Sentence:  From the equilibrium sorption data which are available , it seems logical to expect that polyphosphate <WSD>ions</WSD> would be strongly sorbed on the surface of the dirt ( especially clay soils ) so as to give it a greatly increased negative charge . 
Meaning of the word marked with <WSD> tags:  a particle that is electrically charged (positive or negative); an atom or molecule or group that has lost or gained one or more electrons