Magnesium Depletion in Relation to Some Cropping Systems and Soil Treatments

The concept that plant nutrition is an exchange process between adsorbed ions on the colloidal fraction of the soil and the hydrogen ions on the roots^3,4 naturally directs attention to the cations on the list of the plant nutrients. Among these, calcium and its singular activities have already had fruitful consideration.^1,2,5 Studies have revealed the more or less baffling behavior of potassium that still is challenging attempts at explanation.⁷ Magnesium has been the subject of little research⁹ in the glacial soils, though it has had attention in soils more highly developed.⁸ Consideration may well be given to the magnesium supply and its exchange activities between the soil complex and the plant as these are related to the continued and sufficient delivery of this nutrient for utilization by the plant.

Now that calcium is becoming a common soil treatment, its companion cation, magnesium, deserves attention. This is particularly timely if we are to learn of its behavior in connection with the heavy lime applications given many soils. Then, too, the declining soil fertility manifested in greater magnesium-deficiency symptoms in plants, particularly the soybeans, must naturally demand early attention to magnesium in the soil. Because of these facts, magnesium was given exploratory attention in two soil studies. One consisted of a survey of the magnesium supplies in the soil as revealed by rapid tests in 600 soil samples collected in the State of Missouri during 2 years. Another study involved analytical attention to the soils of some of the plots from the research station at Bethany, Missouri, in cooperation with the Soil Conservation Service. In these, particular attention was given to certain cropping systems and soil treatments as they are related to the depletion of magnesium.

Magnesium in Missouri Soils

Of numerous samples received by the laboratory for tests, those which could be specifically classified as to soil type were used. In addition, samplings were made specifically for this study. Though about 100 soil types of Missouri were represented, the bulk of the samples were from approximately 12 of the most common types.

The tests of magnesium were made according to the rapid method of Baver and Bruner.⁶ The soil was leached with 0.3 N hydrochloric acid, and the magnesium was precipitated as a colloidal hydroxide colored with titian yellow. The values used by Baver and Bruner as standards ranged from 0.25 to 2.00 m.e. magnesium per 100 gm. of soil or from 60 to 480 pounds per acre 2 million. The medium figure was taken as 240 pounds per acre. Soils containing less were adjudged as “low” and those containing more were considered “high” in magnesium.

The results of this inventory are presented graphically in figure 1, which shows the soils as groups in the different geographic areas of the state. Two of the areas, namely, the southeastern part and the southwestern prairie were not represented by a sufficient number of samples. They are included, nevertheless, for their indicative values. The samples of the former of these two areas are all sandy loams. The remainder of the soils for the state were mainly silt loams.

Fig. 1. Exchangeable Magnesium, as Pounds Per Acre, in the Soils in Different Areas of Missouri

 

The less leached soils contained the more magnesium. Those progressively more developed showed lower contents of this plant nutrient. The most significant revelation of this inventory is the fact that even the area of soils with the highest magnesium content did not reveal as much as 240 pounds per acre, considered as medium in amount. The mean for the 600 samples of the state was 168 pounds per 2 million. Thus, either all the soils of Missouri are significantly low in magnesium, in general, or the standards set for this test by Baver and Bruner⁶ are too high.

That the soils of Missouri may be generally low in magnesium is suggested by numerous cases of plant symptoms indicating magnesium deficiencies, particularly on soils with crops in close sequence under reduced amounts of plowing. Further suggestion comes from studies by Graham.¹⁰ When he used the separated silt fraction from an Iowa surface soil after it had been treated with acid clay, the magnesium content of the crop grown on it was no higher than that of the planted seed. These observations suggest that there may be insufficient exchangeable magnesium on the colloidal fraction of the soil, and insufficient active reserve of this element in the silt portion of even the recently glaciated soils. Such facts suggest attention to possible magnesium shortages for crops on Missouri soils of older glaciations and of more intensive development at higher temperatures.

