A fertilizer capable of self-sustaining decomposition is
defined as one in which decomposition initiated in a localized area
will spread throughout the mass. The tendency of a fertilizer offered
for transport to undergo this type of decomposition can be determined
by means of the trough test. In this test localized decomposition
is initiated in a bed of the fertilizer to be contained in a horizontally
mounted trough. The amount of propagation, after removal of the initiating
heat source, of decomposition through the mass is measured.