Soil-plant relationships of magnesium in selected Taranaki yellow-brown loams : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Soil Science at Massey University

Abstract

The exchangeable Mg contents of soils in the Inglewood - New Plymouth area of Taranaki are primarily determined by Mg contents of parent materials. Inglewood coarse sandy loams, formed from Inglewood Tephra contained the lowest, and New Plymouth black loams formed from Oakura Tephra the highest, exchangeable Mg contents. Exchangeable Mg contents of soils formed from Inglewood an Oakura Tephras declined with increasing altitude. The exchangeable Mg contents of Taranaki yellow- brown loams formed from pumiceous parent materials under high rainfall conditions are more similar to those of yellow-brown pumice soils than of Central yellow-brown loams. In a pot experiment, the Mg concentrations of ryegrass plants grown on Burrell gravelly sandy loam (0.24 m.e% exchangeable Mg) or Inglewood coarse sandy loam (0.22 m.e.% exchangeable Mg) were lower than those of plants grown on Egmont black loam (1.22 m.e.% exchangeable Mg) or New Plymouth black loam (1.44 m,e.% exchangeable Mg), Only on the two soils of lower exchangeable Mg content did Mg additions of from 9 to 36 kg Mg ha−¹ result in increased dry matter yields of ryegrass plants. Increases in ryegrass Mg concentrations and total Mg uptakes in response to Mg additions were also more marked on the trails of low exchangeable Mg content than on the soils of high exchangeable Mg content. Ryegrass dry matter yields increased with increasing temperature on all soils, as did plant Mg concentration and uptake, although the latter increases were greater on the soils of low Mg content and did not occur until after an apparent critical minimum temperature of ca. 14 C had been reached. Mg additions had no major effect on plant Ca or K concentrations. The results of the field survey and the pot experiment are discussed in relation to the possible occurrence of Mg deficiency in plants and animals in Taranaki and the possibility of correcting these deficiencies using Mg-containing fertilizers.