. See how that spot compares to your other melons.

Steve in the Adirondacks