 

Table 1–Changes in degrees of saturation of the soil complex by calcium and magnesium and in the organic matter and nitrogen contents of the soil from 1931 to 1937

Crop And Soil Treatment*	Saturation By		Percentage Of
	Calcium	Magnesium	Organic matter	Nitrogen
Bluegrass–no treatment Alfalfa–lime plus phosphate= Rotation–lime plus phosphate Rotation–no treatment  Fallow surface–no treatment  Fallow subsoil–no treatment	Increase  Increase  Increase  Decrease  Decrease  Decrease	Decrease?† Decrease Decrease Decrease?  Increase Increase	Increase Increase Increase Decrease  Decrease  Decrease	Increase Increase Increase? Decrease Decrease Decrease

*Complete data were reported by Whitt and Swanson (13, table 3) for these treatments and crops.

† Where the data are not entirely consistent in their trend for all of the four determinations, 1931, 1933, 1935, 1937, the general trend is given with a question mark.

= Phosphate and lime were applied in 1930, and phosphate at three-year intervals thereafter.

 

Cropping Systems, Soil Treatments, and Possible Magnesium Depletion

In connection with the research work at the Soil Conservation Experiment Station on Shelby loam, a glacial drift soil, the exchangeable calcium and magnesium were determined in differently treated soils sampled to surface depths at three biennial intervals representing a total of 7 years (1931-1937) in the history of the treatments.¹³ Similar determinations were made at the end of the fourth biennial period on successive 1-inch soil layers down to 13 inches in the permanent bluegrass sod in the plot series. Determinations were made, at the same time, of the nitrogen and of the organic matter by means of carbon. The soils sampled and studied for magnesium, included (a) three plots in a rotation of corn, wheat, clover-timothy, with no treatment, (b) similar plots with lime plus phosphate, (c) alfalfa with lime plus phosphate, (d) bluegrass with no treatment, (e) fallow treatment on a surface soil, and (f) similar treatment on an exposed subsoil. It was on only the permanent bluegrass sod that the samples were taken as 1-inch layers in 1939. The other samples were taken as the surface 7 inches in a single sampling, several samplings being composited in each case for the plot.

The changes in the amounts of exchangeable calcium and magnesium with time in these soils under different treatments direct particular attention to the magnesium. These trends along with those for the percentages of carbon and of nitrogen during the 7 years represented by four successive analyses are listed in table 1.

Decrease in magnesium saturation under apparent soil improvement

It is particularly noteworthy that though the shifts in calcium saturation, in organic matter, and in nitrogen were generally in the same direction, the shifts in magnesium saturation seemed to be in the opposite direction. Under those crops and soil treatments which have commonly been considered as soil-building, e.g., the use of lime and phosphate, the growing of alfalfa and bluegrass, or those that demonstrate trends toward increase in the organic matter, in the nitrogen, and in the degree of calcium saturation in the soil, there was a trend toward decrease in the degree of magnesium saturation of the soil. This was true for crops that allowed a minimum of erosion to bring soil from the lower into the upper layers through plowing. Then again, the rotation and the fallowing–all without soil treatment–which give a decrease in the organic matter, in the nitrogen, and in the calcium saturation, serve to give an increase in magnesium saturation. Here higher magnesium saturation in the subsoil layer plowed into the surface as a consequence of truncating the profile by erosion may be responsible. Thus, while soils were improving in calcium saturation and in nitrogen and organic matter content under little or no erosion, the magnesium saturation was decreasing. Contrariwise, while soil depletion with respect to calcium saturation, nitrogen, and organic matter was occurring along with possibly some erosion, the degree of magnesium saturation was increasing in this soil.

This suggests that the lower crop production and the fallowing, which allow exchangeable calcium to be lowered in concentration while some erosion is occurring, serve to increase the degree of saturation of magnesium. Bluegrass, however, without soil treatment has entirely the opposite effects. Its effects on the soil correspond with what happens when either a legume-containing rotation or continuous alfalfa is given lime and phosphate. Bluegrass is thus a soil-builder as shown by comparison of these soil properties.

