Forest soil phosphorus availabilty to seedlings of pinus patula and cupressus lusitanica
Creators: name: Gaitho, Charles Waweru
Publisher: name: University of British Columbia
Year: 2011

description: In a preliminary study, different amounts of iron and aluminum phosphates were added to Bf and Ah horizon materials, and potassium phosphate was added to a mixed L-F-H horizon material. The soil materials were then incubated for 16 days and analysed for extractable Al, Fe and P (Olsen's method). Results after incubation showed that the Bf and Ah horizon material had more extractable Fe and Al than the L-F-H material. Extractable P suggested a P deficiency in Bf soil material and a relatively high level of available P in L-F-H material. Addition of large amounts of iron and aluminum phosphates to the Bf and Ah material yielded only slight increases in extractable P. However, the amounts of extract-able P indicated that the phosphate forms added to these soil materials could remain partially extractable for some time and it was inferred that they resembled native soil phosphates rather than fertilizer phosphates. Next, the availability of phosphorus to two conifer species (Pinus patula and Cupressus lusitanica) was tested for the same three forest soil materials. Different amounts of phosphorus were added to these materials, as before, and seedlings of the two plant species were grown for 178 days. Phosphorus uptake by the seedlings was measured and correlated with the concentration of soil phosphorus extracted by five methods (Bray P-l, modified Bray P-l, Olsen, Truog and Mehlich), in order to identify the most suitable extraction method to serve as an index of soil phosphorus availability to the tree seedlings. Seedling growth (height, diameter, and biomass) was least for seedlings grown in Bf and greatest for those grown in L-F-H materials. Seedlings grown in Bf soil material showed, generally, greater biomass for roots than for foliage or stem, but those grown in the least P-deficient (L-F-H)material had greater biomass for foliage than stem or roots. Analysis of P uptake by seedlings indicated slightly higher P concentration in roots than in stem or foliage. Total P uptake was least from Bf and highest from L-F-H materials. Phosphates added to mineral soil gave a substantial relative increase in total P uptake, especially for Bf materials. However, for L-F-H material, added phosphates caused increases in uptake which were mostly large in absolute value, but small relative to uptake from unamended soil. Cupressus lusitanica seedlings accumulated more phosphorus than Pinus patula seedlings with the low P levels, while the opposite was usually the case with the high P levels. Extractable P after seedling growth was lowest in Bf and highest in L-F-H materials and it increased with increase in the amount of phosphates added. 01 sen's method extracted more P from the three soil materials than any of the other extraction methods. There were slight, inconsistent differences in the amounts of P extracted by the Bray P-l and modified Bray P-l methods. While phosphorus concentration in soil extracted by the five methods from the three soil materials where Pinus patula seedlings were grown correlated with each other, P extracted by Mehlich method from the Ah soil material failed to correlate with the other methods except the Olsen method. For pine, concentration of P extracted by all the methods from the three soil materials yielded significant (P≤0.05 and P≤0.10, respectively) correlation with P uptake from the P-deficient Bf, but failed to yield significant (P≤0.10) correlation with P uptake from Ah material. The Olsen, Truog and Bray P-l methods yielded very significant (P≤0.01) correlation with P uptake by cypress from Ah material. Correlation analysis of foliar P concentration with extractable P concentration in soil showed significant (P≤0.05) correlation for P i n us patula. with all methods. All but the Truog method yielded significance at P≤0.01. For Cupressus lusitanica, correlations were better with Bf than with Ah and L-F-H materials, and only the Olsen method yielded significance (P≤0.10) with all three materials. Overall, considering all soil materials and both species, the Olsen method yielded the most satisfactory relationships with P uptake by the seedlings and with foliar P concentration.