  Agriculture is one of the oldest forms of technology on Earth. The
cultivation of plants requires a terrestrial planet with active hydrological
and carbon cycles and depends on the availability of nitrogen in soil. The
technological innovation of agriculture is the active management of this
nitrogen cycle by applying fertilizer to soil, at first through the production
of manure excesses but later by the Haber-Bosch industrial process. The use of
such fertilizers has increased the atmospheric abundance of nitrogen-containing
species such as NH$_3$ and N$_2$O as agricultural productivity intensifies in
many parts of the world. Both NH$_3$ and N$_2$O are effective greenhouse gases,
and the combined presence of these gases in the atmosphere of a habitable
planet could serve as a remotely detectable spectral signature of technology.
Here we use a synthetic spectral generator to assess the detectability of
NH$_3$ and N$_2$O that would arise from present-day and future global-scale
agriculture. We show that present-day Earth abundances of NH$_3$ and N$_2$O
would be difficult to detect but hypothetical scenarios involving a planet with
30-100 billion people could show a change in transmittance of about 50-70%
compared to pre-agricultural Earth. These calculations suggest the possibility
of considering the simultaneous detection of NH$_3$ and N$_2$O in an atmosphere
that also contains H$_2$O, O$_2$, and CO$_2$ as a technosignature for
extraterrestrial agriculture. The technology of agriculture is one that could
be sustainable across geologic timescales, so the spectral signature of such an
"ExoFarm" is worth considering in the search for technosignatures.
