Mode of action of calcified seaweed on grassland

Calcified seaweed, the particulate remains of a calcareous red algae, is us ed as a soil conditioner; but little is known of its effects. Two glasshous e pot experiments examined the effects of calcified seaweed application on a Hapludalf (Salwick series sandy silt loam) and Lolium perenne. Experiment 1 examined the effects of different particle sizes of calcified seaweed on soil pH. Particle size analysis showed that 81.59% of particles were 1-4 m m in diameter. Significant (P <0.05) increases in soil pH were only found a fter the application of particles <1 mm in diameter. Experiment 2 aimed to elucidate the modes of action of calcified seaweed on soil and Lolium peren ne. Apparent increases were found in shoot and root growth after calcified seaweed application. Results demonstrated that localized changes in soil pH were established, which reflected application rate and particle size distr ibution. Subsequent changes in shoot elemental composition were a function of these localized pH changes. Due to the high CaCO3 content of calcified s eaweed, changes in nutrient availability were primarily due to the addition of CaCO3 and subsequent soil pH changes. Overall, the effect of calcified seaweed on soils and plants was dependent on application rate, particle siz e distribution and rate of nutrient release. A continuum in mode of action appeared to exist with application rate. At the low application rate (0.63 t ha(-1)) calcified seaweed appeared to act as a calcium (Ca)-based soil co nditioner. At higher application rates, the effects were similar to those a ssociated with calcitic lime.