That the bluegrass crop, without soil treatment, is singular in this respect among these limited numbers of cropping systems tested is further supported by the quantitative data from the 1-inch layers of the soil in the grass sod. This is indicated by the distinct differences between the upper seven 1-inch layers of surface soil and the lower six layers of subsurface soil. This was a profile that was not being truncated by erosion and was not given lime and phosphate, yet the surface soil bearing the bluegrass.roots was decidedly low in magnesium saturation in contrast to the subsurface. The data are assembled in table 2.

 

Table 2–Degree of soil saturation by calcium and magnesium, and percentages of organic matter and nitrogen in the surface and subsurface layers of permanent bluegrass*

Bluegrass*	Saturation By				Percentage Of		C/N
	Calcium		Magnesium		Organic matter	Nitrogen
	per cent	m.e.	per cent	m.e.
Surface seven 1-inch layers, mean	61.3	11.74	19.4	3.30	3.73	0.180	11.98
Subsurface six 1-inch layers, mean	56.9	15.52	30.4	8.28	2.06	0.106	11.22
Difference due to bluegrass roots	4.4		11.0		1.67	0.074	0.76
	Increase		Decrease		Increase	Increase	Increase

*This plot was seeded to bluegrass with timothy in 1930. It had been in cultivated crops previously. Complete data for the successive 1-inch layers were reported by Whitt and Swanson (13, table 2).

 

Table 3–Increasing organic matter, nitrogen, and calcium saturation and decreasing magnesium saturation in a treated soil growing alfalfa*

Dates Of Sampling†	Exchange Capacity	Saturation By				Percentage Of
		Calcium		Magnesium		Organic matter	Nitrogen
19311933 1935 1937	m.e. 18.95 19.17 20.02 19.72	percent 58.10 64.79 87.71 77.33	m.e. 11.01 12.42 17.56 15.25	percent 21.16 17.06 17.33 16.84	m.e. 4.01 3.27 3.47 3.32	3.793.91 4.05 4.02	.184.186 .196 .192

*These data were reported by Whitt and Swanson (13, table 3).

†Soil treated in 1930.

 

Here again the soil zone bearing the many bluegrass roots and experiencing the effects of the crop was higher than the subsurface with its few roots, in those respects that represented increase with time (table 1); namely, organic matter, nitrogen, and calcium saturation, whereas it was much lower in magnesium saturation than the subsurface. Bluegrass apparently built up the soil in some respects while it was depleting the active magnesium supply in the absence of erosion.

Alfalfa as a crop with the soil treatments of limestone and phosphate was similar to bluegrass under no treatment in giving increases in the organic matter, the nitrogen, and the degree of calcium saturation, while the degree of magnesium saturation was decreasing. This is shown by the data in table 3.

Increase in calcium saturation and decrease in magnesium saturation after liming

Where the limestone was applied for alfalfa in 1930, as shown in table 3, the calcium saturation, and with it the organic matter and nitrogen, mounted steadily until 1935. By 1937 the calcium saturation was declining, though it was still almost 20 per cent above the value given by the determination in 1931. By that late date the contents of organic matter and of nitrogen were no higher than those of the sampling immediately previous. While these three factors were increasing, the degree of magnesium saturation was decreasing. According to these data, the alfalfa crop on the soil given lime and phosphate was reducing magnesium saturation more drastically than was the bluegrass on the same soil without treatment. The reduction was even more drastic under alfalfa than under rotation on the soil given limestone and phosphate. These reductions in magnesium under alfalfa were coincidental with an increase in the saturation of the soil by calcium.

Bluegrass without limestone applications was bringing about an increase in calcium saturation of the soil colloid similar to that under limed alfalfa. This movement of limestone, for 5 years, from the applied crystalline into the adsorbed form on the soil colloid and the resultant increasing saturation may be the explanation of the better legume crop growth with time after limestone applications. The increased calcium saturation under bluegrass without lime applications has been suggested by Whitt¹² as an explanation for the periodic advent of white clover in bluegrass sods.

