Environment

Soil organic matter and soil nitrogen levels are influenced by several soil management and crop production practices, which include the kind of crop grown and portion of crop harvested; the addition of nutrients to increase total growth and application of manures and crop residues that are not by-products of the fields on which they are applied. Soil organic matter is important in crop production because of its effect on soil physical conditions and the role of organic materials in supplying plant nutrients, especially nitrogen. Agronomists generally agree that when organic matter levels are maintained at as high a level as practical, that the crop production potentials will be maintained at a high level.

Returning crop residues to the soil is a practice generally recommended by agronomists. The principal reasons for such a recommendation are to help maintain soil organic matter levels and to maintain or improve soil aggregation which in turn aids in erosion control. However, because of problems with tillage, cultivation or seeding operations or the possibility of reduced yields following incorporation of residues such as wheat straw burning of fields is a common practices in many areas.

Numerous studies have shown that inorganic nitrogen is immobilized by microorganisms during the decomposition of carbonaceous materials such as wheat straws, resulting in decreased yields due to nitrogen deficiency of the succeeding crop. Russell points out that decomposition of crop residues in warm soils will immobilize more nitrogen than when decomposition takes place in a cold soil (p. 238). The optimum temperature for decomposition of organic residues is in the range of 86-115 degrees F. When temperatures are above or below this range, the rate of decomposition will be decreased. Ferguson and Gorby, and Ferguson have reported that under conditions such as found in southern Manitoba, Canada, where cereal crops are seeded before soil temperatures are high enough for rapid straw decomposition in the spring, that the crop can compete successfully with the microorganisms for the available soil nitrogen. As a result, yields are not often affected by incorporation of straw. Data of grain yields as affected by straw and nitrogen treatments on several southern Manitoba soils are shown in Table 1.

TABLE 1 - YIELDS OF GRAIN WHEN STRAW AND NITROGEN WERE APPLIED ON MINIOTA & ASSISIBOINE SOILS, 1955-58, & MINIOTA & WAS�KADA SOILS, 1959-61 (OATS, 1955-58; WHEAT, 1959-61). (From Fer�guson and Gorby, 1964, Canadian J. of Soil Sci. 44:286-291.)

									Mean
Straw	N	1955	1956	1957	1958	1959	1960	1961	1955-61
Lb/A	Lb/a	--------------------------------------cwt/A----------------------------------------------
00	0	15.6	20.9	12.4	17.0	18.6	8.5	3.3	13.8
	60	26.3	35.3	18.1	17.5	20.8	12.4	5.1	19.4
1500	0	13.9	21.0	13.3	18.2	19.8	7.7	3.3	13.9
	60	26.6	34.7	19.1	16.1	24.2	14.2	3.9	19.8
3000	0	13.6	17.4	11.6	18.5	16.1	6.8	3.3	12.5
	60	25.5	35.2	18.8	13.9	21.4	13.2	4.0	18.9

Other studies have given similar results in the Northern areas of the United States. (Smith et. aI., Oveson, Halsted and Sowden, Smith and Vandecaveye.)

Frequent fallowing in a rotation usually means a greater decline in soil organic matter than found under continuous cropping system. When considering the importance of returning residue to the soil, each year of fallow can be loosely related to burning the stubble during the same year since there is no residue to incorporate into the soil in either case. Ridley and Hedlin found that the organic matter and nitrogen content of a black lacustrine soil near Winnipeg, Canada, was maintained at a relatively high level by continuous cropping to cereal grains over a 37-year period. As the number of fallow periods in a rotation increased, the rate of organic matter loss also increased. Change in organic matter, soil nitrogen levels and resulting average yields per year are shown in Table 2.

In this study, yields after fallow were greater than yields of second or third crop, but greatest production per acre per year was obtained by continuous cropping. Use of farm yard manure applications reduced the rate of decline of organic matter and increased yields.

