Effects of residual phosphorus and potassium fertiliser on organic matter and soil nutrients in aPinus patulaplantation

Abstract

On the Usutu plantation the sustained production from successive Pinus patula rotations has been a focus of active research for nearly 50 y. On 13% ofthe plantation, underlain by gabbro rocks, a 20% growth decline was reported between first (I R) and second (2R) rotations as a result of developing phosphorus (P) and potassium (K) deficiencies. The operational application of fertiliser containing 75 kg P ha-1 and 75 kg K ha-1 corrected the decline in yield. In 1989 P and K fertiliser was randomly applied, at the above rate, to one half of each of a number of trial plots in a 6-y-old, 3R crop. A 4R trial was re-established on the exact location of the former 3R trial during 1999. In 2004 intensive sampling was conducted to ascertain the influence ofthe residual fertiliser on the nutrient content of the foliage, forest floor and topsoil in the 4R crop. Fifteen years after the initial fertiliser application, 0.5 M H 2 SO 4 extracted Pinorganic + P organic in the topsoil was still 41 kg ha-1 higher in the fertilised plots than in the control plots. The total forest floor mass was not affected by the residual fertiliser, although the mass of the partially and fully decomposed forest floor litter increased by 3.5 t ha-1 (l 0% level). The residual fertiliser increased the P concentration in only the various needle components and had no effect on any other components or nutrients in the organic matter. We conclude that residual P fertiliser had a greater effect on foliar, forest floor and soil nutrient content than residual K fertiliser.

The importance of harvest residue and fertiliser on productivity of Pinus patula across various sites in their first, second and third rotations, at Usutu Swaziland

“…The differential decomposition rates of foliage, branches, bark and nonutilisable timber explains the continued nutrient supply (Saarsalmi et al 2010). Other trials at Usutu have shown that the retention of organic matter plays an important role in the nutrient cycling of especially P (Crous et al 2007b(Crous et al , 2008(Crous et al , 2011a. Germishuizen (1979) reported that site C9 (Q-set) was more productive, than A9 (M-set) and D5 (Q/T-set) by 7 and 31% respectively in the F1R and by 18 and 44% in the F2R plot treatments with no fertiliser application respectively.…”

“…In Southern Africa, there is a need to understand the biogeochemical cycling processes for the maintenance of forest productivity as highlighted in studies conducted by du Toit and Scholes (2002), Scholes (2002), Crous et al (2007b) and Dovey et al (2011). This study was, therefore, aimed at investigating the effects of residue removal in comparison with residue retention and fertiliser application on tree growth over three rotations of two trials (F1R & F2R and S2R & S3R) in a pine plantation.…”

“…At Usutu, spot application of 20 kg ha − 1 of P and K fertiliser at planting and broadcast application of 75 kg ha − 1 of P and K fertiliser at first pruning at age 5 years, corrected the yield decline (Morris: Soil fertility and long term productivity of Pinus patula in the Usutu Forest Swaziland, Unpublished). Further research showed that a large proportion of P fertiliser was retained as residual P in the subsequent rotation as the inorganic P fertiliser was converted into organic P (P o ) forms (Crous et al 2007b(Crous et al , 2011a.…”

“…Harvest slash (foliage, branches and non-utilisable timber) is regarded as nutrient rich (Raison 1982;Wall 2012) so its retention on the forest floor results in no loss of growth and also increased volume production over time (Egnell 2011). This practice is followed in the case of Usutu where forest floors are rarely burnt (Crous et al 2007b).…”

“…Whole-tree harvesting has been reported to export more nutrients from sites than conventional harvesting (CH), where slash is retained, resulting in a decline in site productivity (Weber 1985;Olsson et al 1996;Hyvönen et al 2000;Egnell and Valinger 2003;Smaill et al 2008;Eisenbies et al 2009;Titshall et al 2013). The conventional harvesting method applied at Usutu involved removing tree stems with bark from sites but not slash (Crous et al 2007b) and is considered to have a less negative effect on site productivity than slash removal due to low nutrient content losses from stem wood (Mälkönen 1976;Olsson et al 1996;Wall 2012). Harvest slash (foliage, branches and non-utilisable timber) is regarded as nutrient rich (Raison 1982;Wall 2012) so its retention on the forest floor results in no loss of growth and also increased volume production over time (Egnell 2011).…”