While the soils under cropping with no erosion (alfalfa and bluegrass) were undergoing reduction in magnesium saturation and increase in calcium saturation, those under fallow, with erosion, were showing the opposite effects. The cases under study were not numerous enough to warrant generalizations; nevertheless, they suggest that soil-conserving crops and soil treatments are associated with lowered magnesium saturation and that erosive crops are associated with increased magnesium saturation. Though the degree of magnesium saturation was reduced by bluegrass without soil treatment, it was reduced more by rotation with lime and phosphate treatments, and still more by alfalfa with these same soil treatments. Whereas bluegrass raised the percentage of calcium saturation by one fourth, it lowered the magnesium saturation by one twentieth. For the rotation with lime and phosphate, the saturation went upward as much as one third for the calcium and downward one fifth for the magnesium. Under fallow and erosion, the surface soil decreased in calcium saturation equivalent to about one twentieth but increased in magnesium saturation as much as one seventh. The desurfaced soil, or exposed subsoil, under fallow decreased in calcium saturation by about one twelfth but increased in magnesium as much as one third during the 6 years.

Truncation of profile in relation to changes in magnesium saturation

Truncation of the profile by erosion on the 8 per cent slope may offer an explanation of the increase in magnesium saturation in the fallow soils. What truncation was doing was shown by the successive 1-inch layers¹³ which were studied only under the bluegrass sod where no erosion took place. If we assume that the bluegrass crop was without disturbing effects at depths of 7 to 13 inches, and that the profiles of the bluegrass and of the fallow surface plots were the same at the outset, then if truncation from 1930 to 1937 were to increase the magnesium saturation of the fallow surface plot to the figure of 23.37 per cent as a mean of the upper seven 1-inch layers, this would represent a shift downward in the profile to approach the mean magnesium figure of 22.63 per cent under bluegrass for the seven 1-inch layers ranging from the depths 3 to 9 inches inclusive, or the mean figure 24.57 per cent for the depths from 4 to 10 inches inclusive. This would demand truncation by erosion to the extent of 3 to 4 inches in 7 years, or the surface 7 inches in 14 years. Coincidently, the erosion data for these 7 years¹³ showed that this plot was losing soil at the rate of more than 86 tons per acre per annum. This would mean the loss of the surface 7 inches in about 12 years. Such a close agreement with the figure of 14 years, as calculated from differences in magnesium saturation, suggests that magnesium saturation was maintained by truncation of the profile through erosion.

In the fallow subsoil, the magnesium saturation was 29.75 per cent in 1937. If this subsoil corresponds to the section in the bluegrass profile from 7 to 13 inches, then the mean magnesium saturation of these separate 1-inch layers would have been 28.69 per cent, or if the collection were made as a 7-inch sample it would have been 30.36 per cent. The fact that this subsoil was eroding at the rate of 7 inches in less than 17 years¹³ indicates an approach toward a relatively constant degree of magnesium saturation with increasing depth in the profile below the thirteenth 1-inch layer, the limit of the study.

That magnesium saturation is higher in the subsoil than in the surface soil in another glacial drift profile was demonstrated in some studies by Harris and Drew.¹¹ Though erosion produced a soil with a clay content almost twice as great in their studies, the exchange capacity had become correspondingly larger. The exchangeable magnesium increased slightly more than the clay content, to show increasing magnesium saturation in going from the surface to the subsoil. Other soil properties, such as exchangeable calcium, exchangeable potassium, total nitrogen, and organic matter did not increase accordingly. In some of these there were decided decreases. The exchangeable hydrogen increased much more than the clay content, suggesting that the magnesium saturation was maintained in the presence of increasing hydrogen saturation and of decreasing saturation by some of the other cations.

In the deep, virgin loess as it is more highly weathered under more intense climatic forces near Vicksburg, Mississippi (reported by H.B. Vanderford in a private communication), than under the Missouri conditions concerned in this study, the exchangeable magnesium increases more than threefold with depth in the solum. The calcium increases by only one eighth. At a depth of 20 feet the exchangeable calcium is tenfold and the magnesium over sixfold that in the surface horizon. Here again, truncation of the profile to the extent of 12 inches would mean little change in the calcium saturation but more than doubling in concentration of exchangeable magnesium.

