Measuring organic matter: Weight or volume?

Organic matter is the most valuable part, by weight, of soil. But organic sources in soil mixes are specified by volume which is mostly air. What is the price of the air part?

What made me wonder was an 80:20 sand:peat mix that a friend bought, which was assessed at only 0.66% organic matter. Soil is blended by volume, so my friend’s mix was supposed to be 20% peat by volume. But soil scientists analyze organic matter by weight. How can a buyer figure out if 20% peat by volume is only 0.66% organic matter by weight? Or if the buyer is paying mostly for air?

Organic sources are light, having low bulk density or low weight per volume. So, the first math problem is that when light organic sources such as peat are mixed with sand with high bulk density, the dilution by weight is many times larger than the dilution by volume. The second math problem is that organic sources such as peat are not 100% organic matter. So we also need to know the % organic matter of the source.

The most valuable part of the soil mix that you buy is the organic matter. Reporting organic matter by weight would make somewhat more sense than reporting by volume, because the vast majority of that volume is air.

Not surprisingly, vendors of soil mix not only don’t report the weight organic matter of the mix, they don’t report the bulk density (weight per volume) of the organic source, its organic matter content, or other details of interest, such as the fiber content. Before getting more into the math of organic matter and bulk density, what makes organic matter so valuable?

What makes organic matter valuable?

Finer loamy and clayey soils, most soils, have no structure and no tilth, except if there is organic matter. Without organic matter, such soils are not crumbly and soft but are hard as a brick, flat as a pancake, and generally impenetrable to plant roots.

For coarser sandy soils there is the opposite problem. Without organic matter, water and nutrients rush through like a sieve. In sand soils, cation exchange capacity which holds onto mineral nutrients, and much of the water holding capacity, is almost entirely due to organic matter. Repeated studies have shown that organic and some inorganic amendments can increase water holding capacity and nutrient retention and increase capillary porosity. Irrigate a sand soil without organic matter and where does much of the nitrate go? It can leach into the water table and/or pollute coastal estuaries.

Irrigate a finer soil without organic matter and much of the water may not go into the soil but will run off and carry fertilizer phosphorus horizontally by sheet flow, to potentially pollute freshwater ecosystems. Without organic matter you are in one kind of fix or another. That’s the chemical side related to water.

The physical side is also serious. Sand greens and sports turf fields without organic matter have little stability and shift around. In 1998, I visited the University of Miami Fields that had recently been constructed for NFL practice, for SuperBowl XXXIII in Miami. Large athletes were creating divots a couple of feet across. When a running athlete made a quick turn, a huge chunk of soil would slip sideways creating a crater. The problem was that these were pure sand fields, with no organic matter added.

Finally, for all soils, almost the entire nitrogen composition is in the organic matter. No organic matter, e.g., tropical lateritic soils, and there is no soil fertility.

Organic sources differ in bulk density and organic matter content

The most desirable organic sources for amending sand greens and sand fields are sphagnum peat and reed-sedge peat (Table 1). These are “fibric” sources having high fiber content and very low bulk densities. For sphagnum peat the bulk densities range mostly 0.07 to 0.16 with a median of 0.13 grams/cubic centimeter. Organic matter content is mostly 89% to 99% with a median of 95%. (The sources and explanation of these values is in Table 1, below.)

Reed-sedge peat bulk densities have a median of 0.23, and organic matter content about 90%. More well decomposed muck with little visible fiber is “sapric” and has much higher bulk density, a median of 0.50 grams/cubic centimeters, and organic matter content is about 50%. Bulk density, or specific gravity, is relative to water, 1.00 under standard conditions. Unamended sand is relatively heavy, with bulk density about 1.6 grams/cubic centimeter.

As a quick and rough conversion of volume-to-weight conversion, you can use the ratios of bulk densities. For example, divide the bulk density of sphagnum peat, 0.13, by the bulk density of sand, 1.60, to get a 1/12 conversion from volume to weight. A 20% peat mix by volume should have about 1/12th the organic matter by weight, or 1.6%, or 1.5% in accounting for only 95% organic matter in peat. That’s still more than twice what my friend had. If a low-grade muck had been used (this was Florida), say 0.50 bulk density, then we would divide 20% by 3.2 to show that the organic matter in the mix should have been 6.2% or ten times higher than was paid for. Muck is only about 50% organic matter, so even with that adjustment there should still be 3.1% organic matter by weight in the soil mix. Either way, these numbers do not add up. It could be argued mathematically that most of the price was for air.

