Growth and Composition of Sudangrass on High-Calcium, Low-Magnesium Soil

Unsatisfactory growth of monocot crops on irrigated areas of Haverson soil, which is classified as a Ustic Torrifluvent fine-loamy mixed (calcareous), mesic, and known occurrence of Mg deficiency and grass tetany in the area encouraged the authors to undertake this study. The objectives in the work were to determine plant performance through a considerable range of soil Ca:Mg and establish, if possible, minimum soil Mg levels for adequate nutrition of Sudangrass (Sorghum sudanense L.), an important forage of the area. ‘Piper’ Sudangrass was grown in a greenhouse on three soil sources from the area: 1) surface soil previously Mg-treated, 2) surface soil, 3) subsurface. The sources were all very slightly calcareous and had ratios of extractable Ca:Mg as high as 30:1 and as much as 4.5 meq exchangeable K/100 g. Mg was added at rates of O, 2, 4, 8 meq, and K at 0 and 2 meq/100 g soil to attempt a remedy of field-observed chlorosis and poor yields. Iron sulfate foliar sprays were used periodically to alleviate an iron chlorosis which developed in most cultures hi the experiment. The Mg content of tissue grown on untreated soil ranged from 0.13 to 0.20%, generally one-third to one-half the Ca content. Soil K additions increased plant Ca content considerably and without exception, and increased plant Mg contentin the second and third harvest with all three soil sources. Soil Mg additions increased plant Mg as much as three-fold, but depressed Ca appreciably only with the topsoil having field-applied Mg a year previously, and only slightly at harvests 1 and 2 with the other two soil sources. Mg depressed plant K in harvests 1 and 2. Plant Mg and K contents increased approximately proportional to soil additions of these ions. Plant materials harvested were generally below suggested minimums of Mg content for grass tetany control, and the K/Ca + Mg meq ratio exceeded 1.80. Results of these investigations indicate that for sudangrass forage of satisfactory quality, extractable Mg should be raised by soil additions to one-half or equal the soil K when conditions of extractable Ca and K similar to these are found. Although yields were not affected the first harvest, Mg content of forage was increased and yields and Mg were both improved in subsequent harvests. Additions to bring Mg equal to native soil K and reduce Ca:Mg to 10:1 resulted in forage Mg content above 0.20%.