Previous research indicated that the application of phosphorus (P) and potassium (K) fertilizer to Mexican weeping pine (Pinus patula Schiede ex Schltdl. & Cham.) on gabbro-derived soils was necessary to correct
a second rotation yield decline. To plan efficient fertilization of subsequent rotations, the fate and utilization of the applied K fertilizer was investigated using the rubidium (Rb)/K reverse tracer technique whereby the naturally occurring Rb in the soil was used as an indicator of native
soil K. At the age of 15 years, samples from the soil, forest floor, bark, wood and live foliage were collected from a trial in which various amounts of P and K fertilizer were applied at different stages of stand development. No significant quantity of the applied K remained in the soil at
the end of the 15-year rotation, but sufficient plant-available P remained on plots where 80 kg ha−1 P and K was applied. Significantly greater quantities of P per ha were found in the forest floor, wood, and foliage on the plots that received 40 or 80 kg ha−1
P and K compared with the control. The application of 80 kg ha−1 P and K increased the P content of the wood and foliage over that recorded in the 40 kg ha−1 P and K treatment. The application of 80 kg ha−1 P and K increased the K content
of the forest floor and foliage compared with that of the control. Across all PK quantities, fertilizer K contributed 28, 17, and 8% to the quantity of K in the wood, bark, and foliage, respectively. The export of up to 52 kg K ha−1 from the site in wood and bark will necessitate
the reapplication of at least 104 kg K ha−1 to the subsequent rotation to produce timber sustainably on the gabbro-derived soils at Usutu.