Potassium doses and frequency of application to alfalfa in tropical soil.

Providing an adequate supply of nutrients is important for alfalfa production and is essential to maintain high quality and profitable yields. Potassium fertilization is essential for alfalfa production (Rassini and Freitas, 1998) and is the most common nutrient input for this crop in the high weathered, low-fertile and acids soils of tropical regions. High-yielding alfalfa removes large amounts of potassium from the field in each cutting (Smith, 1975 and Lanyon and Smith, 1985). Lloveras et al. (2001) found extractions from 1500 to 1700 kg.ha·1 (with productivity of21.5 t.ha"l DM) in soi! ofhigh fertility. Potassium is taken up by plants in ionic form, and diffusion to plant roots accounts for the majority of plant uptake, while mass flow contributes to only a small fraction of total plant K. The salts of K in general show high solubility and can reach high concentrations in soil solution, which may lead to depletion by leaching and excessive absorption by plants (Havlin et al., 1999)