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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 3- Residue Management, No. 2, December-January 1985


Winter Wheat Straw Decomposition Rate

Roger Veseth

The length of time required for decomposition of winter wheat straw under conservation tillage has several important management implications. Fertilizer tie-up and nutrient release for the following crops, influence on soil temperature and moisture conditions and carryover of straw-borne diseases such as Cephalosporium stripe are just a few crop management considerations influenced by the rate of residue decomposition.

Preliminary research results indicate that wheat straw may decompose faster under no-till and minimum tillage than previously thought. A study monitoring the rate of winter wheat straw decomposition was initiated on the Palouse Conservation Field Station near Pullman in September 1983. It is being conducted by STEEP Researcher Diane Stott, USDA-ARS soil microbiologist at Pullman.

Straw samples were collected and weighed periodically in 1984 to determine the rate of decomposition. The site was left undisturbed through the year. Weeds were controlled with glyphosate herbicide (Roundup). Table 1 illustrates 1984 data on the rate of decomposition and distribution of the remaining straw.

In August, 11 months after harvest, 74 percent of the original 8,100 pounds per acre of wheat straw had decomposed, Straw lying on the soil surface decomposed at a much faster rate than standing straw, as indicated by the major proportional change in straw position distribution from March to August. In March, 39 percent was standing stubble and 61 percent was on the surface. Of the remaining 26 percent of wheat straw in August, 86 percent was standing stubble and only 14 percent was on the soil surface.

With annual cropping under no-till and reduced tillage, a higher portion of the straw would be on the soil surface or incorporated after the field operations, further speeding the decomposition rate. In an adjacent plot where spring barley was no-till seeded into the winter wheat stubble, only 19 percent of the wheat straw remained under-composed after 11 months compared to 26 percent under chemical fallow. Both the chemical fallow and no-till spring barley plots were no-till seeded to winter wheat in September. Nearly all the straw from the previous winter wheat crop was on the soil surface or shallowly incorporated after seeding.

The results of this research suggest that in a 3-year annual crop rotation under no-till, very little winter wheat straw would remain at the end of the second year after harvest. The study will be continued in 1985 and expanded to include the evaluation straw decomposition under several tillage systems.

Table 1. Decomposition rate of 8,100 pound-per-acre winter wheat straw during the chemical fallow year after harvest, September 1983, Pullman, WA (Stott, USDA-ARS, Pullman).

Time Residue Level

(lb/acre)

Amount Decomposed

(%)

Distribution of remaining straw
Soil Surface

(%)

Standing

(%)

March (6 months) 5,201 36 61 39
June (9 months) 3,789 53 33 67
August (11 months) 2,106 74 14 86
     
 

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