Phosphorus and potassium are essential for good alfalfa growth and quality. Plant tissue tests have not been reliable in predicting P and K responses with alfalfa harvested at the bud stage. The current standard critical level for tissue testing is to take the sample when the alfalfa reaches the maturity stage of 1/10th bloom. In 2003-2004, a total of 6 replicated small plot fertilizer trials were established in grower fields with soil P levels between 12 and 2 ppm. Phosphorus treatments were 0, 100, 200 and 400 lbs. P2O5 per acre. With yields indicating a positive response to P, many PO4-P tissue levels of the 0 P plots were above the critical level. Two locations were chosen for sequential tissue sampling at different growth stages to determine the rate of nutrient dilution as the alfalfa matured. Eleven cuttings were sequentially sampled in 2004 and 2005 starting at the vegetative stage, with two cutting/locations final sampling at 10% bloom. Alfalfa stage at sampling was determined by Mean Stage Count (MSC), most mature stem, predominant stage, height of tallest stem and canopy height. PO4-P and K analysis was done on the mid-stem portion of the samples. Data from the 11 cutting/ locations indicate that the rate of P and K decline is much steeper than expected from the previous research. For samples taken before any bloom appeared in the field, PO4-P declined greater than 100 ppm per day. Harvesting a few days early can significantly affect the P and K tissue levels. Of the 4 methods used to describe the maturity of alfalfa at sampling, the canopy height method provided the best correlation with the decline in PO4-P and K concentrations. Stage of the most mature stem provided a consistent method of staging the maturity of the alfalfa.