Due to their structure the space between individual clay particles is small and already restricts the passage of water, reducing infiltration[2] and hence drainage.
Common salts such as sodium ions contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types.
It has been observed that increasing the amount of soil organic matter through the working-in of manure, compost or peat can both improve local drainage and promote the proliferation of earth worms that can, over time, break relatively thin hardpan layers.
This combination can help loosen clay particles bound into a hardpan by the actions of hard salts such as iron, calcium carbonate and sodium, by promoting their mobility through a higher pH while proving a suitable source of exchanging minerals (the gypsum).
This works because gypsum salts, although not "soft", are still water permeable and have a larger, more open structure, the results of which do not promote as hard a matrix as was replaced.