Relations between soil factors and herbage yields of natural grassland on sandy soils in the south‐eastern Transvaal

Abstract

Forty systematically‐placed observation points were established in a 1.5‐ha area of floristically unifoim veld in which productivity varied spatially. At each point grass yields were determined by harvesting quadrats at eight‐week intervals during the growing season, and the volumetric water content of the soil was measured weekly. In addition, root mass and root‐N content, and soil variables involving C and N contents, extractable P, K, Ca and Mg, cation exchange capacity and base saturation, pH, clay content and soil depth, were determined. The data were examined using linear correlations between variables, principal components analysis and multiple regression. The major factors associated with variations in yield were, in order of importance, soil water content, the N supply (as expressed by root‐N content or root‐N mass) and the cation supply (as expressed by the topsoil variables, extractable Ca or cation exchange capacity or percentage base saturation). There was a relatively weak positive association between yields and the H⁺ concentration in the subsoil, and a weak negative association between yields and the clay content of the topsoil. A multiple regression function involving all five factors accounted for 77.3% of the variation in dry matter yield.

Veertig sistematies‐geplaasde waarnemingspunte is op ‘n 1.5‐ha area van floristies‐eenvormige veld, waarin produkti‐witeit ruimtelik varieer, geplaas. Grasopbrengs is by elke punt deur die oes van kwadrate met tussenperiodes van agt weke gedurende die groeiseisoen bepaal, en volumetriese waterinhoud van die grond is weekliks gemeet. Daarbene‐wens is wortelmassa en wortel‐N inhoud, en grond veranderlikes betrokke by C‐ en N‐inhoud, ekstraheerbare P, K, Ca en Mg, katioonuitruilingsvermoë en basisversadiging, pH, klei‐inhoud en gronddiepte bepaal. Die data is met behulp van liniêre korrelasies tussen veranderlikes, hoofkomponentanalise en meervoudige regressie‐analise ondersoek. Die hoof faktore betrokke by opbrengsveranderinge is, in volgorde van belangrikheid, as grondwaterinhoud, N‐voorsiening (uitgedruk as wortel‐N‐inhoud of wortel‐N‐massa) en katioonvoorsiening (uitgedruk as die bogrondse veranderlikes, ekstraheerbare Ca of katioonuitruilingsvermoë of persentasie basisversadiging) geïdentifiseer. ‘n Relatiewe swak positiewe assosiasie tussen opbrengs en die H⁺ konsentrasie van die ondergrond, en ‘n swak negatiewe assosiasie tussen opbrengs en die klei‐inhoud van die bogrond is ook bespeur. ‘n Meervoudige regressie funksie wat al vyf faktore insluit verklaar 77.3% van die variasie in drogmateriaal opbrengs.

Additional index words:

• Cation exchange capacity
• nitrogen supply
• root mass
• soil acidity
• soil water content