In 24 dairies, located on trumao Osorno soils, the seasonal variation of the content of P in the soil, forrage, and milk and of Ca in the forrage and milk, was determined, taking samples in January (summer), April (autumn), July (winter), and October (spring). The level of P in the soil was, in general, aboye medium (8-15 ppm); but 32% of the samples were below this level in winter, 33% in spring, and 14% in autumn. In the forrage, P was defficient (below 0.33%, required by 20 lt/day/cows) in more than 90% of the samples, except in autumn, when this percentage was only 50. In the milk, P was also below normality (0.09-0.10%), in general: 100% of thesamples in autumn to 54% in spring. The level of Ca was less critical; 42% of the forrage sample were low in autumn, but only 4% in spring. Nevertheless, alI the milk sampleswere aboye normal (0.11-0.13%). h was concluded that milking cows should be supplemented with P through ah the year, specially in summer, and also with Ca, specially in autumn and winter.