A study of soil sequences in relation to the growth of pinus radiata in Nelson

Abstract

This study has two principal aims; firstly to investigate the effect of topography and in particular erosion as a soil-forming factor and secondly to investigate the reasons for the existence of Pinus radiata showing symptoms of nutrient deficiency in Nelson. Hans Jenny in his book "Factors of Soil Formation", wrote "Topography as a soil-forming factor has not received the attention it deserves. It is true, of course, that a considerable amount of information on runoff and erosion in relation to slope is at hand, but it deals primarily with the removal and destruction of soil and not with soil formation." This statement, made nearly 30 years ago, is still true. One method of assessing the importance of topography as a soil-forming factor is the recognition and investigation of a Toposequence wherein the remaining four soil-forming factors are kept constant or ineffectively varying. Observed differences between the soils of the sequence are then considered to be the result of differences in topography since the initial point of soil development. One such sequence had been identified in 1964 by Mr.E.J.B. Cutler of the Soil Science Department, Lincoln College on strongly weathered granite at Kaiteriteri. In addition, another was postulated within the Mapua hill soils developed on the seaward end of the Moutere gravels, an area of very old, very strongly weathered and leached alluvial greywacke gravels in Nelson. Associated with both these postulated sequences was Pinus radiata showing nutrient deficiencies and consequent variations in growth. This study is probably the first to thoroughly investigate differences in soil development attributable to the down-cutting effects of erosion exposing fresh parent material for soil development. Each erosion cycle has provided a new groundsurface for profile formation and, taken together, the various groundsurfaces identified provide a soil sequence where the gains, losses, transformations and redistribution of many organic and inorganic parameters are attributable to the effects of relief in general and erosion specifically. The strong weathering and leaching undergone by even the youngest groundsurface in both sequences means that the stage of soil development encompassed in this study is considerably more advanced than in most other sequence studies. These have tended to concentrate on the rate of build-up of organic matter and associated fractions in relatively young soils. One exception is the study by P.R. Stevens of a chronosequence of soils near the Franz Josef glacier in Westland, which covers a range of soil development from zero time to a strongly podzolised profile. The present study includes stages of soil development even more advanced than those in Stevens' work. The second part of the study is part of a widespread research programme being carried out by members of the New Zealand Forest Research Institute and Lincoln College into the underlying causes of a possible decline in productivity on the poorest sites in Nelson, following felling of the first crop of Pinus radiata. If this fall-off in productivity is indicative of a future general decline in other areas of low fertility the implications for the expanding New Zealand forestry industry would be quite serious. For this reason, considerable effort has been and is being expended to investigate the problem in Nelson. Recent work by Dr. H. Holstener-Jorgensen in the area has shown that the postulated productivity decline in the area may be a result of different establishment practices between the two crops rather than an irreversible decline. Consequently the present study has tended to concentrate on the causes of the undoubted growth differences present within the regeneration crop. A combination of methods was used to investigate this problem as well as the causes of similar nutrient deficiencies in young planted Pinus radiata growing on areas of the Kaiteriteri granite soils. The thesis follows the traditional form with an extensive review of literature covering previous studies of topo - and catenary sequences with a section devoted to the influence of relief as a soil-forming factor and the evolution of landscapes, particularly hillslopes. Field work and analytical procedures are covered in the section on materials and methods which is followed by the presentation of results and ensuing discussion. Literature references, Tables, Figures, Plates and Appendices are bound in a separate volume. It is relevant to comment upon the value of such a study. The identification and study of monofunctional soil sequences is a prerequisite to the understanding of the processes of soil formation. All sequence studies suffer from the assumption that it is possible to say definitely that all other factors of soil formation have remained constant during the development of the soils concerned. However, this does not diminish the usefulness of the physical, chemical and mineralogical information which can be gained from the study of such sequences. The information concerning these soils and the foliage data obtained should prove valuable in helping to elucidate the problems of forestry management which have and will undoubtedly continue to appear in future years in the Nelson province.