At the Northern Great Plains Field Station located at Mandan, North Dakota, a study of the total soil nitrogen levels after 30 years also shows how different rotations and soil treatments can affect the rate of organic matter loss (Table 3).

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Table 2. Organic Matter and Total Nitrogen Content After 37 Years of Wheat Rotation and Average Yields for the Same Period (From Ridley and Hedlin, 1967, Canadian J. of the Soil Sci. 48:315-322.)

Organic Matter (%) Total Nitrogen (%) Yield (Bu/A/Yr)
Crop Rotation Control Manure Control Manure Control Manure

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Table 3 - Effect of Thirty Years Cropping on Nitrogen in the Soil of Selected Plots from the Northern Great Plains Station, Mandan, North Dakota.
Loss or Gain of Total Nitrogen as a Percentage of Calculated Original Value

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There are many studies showing similar effects of cropping systems on soil organic matter, nitrogen levels, and various soil amendments on yields. (Smith, et.al., Oveson, Spratt, Pratt, et. al., Larson, et. al., Smith and Vandecaveye, Young, et. al., Sander.)

Luebs reporting on a 10-year burning and tillage study at the Fort Hays (Kansas) Branch Experiment Station found that burning did not affect average wheat yields. (Table 4.)

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Table 4. Effect of Burning on Wheat Yields in a Continuous Cropping System, Hays, Kansas, 1945-54. (From R.E. Luebs, 1962, Kansas Agricultural Experiment Station Bulletin 449.)

Residue and Tillage Treatment                                Average Yield/Bushels Per Acre

Plow                                                                               17.5
Burn and plow                                                                18.0
List                                                                                 15.6
Burn and list                                                                    17.6
Disk                                                                                19.7
Burn and disk                                                                  18.5
Burn and duckfoot                                                           17.0
Burn and chisel                                                                18.4

Average----no burning                                                     17.6
Average----burning                                                          17.9
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Oveson evaluated the effect wheat straw had on soil nitrogen and on the yield of wheat in wheat-summerfallow system over a 34-year period on a Walla Walla silt loam near Pendleton, Oregon. Table 5 shows that average yield of wheat, where all straw produced was plowed down, was no better than where straw was burned in the fall after harvest and was lower in yield than where straw was burned in the spring. Additions of nitrogen as ammonium sulfate, pea, vines or manure increased yields, with the 10-ton manure application giving the best results. Burning of straw after harvest resulted in the greatest loss of nitrogen from the surface foot of soil. Spring burning and plowing under of straw also resulted in significant soil nitrogen losses. When nitrogen fertilizer was added, soil nitrogen losses were greatly reduced, and when 10 tons manure was applied annually with 30 pounds of fertilizer nitrogen, there was a 7.5 percent gain in soil nitrogen.

Russell (pages 280-285) and Millar (page 101) discuss the effect of various cropping systems and soil amendments on the level of soil organic matter and nitrogen. They indicate that the amount of organic matter and nitrogen will eventually reach an equilibrium level. The equilibrium value reached will depend upon factors such as soil texture, drainage, climatic conditions and the cropping system followed.

Results of a longtime dryland soil study in Kansas as reviewed by Hobbs and Thompson showed that:

1. Nitrogen and organic matter contents of surface soils decreased with continued cultivation.

2. Magnitude of losses from cultivated land was related directly to the quantity of organic matter in the soil originally and to cultivation intensity (Table 6).

3. Organic matter losses followed a curvilinear trend, being large early in the cultivation period and becoming smaller with continued cultivation and approaching an equilibrium-nitrogen content of 0.103 percent.

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TABLE 5 - Average Field Plot Yields, Percent Nitrogen in the Surface Foot of Soil and Loss or Gain of Nitrogen in Percent and Pounds Per Acre After 34 Years of cropping to Wheat and Summerfallow on Plots Receiving Different Treatments of the Wheat, Straw and Other Residues. (From M. M. Oveson, 1966, Agronomy J. 58:444-447.)