Peat is very compressible and the bulk density varies under natural conditions with depth and other factors. By its nature peat develops under wet conditions but once it dries it can be fluffed up. Maybe the vendor tried really hard to fluff up the peat before incorporating it. Even so, the numbers don’t add up. Trying to give every benefit of doubt to the vendor, the bulk density, the fluff, the organic source, and it still doesn’t add up. Ideally peat and other organic sources should be measured by weight so we would not have to go through arithmetic contortions to figure out how much there really is.

How can we meet organic matter goals?

Sand fields are amended with 5% to 20% of organic matter in the blend to substantially alter physical and chemical properties (McCoy, 2013). Commonly sold for Florida sports fields is an 80:20 mix, 20% organic source by volume. The source of organic matter can be peat such as Canadian sphagnum peat, reed-sedge peat, highly decomposed organic source such as muck, compost, and other sources, which vary greatly in bulk density and organic matter content (Table 1).

Taylor et al. (1997) showed that an 80:20 mix with 87% organic matter sphagnum peat resulted in 1.34% soil organic matter and an 80:20 mix with a 94% organic matter reed-sedge peat resulted in 2.98% organic matter. Bulk density of peats was not indicated. McCoy (2013) pointed out that about 15% by volume of a fibric sphagnum or 7.5% by volume of a hemic reed-sedge peat would result in 1.5% organic matter. Bulk density of peats was not indicated. Bigelow mixed sphagnum peat, with bulk density 0.15 and 97.3% organic matter, with fine, medium, and coarse sand with bulk densities 1.42, 1.47, and 1.59, to result in 0.51% and 1.20% organic matter by weight for a 10% and 20% by volume of sphagnum. It is not easy to mentally interpret these numbers except with formulae below.

McCoy (1992) reported the organic matter content of 70%, 80%, and 90% coarse and fine sand with 1.6 gram/cubic centimeter bulk density, and 0.07 and 0.06% organic matter, to which was added 30%, 20%, or 10% sphagnum and reed-sedge peat with organic matter content of 87% and 94%, respectively. Because the bulk density of organic sources was not reported, it was calculated from the formulae below, using least squares differences and the results are included in Table 1 along with other reported values.

Take the example of my friend who paid for an 80:20 mix which resulted in only 0.66% organic matter by weight. With either the formulae or the converter we can go backwards to solve for the bulk density of the organic material. If the 80% sand was a zero-organic matter sand with bulk density of 1.6 and the organic source was 95% organic matter, the starting bulk density would have had to have been 0.045 grams per cubic centimeter. This is outside the lower limit for the highest quality peat. More realistically, I suspect that a lower quality organic source was probably used, at much less than a 20% mix. For example, if 40% organic matter muck with 0.50 bulk density had been used at 5.1% it would have projected an estimated 0.66% organic matter in the mix, which is what was observed, notwithstanding aggressive fluffing by the vendor.

Recommendations

In specifying soil mixtures, whether in purchases and sales or in experiments, the organic matter content by weight should be indicated along with the volume mixture and the type of organic matter used. The USGA Recommendations for Putting Green Construction are to have 1% – 5% (ideally 2% – 4%) organic matter by weight; it is clear that McCoy (2013) was aiming at 1.5% by weight and McCoy (1992) was aiming at 3.5% organic matter by weight in the soil mix. For example, at a minimum, a sphagnum or reed-sedge peat would be specified as to the organic matter content, at least 85%, and bulk density of the peat would be measured at the time of blending. Ideally, the fiber content of the organic source would also be specified along with the characteristics of the sand source before blending, including organic matter content and bulk density.

In conclusion, when soil mixes are specified and blended, it is done by volume. That’s easy and looks more precise than weight because organic sources such as peat often contain a lot of water. Who wants to pay a high price for water? But if you go by volume, you pay a high price for air.

In contrast, when soils are assessed in the laboratory, it’s done by weight. If everyone used weight AND volume, it would make it easier to specify, and easier for the buyer to know what was purchased, and it would make interpretations and comparisons across experiments more meaningful.

