Effect of potassium fertiliser., soil magnesium status, and soil type on uptake of magnesium by pasture plants from magnesium fertilisers

Abstract

Comparisons were made between low rates of dolomite (D) and superphosphate (SP) applied separately or in a dry mixture (SP + D), dolomite- “reverted” superphosphate (DSP), serpentine-reverted superphosphate (SSP), superphosphate plus ground serpentine rock (SR), and superphosphate plus calcined magnesite (MgO), supplying 27.5 to 55kg total Mg/ha, in the presence and absence of K fertiliser, as sources of Mg for uptake by pasture plants in 23 experiments on a range of soil types (pH 5.2 to 6.6) of variable Mg status (0.28 to 4.17 me. exchangeable Mg per 100g soil). Dolomite and ground limestone (L) at 2510 kg/ha were included in 13 experiments.

No consistent yield responses were obtained from any of the Mg treatments or L. but some significant K responses were found.

Mg concentrations in herbage from control (no Mg) treatments ranged from 0.16 to 0.26% in spring and from 0.23 to 0.35% in autumn.

At the low rates used, mean absolute increases in herbage Mg concentrations were very small, ranging from nil to 0.01% dry matter (0 to 4% increase) from one application of 27.5 kg total Mg/ha and 0.005% to 0.02% (2 to 9% increase) from two applications. Dolomite at 10 times the low Mg rate increased herbage Mg on average by 0.036% (range 0.011% to 0.080%) (16% mean increase).

Results differed between soil types and sites. In general, effects were greater on soils from pumice than on other soils, but the average Mg status of the pumice soils was less than half that of the non-pumice soils.

The heavy rate of finely ground dolomite produced maximum effects between 4 and 9 months after application on soils from pumice, but between 10 and 14 months on other soils.

Limestone slightly depressed herbage Mg levels, but significantly increased Ca levels.

MgO gave greater increases than other comparable treatments.

No consistent effects of the Mg or L treatments on herbage K, Na, and P concentrations were detected.

K depressed Na > Mg > Ca. Larger effects of K on Na and Mg were found in grasses than in associated clovers. K effects were greater when K deficiency was corrected than when K was in the adequacy range.

N increased herbage Mg in two experiments and decreased it in a third.

Herbage Mg concentrations appeared to be better correlated with log. soil Mg values than with square root of soil Mg levels or untransformed values, but the best relationship accounted for only 46–61% of the variation. Herbage Mg concentrations are thus poorly predicted from soil analysis values by any of the relationships examined.