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Effect of Biosolids Application on Soil Quality

Gary Poole
with David Granatstein, Ann Kennedy, Astrid Andersen

Objective

To determine what, if any, measurable impacts a single application of biosolids has on soil quality and erosion potential on dryland grain fields.

Location: West of Mansfield, WA (Douglas Co.)
Annual precipitation: 10.5 inches
Soil: Touhey fine sandy loam
Rotation: Winter wheat/summerfallow

Treatments

Check (no fertilizer)
50 lb N/ac as anhydrous ammonia
5 dry tons/ac biosolids
10 dry tons/ac biosolids

Comments

This project consisted of measuring a number of physical, chemical, and microbial parameters of soil quality on plots already established (see report from Gary Poole). Green cover was measured with a residue rope in November 1993. Infiltration was estimated by a measurement of initial absorption with a single-ring infiltrometer. The results are reported as the number of minutes required for infiltration of a ponded inch of water. Straw samples were collected just prior to harvest in July. Soil samples (0-4" depth) were collected in April and August for chemical and microbial analysis.

Data

NS=not statistically significant

Green Infiltration Rate Organic

Treatment Cover
% Apr 94
min/in Aug 94
min/in Straw
lb/ac Straw:
Grain Matter
% pH P
ppm Zn
ppm

Check 52a 5.9 7.2a 5033a 1.93 1.7 5.5a 17.2a 0.6a

Anhydrous 54a 5.6 4.8ab 9176ab 1.83 1.8 5.4a 18.0a 0.9a

5T biosolids 70b 3.7 6.8a 10144b 2.23 2.1 4.9b 51.2b 3.5b

10T biosolids 74b 3.2 2.6b 12334b 2.67 2.2 4.7b 57.1b 4.5b

LSD (5%) 10.3 NS 3.42 4168 NS NS 0.27 17.01 1.35

CV % 8 71 32 23 19 15 3 24 28

Conclusion

Overall, we were surprised to find measurable changes in such parameters as infiltration and soil organic matter which are generally slow to change. The reduction in pH with increasing biosolids rate is probably due to the sulfur and nitrogen oxidation and the increase in soluble salts. Also, biosolids tended to suppress the microbial populations (data not shown) perhaps due to the same cause. The greater green cover, higher infiltration, and increased straw production are positive effects on soil conservation. Levels of both P and Zn were elevated by biosolids, but a single agronomic application (3-4 T/ac) should not lead to any problems with these elements and would supply adequate P and Zn where deficiencies exist.

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