Investigations of nutrient stress in some forestry areas of South Africa

[摘要] ENGLISH ABSTRACT:One of the greatest .limitations to the productivity of a plantation forest is poornutrient status of the soil. Empirical application of corrective treatments aremarginally successful in some cases, but because of limited understanding of the soiltreesystem, most nutritional problems go unnoticed or are accepted as a conceivablegrowth constraint. The aim of this investigation was to identify nutrient growthproblems through field observations and to determine means of confirming thesenutrient imbalances.Poor and abnormal growth of pine trees in the following areas were investigated:(i) The North Eastern Cape: Ugie and Maclear Districts(ii) The Natal Midlands: Mooi River area(iii) Mpumalanga: .Graskop and Kaapsche Hoop areas(iv) Southwestern Cape: Jonkershoek PlantationThe study was conducted as nursery trials at the University of Stellenbosch and fieldtrials at the various locations. Soils from the different regions were collected and usedas growing media to test the growth response of five timber species (Pinus patula, P.elliottii, P. taeda, P. greggii and Eucalyptus nitens) under controlled conditions tovarious nutrient treatments. Indicator plants (cauliflower and soya) were included inthese bio-assays. In the field trials nutrients were applied to seedlings and establishedstands of various ages by means of foliar and soil application.In the nursery trials and the trials where seedlings were planted in the field, plantperformance was measured by quantitative means. The reaction of established standsto nutrient treatments were however less vigorous and qualitative means (e.g. colourchanges) were used for assessment.Field observations in the North Eastern Cape and the Natal Midlands indicatedpossible boron, iron and molybdenum deficiencies and thus the work concentrates onthese elements. Foliar and soil analyses reinforced these observations with marginal todeficient levels for boron and molybdenum being detected. The yellowing of foliageduring the dry season was symptomatic of ineffective nitrogen assimilation andpointed to a molybdenum deficiency while seasonal growth tip dieback, resmexudation, sinuous tree limbs and bushy trees were classical boron deficiencysymptoms. Potted trials indicated positive reaction to the application of boron andmolybdenum deficiency symptoms were observed on indicator plants. Conclusiveevidence of a boron deficiency in some of the pot trials, the planted field trials and thetree evaluation field trials remain elusive due to toxicity experienced as a result of anover-application of the nutrient. The occurrence of multiple deficiencies (phosphorousand calcium) further complicated the findings.The Mpumalanga observations indicated severe nutrient imbalance due to manganesetoxicity (strong iron antagonism). The discolouring of the foliage on some sitestowards the end of the winter was thought to be an induced molybdenum deficiencyon the weathered and acidic red soils. Positive reaction to molybdenum applicationoccurred in a single. tree evaluation trial, but because of soil oxidisation duringcollection, the effect of manganese toxicity was diluted in the pot trials. Foliaranalyses indicated that foliar application of iron was unsuccessful and that othermeans should be considered to ensure uptake of this nutrient.

[发布机构] Stellenbosch University