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Forage & Grains

Magnesium for Dairy Pastures

By Neal Kinsey, Kinsey Ag Services

Neal Kinsey is owner of Kinsey Agricultural Services, Inc. and is the author of ‘Hands-On Agronomy’ and the ‘Hands-On Agronomy Workshop’ DVD. He can be reached by phone, email or through his website: phone (573) 683-3880, Email, Website

Added April 7, 2014. On most dairy farms, too little magnesium in the crops, even where farmers are just growing pasture and forages, is generally a far greater issue than most realize. Based on soil and plant testing performed throughout the U.S. and many other parts of the world, the problem of too little magnesium in the feed is present on a majority of all dairy farms. For most organic dairies testing for magnesium in the plants used for feed will confirm such a deficiency. But the solution to the problem is not always as simple as adding more magnesium to the soil. There is more than one reason for a magnesium deficiency in the crops and feeds grown on dairy farms and in too many cases just adding more magnesium to the soil does not solve the problem.

By use of a very detailed soil analysis we have tested dairy farms located in Eastern Europe, Ireland and New Zealand, where the majority suffer from too little magnesium in the feed due to a deficiency of magnesium in the soil. Though not as concentrated in terms of numbers, dairy soils in the U.S. and other areas of the world can also have the same problem, and this is especially true for sandy soils and some highly eroded clay soils. When magnesium levels are less than 10% of the soil’s total nutrient holding capacity (percent saturation based on cation exchange capacity) all plants growing in such soils will be deficient in magnesium. In cases such as these, magnesium problems can be solved just by adding the proper magnesium fertilizer to those soils. But care should be taken as to what materials are needed and how much to apply when this problem exists on your farm. Magnesium deficiency can also be caused initially from the application of too much high calcium lime – especially on sandy soils. Know what your soils need first and then be sure to apply the correct requirement for each nutrient. If not, it may cause an even bigger price to pay from too much of a magnesium build-up in the soil. This is because soils that have too much magnesium can cause some of the same problems that are caused by the soils that have too little. Doing it right the first time can truly save even more added trouble, time and expense.

When a soil has too little magnesium, the plants will not get enough. But the same is true if there is too much magnesium in the soil. When the saturation of magnesium is too high in any soil, the plants, including pasture grasses, hay meadows and silage fields will by analysis show to be deficient in magnesium. Adding more magnesium to such soils will only contribute more to the problem. And mark this well, a good soil pH does not assure that your soil has the correct amount of magnesium. When soils have a pH in the 5.5 to 7.0 range their principal make up is determined by four elements – calcium, magnesium, potassium and sodium. However, in this combination, too much of one or more of these elements means there will always be too little of one or more of the others. This can cause the pH to look good when in fact the soil is too high in magnesium and lacking one or more of the other three elements (calcium, potassium or sodium) in sufficient enough amounts to provide good nutrition for the livestock grazing there.

If a soil has too much of one element, it will not have enough of something else. A good soil test should tell farmers what is causing the pH to be where it is. Such tests will enable farmers and their consultant or fertilizer dealer to identify any excesses and deficiencies in each pasture, hay meadow or field of silage. To control any nutrient that is excessive in the soil, including too much magnesium, begin by making the required corrections to supply whatever materials are lacking in order to correct deficiencies of those nutrients that are shown as needed for that soil. This is where true soil balance begins. Without the ability to measure and understand such relationships, soil balance is just a term used as an excuse as to why something does not work as expected, but without any real idea of where the true solution to the problem lies, and likely with no idea or direction of even where to start.
Supply any deficient nutrients to control any excessive nutrients. This is the true beginning for building balanced soil nutrition. The soil is the plant’s stomach.

This balancing is not just for supplying the needs of the cows, but the needs of all those soil organisms that serve to feed the soil that feeds the cows. When a fertility program is employed that only considers the needs of the crop, and not the needs of the soil, that neglected soil, the crop and the cows will all pay a steep price.

There are those who will maintain that magnesium is not a real factor to be considered in regard to soil fertility for dairy cattle either in terms of yield or top quality feed production. Once the relationship of magnesium to nitrogen and phosphate utilization by plants is considered, it becomes clear that such thinking is not accurate. When plants lack magnesium, whether from soils that are too low or too high, then nitrogen utilization suffers because magnesium composes the center for chlorophyll formation and nitrogen attaches around that Mg center. In addition, plants need magnesium to metabolize phosphate. So when a soil is too high or too low in magnesium, nitrogen and phosphorous cannot provide the full benefits they normally could for the crop. In other words, without sufficient magnesium farmers do not receive full value from any nitrogen and phosphate fertilizers they have applied.

There are certain characteristics that can identify clay soils that are too high in magnesium even though the plants growing there are not getting enough. The magnesium content of such soils can even affect physical structure and related problems. When wet, clay soils are sticky and very slick. Then as these same soils dry out, they begin to show cracks in the dry soil as it begins to pull apart. Far too many farmers assume that when it comes to physical structure, the soil they have can never be changed for the better. However, once the nutrient make up of the soil is correctly achieved, and not just a good soil pH, even pasture soils with physical structure problems will show changes for the better.

Pugging, or soil that is easily tromped down by livestock while wet, is a good example. Note that some fields, or in some cases, even areas of the same field, tend to have the problem more than others. Check the areas that have the worst problem with pugging as compared to those that do best under the same conditions. Use any good soil test that shows the percentage of calcium, magnesium, potassium and sodium. Compare the numbers and see what they show. The closer each one of these specified elements are to the needed percentage the less pugging should be evident there.

For example, we find many dairy farms that have quite good levels of calcium, but are very low in magnesium which shows more trampling or pugging damage. Yet on the other hand, many heavy clay soils have extreme levels of magnesium and insufficient calcium to allow proper drainage and they too suffer inordinately from pugging problems. The correct amount of each nutrient in the soil, in particular that of calcium and magnesium, actually form a stronger support system to better withstand the foot traffic of cattle or any other livestock grazing there. Such needs are not solved just by knowing the pH, but they can be by measuring the percent saturation of the elements that most affect how the soil holds up under wet conditions.

Sources of magnesium and their value can vary greatly. Where both calcium and magnesium are required, dolomite lime can be utilized, and even some manures and composts also contain relatively high amounts. But on soils where calcium is adequate to high, dolomite or anything with significant calcium content should be avoided. In such cases, Epsom salts or other forms of magnesium sulfate can be used, or if potassium and sulfur are needed, sulfate of potash/magnesia (marketed as K-Mag or Sul-Po-Mag in the US) can be used.

Soils should be properly analyzed to establish any true need for magnesium and to determine the correct material and the prescribed amount required. Whether it is lime or some other material that needs to be applied, all such products should be considered based on actual nutrient content and their use managed accordingly. If lime is being considered as a magnesium source, there are several factors to consider when determining the needs of different soils (principally supplying needed calcium and magnesium) and how much is enough. This includes the nutrient-holding capacity of the soil, the calcium and magnesium content of liming material to be used, how much is needed to supply what each soil lacks and the fineness of grind of whatever lime will be used to determine what amount it can ultimately supply. All of these factors should be considered for each different type of lime and each different soil where it might be applied.

Then the last and most important point – to provide the correct magnesium for the soil each farmer should be sure that the soil tests being utilized are capable of accurately determining true magnesium needs in order to secure the correct material to sufficiently provide for any needed corrections or changes.