Seminar Series Abstract

Wood Fibers (Sawdust) Effect on Soil Reaction

Sam O. Dennis*¹, Richard G. Buggeln², and Simeon K. Rotich¹

¹ Cooperative Agricultural Research Program
Tennessee State University, Nashville, TN
² Center for Industrial Services
The University of Tennessee, Knoxville, TN
October 2, 2002

Wood fibers such as sawdust accumulate at sawmills and are sometimes sold at marginal cost to farmers. If sawdust is properly utilized it can be of great value to growers. Sawdust is relatively high in organic carbon and can, therefore, increase soil organic matter with time. Most soils are greatly in need of organic matter. Organic matter can increase the water holding capacity of soils as well as serve as the main source of food and energy for soil organisms. However, there is an old wives’ tale among some growers that “sawdust makes the soil acid.”

Soil reaction is a term used to indicate the acidity or alkalinity of a soil. The degree of acidity or alkalinity is determined by the hydrogen ion, H⁺, concentration in the soil solution. In acid soils, H⁺ concentration is greater than OH^- concentration, whereas in alkaline soils, H⁺ concentration is less. In soils with neutral reaction, H⁺ = OH^-.

Sawdust application to soil can cause nitrogen deficiency in plants because of the high C/N ratio of the sawdust materials. The apparent symptoms resulting from nitrogen deficiency can be mitigated with proper nitrogen and water management. The question becomes, why do growers think this is a pH effect? It is worth mentioning that, in many instances, the soils in question are acidic to start with.

The goal of the study was to generate sufficient data to show that the wives’ tale has factual or no factual foundation. Chances are, the basis for the growers’ “mantra” is the historical perspectives of early uses of wood fibers (e.g., oak bark was formerly used to tan leather and bark contains tannic acid).

The study was conducted in the laboratory using soil columns constructed from polyvinyl chloride (PVC) tubes (10-cm i.d. X 20-cm long). Four different soil types collected from growers’ fields were used to pack the soil columns to a bulk density of 1.25 gm/cm³. Two different types of sawdust samples were used in the study. One of the samples was a one-year-old sawdust (Osd) while the other was a one-day-old sawdust (Nsd) collected from a local sawmill. A 1.27-cm layer of the sawdust samples was applied to the surface of the soils. Water was applied with a customized water application device at the rate of 50 ml per day, three times a week throughout the experiment. The pH of the soils was taken at 0, 30, 60, and 90 days.

There is little or no quantitative data in published literature on the effect of sawdust on soil pH. The initial pH of the soils was 5.03, 5.70, 6.65, and 6.28. The pH of the sawdust samples was 4.53 for the old sawdust and 7.78 for the new sawdust. Our results show that the effect of sawdust on soil pH was dependent on the initial soil pH and the initial pH of the sawdust. Based on a mathematical model, we predicted that it would take 185 days for soil with the old sawdust to increase acidity to the initial pH of the old sawdust (4.53). Conversely, it will take 142 days for soil with the new sawdust to attain the initial pH of the old sawdust. Studies of this nature tend to shed some light on the old wives tale that “sawdust makes the soil acid.”

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