What About Calcium and Magnesium Ratios?

Each year we get many questions about calcium/magnesium ratios on soil tests.  What is the ideal ratio?  How high is too high for magnesium?  What can I do if I have a very high magnesium level?  Etc.

You also may read articles that will say that magnesium levels don’t matter unless you are on sandy soils and have a magnesium deficiency.  I hopefully can clarify some of these “murky waters”.  First, I will say that calcium/magnesium ratios are not about assessing whether you have sufficient nutrient levels of both.  It is about soil and water management.  The calcium/magnesium ratios will be more of a factor in stressful years than in non-stress years.

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 reason for this is that calcium will flocculate (group together) the soil clays into aggregates.  This means that there will be more pore space, better aeration and more root development.  Magnesium molecules, due to their large hydrated radius (as much as 22 molecules of water around each magnesium molecule), will cause clay particles to stay farther apart.  This means that the soil has less pore space, becomes a more dense, tighter soil that will decrease microbial activity and root growth.  When these soils dry, they also become very hard which also reduces root growth and nutrient uptake.

These soils, due to the soil condition created by the higher magnesium, will have a slower rate of infiltration and internal drainage.  This means these soils will have greater runoff during rainfall events.  If this condition is on flat river valley soil more water ponding will occur and will be slower to drain and dry.  Within 48 hours after a rain, oxygen in rainwater can be used up by microbes unless you have aeration that will re-oxygenate soil water.  Nitrogen efficiency is also decreased.  By increasing the calcium levels in these soils with gypsum you can change the soil structure and improve the soil conditions improving oxygen supplies in the soil and making soils “field ready” faster.  The soils are more pliable and softer.  They have greater soil health since air is the most critical factor to life, whether it be animals, microbes or plants.

Increasing calcium with gypsum will also form stronger soil aggregates by stabilizing organic and humic polymers.  Raindrops and tillage will be less destructive of these soil aggregates meaning less soil crusting.  Tile waters run more clear (not murky) when the clays are flocculated by gypsum.  This means less nutrients leaving the fields.

Those who have never tried gypsum at soil amendment rates will question its validity, however, our customers who have used PRO CAL 40 gypsum on these soils will support these facts.