I looked around for soil tests by Univ. of Nebraska, but I only found
a page for calculating fertilizer recommendations from tests that
you had done by private labs, etc. (Probably big agriculture synthetic
fertilizer recommendations at that.)
Others have recommended Logan Labs in Ohio for this, but their
web site seems to have issues at the moment.
(
http://www.loganlabs.com/ )
You can overdo anything, manure, leaves, straw, other kinds of organic
matter, but I would not expect a foot of alpaca manure to be overdoing
it in a Nebraska native soil. I would not hesitate to use that in my garden.
(Although these days I would mix a shovelful or two of it into container
mix and plant some fast-growing test plant like buckwheat in it first. You
can do that test indoors in winter. Even if the alpaca owner does not use
any herbicides in their pastures, it might be in hay bales that they buy to
feed the alpacas in winter.)