For optimum soil and water management it is best to have the base saturation of magnesium around 15% and the base saturation of calcium above 75%. The amount and relative proportion usually reflect the soil's parent materials. The calcium/magnesium ratios will be more of a factor in stressful years than in non-stress years. The question of balance between soil magnesium and calcium levels seems to revive among farmers every few years. High-magnesium soils form a hard coating, and water will run off the soil's surface. Magnesium causes the soil particles to bind together, but calcium causes the particles to separate. Soil should have a calcium-to-magnesium ratio of 4:1 to 7:1, according to Calcium Products Incorporated. All soils contain calcium ions (Ca 2 +) and magnesium (Mg 2 +) cations (positively charged ions) attracted to the negative exchange sites on clays and organic matter (cation exchange complex of the soil).