Exercise 10: Determination of Soil Organic Matter

Organic matter influences soil properties and processes in many ways. Physically, the organic matter helps bind the soil into structural units that contribute to the soil’s permeability to water and air. Chemically, soil organic matter provides cation exchange capacity to the soil. Biologically, soil organic matter includes living soil organisms and materials that serve as a source of carbon and energy for soil microorganisms. And, the organic matter provides a storehouse for all essential plant nutrients, either through its cation exchange sites or built into its molecular structure. In several states, soil organic matter contents are considered for nitrogen, sulfur, and herbicide recommendations.

In spite of its importance, there is much that is still unknown about soil organic matter. The molecular structures present are so complex that they are only partly known. The exact ways in which organic matter performs some of its functions also are difficult to establish. Even the amount of organic matter present has proven difficult to measure exactly. Some methods tend to overestimate the organic matter content, whereas others tend to underestimate it.

Soil organic matter determinations normally begin by burning the organic matter out of a weighed sample. The burning either may be by dry combustion or by wet combustion. The dry combustion process uses a furnace to heat the sample to 600-1500^oC, so oxygen from the air can burn out the organic matter. Problems exist with both types of procedures. The dry combustion process is likely to drive off clay-bound or hydrated water along with the organic matter at higher temperatures. Wet combustion, on the other hand, is likely to leave part of the organic matter unoxidized. In addition, certain soil constituents such as Cl^–, Fe²⁺, and higher oxides of Mn that undergo oxidation-reduction reactions in a chromic mixture can lead to incorrect results in organic C for some soils.

The measurement following the oxidation process can take several forms. Samples tested by the dry combustion process may simply be weighed before and after ignition, and the weight loss calculated as the percentage of organic matter. Wet combustion can be carried out as a titration procedure in which a known amount of oxidizing agent is added and the unconsumed amount is back titrated with a reducing agent. An alternative in either type of process is to measure one of the products of combustion. For example, carbon dioxide may be caught in a standard base and the excess or unreacted base is titrated, as will be done in the CO₂ evolution experiment to measure microbial activity. The amount of carbon dioxide can be converted to organic matter by multiplying by a factor based on the percentage C in the organic matter. This percentage often has been assumed to be 58%. Dividing 100% by 58% gives 1.724, a factor so well established that it has been written into several laws as the legal means for calculating organic matter. Unfortunately, the errors associated with measuring carbon do not justify four significant figures in the conversion factor (1.7 would be fine). Moreover, the value of 58% takes into account forms of soil organic carbon that strongly resist oxidation, such as charcoal residues. A factor of about 1.9 would be about right as an average for most of the data now available. Simply using % C x 2 = % O.M. would usually be more accurate than using 1.724.

Another combustion product that is sometimes measured is nitrogen in the form of ammonia. The organic nitrogen released during the Kjeldahl procedure is converted to ammonia. The ammonia can be released from solution by making the solution strongly alkaline and steam distilling the ammonia into an acid solution where it is titrated. Another method is to measure the ammonia with a special ammonia electrode similar to those used for measuring pH. In either case, the % N must then be multiplied by a factor to calculate the percentage of organic matter. The soil organic matter averages about 5% N, so the usual factor is % N x 20 = % O.M. Of course, uncertainty about the percentage N in the organic matter makes the factor 20 even more questionable than the factor for carbon.

In this exercise, we will use the dry-combustion but not the wet-oxidation methods. The dry-combustion method is based on weight loss upon heating. The wet-oxidation procedure, often called the Walkley-Black procedure, oxidizes the organic matter with chromic acid and results in a green-colored solution (chromic sulfate) that absorbs the yellow-orange part of the visual light spectrum and allows green light to be transmitted and observed by the human eye. The amount of yellow-orange light absorbed by the sample can be measured using a spectrophotometer, or the amount of unreacted chromic acid can be determined by back titration. We will use back titration in this exercise. External heat will help speed up the reaction and drive the reaction to completion but even with this some of the original organic matter likely will remain unoxidized. The Walkley-Black procedure was formerly widely used because it is simple, rapid, and has minimal equipment needs. On the other hand, it requires handling a concentrated strong acid and creates a hazardous waste that requires special procedures for disposal.

Procedure

I. Loss-on-ignition (LOI)

1. Obtain and weigh a porcelain crucible to 0.0001 g using an analytical balance.

Take note of the code/label of your crucible. Each group will prepare one replicate only.

2. Weigh approximately 5.0000 ± 0.0050 g (record exact weight) of your assigned soil sample (< 0.25 mm or 60 mesh). If sample is high in organic matter, use 2.5000 g (if appears to be high in organic matter, check with your instructor).
3. Set aside the weighed crucible that will be moved to an oven set at 105°C after class. After 24 hr, your instructor will determine the oven-dried weight using the same analytical balance.

This step removes free water from the soil to give the starting weight for organic matter loss upon ignition.

4. 4.Once the oven-dried weight is obtained, your instructor will transfer the samples to a muffle furnace and combust them at 400°C overnight or for 16 hr.

The organic matter is oxidized to carbon dioxide at this temperature.

5. When the furnace has cooled, your instructor will transfer your crucible to a desiccator. The following lab period, you need to determine the weight of the cooled crucible (nearest 0.0001 g) and determine the mineral content (residue after ignition) as a percentage of the oven-dried sample weight.

A number of substances are hygroscopic and when set in the open will pick up water from humidity in the air. A desiccator is an enclosed container that contains a desiccant, such as silica gel, to lower the humidity inside the container.

6. Calculate % organic matter.

7. Estimate the pounds of organic N per acre in this soil.

We can assume that an acre-furrow-slice of soil weighs 2 x 10⁶ lb. Also, we can assume that N content of soil organic matter is about 5%. Thus, we can calculate the percentage N in the soil and multiplying by 2 x 10⁶ lb to get an estimate of the lb of N per acre (in Iowa, we would expect about 2-3% of this N to be mineralized annually).

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