The magnesium contents of soils and crops

Abstract

Abstract Preliminary work shows that under 20% of soils in the northern half of Scotland contain less than 3.1 mg. of Mg per 100 g. Very similar amounts are extracted by 2.5% acetic acid and ammonium acetate. Experiments comparing a range of sources of Mg measure effects on the Mg contents of crops and determine the level in the soil at which crop yield responses can be expected. The readily soluble Mg content in the soil is likely to be below 3 mg. of Mg per 100 g. of soil before any yield response can be expected. Normal fertiliser dressings have very little effect on the percentage of Mg in cereals, potatoes and swedes, but crops grown on different soils can show large variations. Dressings of K required to maintain yields of grassland herbage have very little influence on its Mg content. There is a marked seasonal increase in the percentage of this element in the grasses separated from regularly cut mixed herbage but practically none in the clovers. The range of Mg contents of herbage and soil samples are similar for each one of five soil types. Mg contents of herbage and of soil samples are poorly correlated, but the relationship is generally better with clover than with ryegrass. For slightly acid mineral soils a tentative minimum value of 16 mg. of Mg per 100 g. of soil is suggested for the level required to ensure that the herbage content of Mg is near the maximum.