Summary

The data presented in these studies, concerned with magnesium saturation of the soils under cropping with no erosion and in fallow soils with significant erosion, bring magnesium into the picture as a nutrient that is reduced significantly by cropping. They also suggest that erosion may have been serving in the past to hide what may be an impending serious deficiency in soil fertility. They point out further that the performances by bluegrass sod without soil treatment and by alfalfa and by rotation, both with lime and phosphate, ordinarily considered as soil-building effects, are decidedly depleting for magnesium, if decreasing degree of soil saturation of this nutrient may be taken as a criterion.

As our efforts in soil-building, particularly by liming, phosphating, cover cropping, and erosion prevention, serve with benefit to the active calcium and humus contents of the soil, they may be a detriment to the available magnesium supply. A liberal virgin store of magnesium in the more active form or a large stock in the mineral reserve may have been saving us from trouble in the past with respect to shortages of this nutrient. On some of the more highly developed soils of which the silt and sand fractions are very low in minerals other than quartz, and where such soils are now being maintained against erosion by conservation practices, it may not be long before the degree of magnesium saturation will be too low to guarantee good yields of crops. For some soils, as the Shelby silt loam used in this study, the practices in the past of using erosive systems of cropping, and of fallowing with its intensive erosion, may have prevented such troubles. Under intensive cropping with erosion prevention and the application of other fertilizer elements, however, the need for using magnesium as a soil treatment may soon become comparable to the present needs for calcium, potassium, and other fertilizer cations. The same fundamental principles underlying these in their plant and soil relationships will then help us to understand and manage the behavior of magnesium.

 

References Cited:

1. Albrecht, W. A.: “Calcium and hydrogen ion concentration in the growth and inoculation of soybeans”. Jour. Amer. Soc. Agron., 24: 793-806, 1932.
2. Albrecht, W. A.: “Inoculation of legumes as related to soil acidity.” Jour. Amer. Soc. Agron., 25: 512-522, 1933.
3. Albrecht, W. A., and McCalla, T. M.: “The colloidal clay fraction of soil as a cultural medium.” Amer. Jour. Bot., 25: 403-407, 1938.
4. Albrecht, W. A.: “Adsorbed ions on the colloidal complex and plant nutrition.” Proc. Soil Sci. Soc. Amer., 5: 8-16, 1940.
5. Albrecht, W. A.: “Plants and the exchangeable calcium of the soil.” Amer.Tour. Bot., 28: 394-402, 1941.
6. Baver, L. D., and Bruner, F. H.: “Rapid soil tests for estimating the fertility needs of Missouri soils.” Missouri Agr. Exp. Sta. Bul., 404, 1939.
7. Ferguson, C. E., and Albrecht, W. A.: “Nitrogen fixation and soil fertility exhaustion under different levels of potassium.” Missouri Agr. Exp. Sta. Res. Bul., 330, 1941.
8. Garner, W. W., et al.: “Sand-drown, a chlorosis of tobacco due to magnesium deficiency, and the relation of sulfates and chlorides of potassium to the disease.” Jour. Agr. Res., 33: 1-30, 1923.
9. Graham, E. R.: “Magnesium as a factor in nitrogen fixation by soybeans.” Missouri Agr. Exp. Sta. Res. Bul., 288: 1-30, 1938.
10. Graham, E. R.: “Soil development and plant nutrition: II. Composition of sand and silt separates in relation to the growth and chemical composition of soybeans.” Soil Sci., 55: 265-273, 1943.
11. Harris, H. L., and Drew, W. B.: “The establishment and growth of certain legumes on eroded and uneroded sites.” Ecology. [In press.]
12. Whitt, D. M.: “The role of blue-grass in the conservation of the soil and its fertility.” Proc. Soil Sci. Soc. Amer., 6: 309-311, 1941.
13. Whitt, D. M., and Swanson, C. L. W.: “Effect of erosion on changes in fertility of the Shelby loam profile.” Jour. Agr. Res., 65: 283-298, 1942.