WordNet records no synonym and no antonym for soil conditioner. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a chemical substance used to improve the structure of the soil and increase its porosity
“gypsum can be used as a soil conditioner”
Synonyms, antonyms and the broader and narrower relations for soil conditioner come from Princeton University’s WordNet. Full sources and licences.