Seedbed: Use the cheapest and nearest source of magnesium for your soil

Monday, December 1, 2008

Lab studies about the disadvantages of dolomitic magnesium ignore the realities in the field

by KEITH REID

Just the other day, I had an email from an agronomist looking for information about magnesium, since there were salesmen in his area trying to sell calcitic lime, despite the fact that they were sitting next to a quarry producing high-quality dolomitic lime.

I was able to respond that there is no advantage to the calcitic lime, and that growers in his area should use the most economical source of lime.

Since trucking is the largest part of the cost of lime, this generally means the source closest to home.

The exception is on soils which are low in magnesium, where it will pay to truck dolomitic lime an extra distance to meet the magnesium requirements of succeeding crops.

But where do the claims about the harmful effects of magnesium come from? As with most of these myths, it is the case of a small nugget of fact blown way out of proportion.

One of the pieces to the soil structure puzzle is the interaction between the negative charges on the clay surfaces and the positive charges in the cations that are held to the clays, along with the water in the soil.

The water molecule is "polar," having more positive charge at one end of the molecule (the hydrogen end) and more negative charge at the other (the oxygen end). This means that water will form shells around any cation in solution, with the oxygens pointing in and the hydrogens pointing out.

Cations with two or more positive charges are attracted to the negative clay particles enough to push this water layer out of the way, pulling the clay particles together or flocculating them. Single charge cations, like sodium, tend to keep their water layer, pushing the lay particles apart (dispersion). Dispersion of the clays in soil is not a good thing, since it means that the structure of the soil falls apart every time it rains, reducing water infiltration and drainage and limiting root growth. 

The concern about the role of magnesium in soil structure is based on the difference in flocculating power between calcium and magnesium. Calcium is 43 times as effective at flocculating clay particles as sodium, while magnesium is only 27 times as effective. This means that the soil structure will be weaker in soils dominated by magnesium rather than by calcium. Lab studies have confirmed this difference in clay fractions isolated out of some bulk soils and then saturated with various ratios of calcium to magnesium. They have even shown decreases in water infiltration and increases in soil erosion when the soil contains a high proportion of magnesium, although this was not consistent across all soil types.

The researchers concluded from this that dolomitic limestone could degrade soil structure. But the lab studies ignored three very important factors which will influence whether you can scale up these findings from lab to field. 

The first is that soils are not saturated with a single cation, but have a mixture of calcium, magnesium, potassium, sodium and other cations. There are soils in Ontario with more magnesium than calcium which suffer from poor structure, but these are also soils that have a significant concentration of sodium. The poor structure is likely related more to the sodium than the magnesium.

The second factor is that the relative quantities of calcium and magnesium in a bulk soil are very difficult to change. Adding dolomitic lime is going to add equal or more calcium than magnesium, so the ratio in the soil is not going to change.

Switching to a calcitic lime will increase the calcium slightly, but it will take much higher application rates than we need for liming to make an appreciable difference in the relative amounts of calcium and magnesium.

Finally, the studies ignore the biological factors that go into creating a stable soil structure. The gums and glues from organic matter breakdown have a much greater influence on soil structure stability than the mineral make-up, and the physical effect of a lattice of crop roots and fungal hyphae is added on top of this. 

In summary, even though we can show differences in soil structural stability in the lab, it does not relate to significant changes in the field. In an area where dolomitic lime is cheaper and more readily available than calcitic, that is the form of lime you should be using. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs based in Stratford. keith.reid@ontario.ca