Table 1. Physical characteristics of organic source used in turfgrass root-zone mixes bulk density, organic matter content by weight, and comments with references.
Organic source Bulk density Organic matter Comments

• – – – Representative values – – – – –
  Sphagnum peat 0.13 95% Median of below
  Reed-sedge peat 0.23 90% Median of below
  Muck 0.53 50% Median of below
• – – – Sources – – – – –
  Fibric and most histic 0.05 to 0.15 Lynn, 1974, reported by Lucas, 1982
  Sphagnum peat 0.08 87% Db derived from Taylor et al., 1997
  Sphagnum peat 0.07 to 0.11 95% to 99% Lucas et al., 1965, cited by Waddington, 1992
  Various peats 0.15 Range 0.05 to 0.23 per Price et al. 2015
  Canadian sphagnum peat 0.16 96% Db derived from McCoy 1992
  Canadian sphagnum peat 0.15 97.3% Bigelow et al., 2004
  Canadian sphagnum peat 0.13 93.4% Waltz et al., 2003
  Reed-sedge peat 0.17 94% Db derived from Taylor et al., 1997
  Michigan sphagnum peat 0.25 91% Db derived from McCoy 1992
  Reed-sedge peat 0.16 to 0.29 82% to 95% Lucas et al., 1965, cited by Waddington, 1992
  Compost 0.39 64% Db derived from McCoy 1992
  Reed-sedge peat 0.41 86% Db derived from McCoy 1992
  Muck 0.53 40% Db derived from McCoy 1992
  Peat humus 0.32 to 0.64 50% to 90% Lucas et al., 1965, cited by Waddington, 1992
  Florida histosols 0.26 to 0.73 Zelazny et al., 1974 reported by Lucas
  Okeelanta muck 54% Lucas, 1982
  Torry muck 45% Lucas, 1982

References

Bigelow, C. A., D. C. Bowman, and D. K. Cassel. 2004. Physical properties of three sand size classes amended with inorganic materials or sphagnum peat moss for putting green rootzones. Crop Sci. 44:900-907.

Lucas, R. E. 1982. Organic soils (Histosols) formation, distribution, physical and chemical properties, and management for crop production. Research Report 435, Michigan State University Agricultural Experiment Station, East Lansing. 77 p.

Lynn, W. C., W. E. McKinzie, and R. B. Grossman. 1974. Field laboratory tests for characterization of Histosols. In Histosols, Chapter 2, Soil Science Society of America Special Publication, No. 6, Madison, WI.

McCoy, E. L. 1992. Quantitative physical assessment of organic materials used in sports turf rootzone mixes. Agron. J. 84:375-381.

McCoy, E. L. 2013. Commercial amendments for sand-based root zones: Review and interpretation. HortTechnology 23:803-813.

Taylor, D. H., C. F. Williams, and S. D. Nelson. 1997. Water retention in root-zone soil mixtures of layered profiles used for sports turf. HortScience 32:82-85. USGA Green Section Staff. 1993.

USGA Recommendations for Putting Green Construction: The 1993 Revision. USGA Green Section Record. March/April 1993. 4 p.

Waddington, D. V. 1992. Soils, soil mixes, and soil amendments. pp. 331-383 in: in: Waddington, D. V., R. N. Carrow, and R. C. Shearman (eds.). Turfgrass–Agronomy Monograph 32, American Society of Agronomy, Madison, WI.

Waltz, F. C., V. L. Quisenberry, and L. B. McCarty. 2003. Physical and hydraulic properties of rootzone mixes amended with inorganics for golf putting greens. Agron. J. 95:395-404.

Zelazny, L. W. and W. H. Carlisle. 1974. Physical, chemical, elemental and oxygen-containing functional group analysis of selected Florida Histosols. In Histosols, Chapter 6, Soil Science Society of America Special Publication, No. 6, Madison, WI. pp. 63-78

Formulae for converting organic source by volume to organic matter by weight

If you prefer not to use the formulae, try the organic matter – organic source converter

The formula for organic matter in an organic source-amended sand blend is:

OM3 = (Db1 * r * OM1 + Db2 * (1-r) *OM2 ) / (Db1 *r + Db2 * (1-r))

where:

OM3 = organic matter content by weight of blend
OM1 = organic matter content by weight of organic source
OM2 = organic matter content by weight of sand
r = mixing ratio by volume
Db1 and Db2 are bulk densities or organic source and sand, respectively

If the sand has no organic matter, this can be simplified:

OM3 = (Db1 * r * OM1) / (Db1 *r + Db2 * (1-r))

And Db1 can be solved if we know OM3, OM1, Db2, and r:

Db1 = (OM3 * Db2 * (1-r)) /( (OM1 – OM3) * r)