Achieving Solid Soil Structure Is Keystone to Healthy Soils

One important compound worth mentioning is glomalin, a sticky glycoprotein that facilitates the formation of water stable aggregates (Wright and Upadhyaya, 1998). Without this compound, the ability of soil to form aggregates would be severely limited. Adding clippings, compost or degraded manure will increase the amount of these compounds and promote aggregate formation. In short, you're "feeding" the soil microorganisms in order for them to facilitate soil aggregation.

While adding organic matter can feed soil microorganisms, the application of artificial nitrogen fertilizer can alter their activity. Excessive or over-fertilization may inhibit the activity of these important organisms. The glomalin producing microorganism is actually arbuscular mycorrhizae (AM), a fungus that forms a symbiotic relationship with plant roots. Plants supply the mycorrhizae with energy (i.e., carbon) and the mycorrhizae supplies plants with nutrients mined from organic matter.

If grasses have excess amounts of nutrients, they generally allocate more photosynthate to leaves and less to roots or AM fungi (Johnson et al., 2003). In regards to soil structure, research shows that adding just 100 kilograms of nitrogen per square hectare (kg N ha-1) is known to increase the growth of AM fungi that excretes glomalin (Wilson et al., 2009).

However, another study found that adding 200 kilograms of kg N ha-1 can decrease the AM fungus that produces glomalin, but not always (Treseder et al., 2007). Nevertheless, a general rule of thumb is excessive fertilization will reduce AM fungi biomass (Johnson et al., 2003), and thus could reduce the production of glomalin and the formation of soil aggregates.

Jared L. DeForest, Ph.D., is an assistant professor of soil ecology in the department of environmental and plant biology at Ohio University. He can be reached at deforest@ohio.edu.

REFERENCES

Brady, N. and Weil R. 2008. The Nature and Properties of Soils. 14 ed. Prentice Hall, Upper Saddle River, NJ.

Díaz-Zorita, M., Perfect E., and Grove J.H. 2002. Disruptive methods for assessing soil structure. Soil and Tillage Research. 64, 3-22.

Wright, S.F. and Upadhyaya, A. 1998. A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi. Plant and Soil. 198, 97-107.

Johnson, N.C., Rowland D.L., Corkidi L., Egerton-Warburton L.M., Allen E.B. 2003. Nitrogen enrichment alters mycorrhizal allocation at five mesic to semiarid grasslands. Ecology. 84, 1895-1908.

Wilson, G.T.W., Rice C.W., Rilling M.C., Springer A, and Harnett D.C. 2009. Soil aggregation and carbon sequestration are tightly correlated with the abundance of arbuscular mycorrhizal fungi: results from long-term field experiments. Ecology Letters. 12, 452-461.

Treseder, K.K., Turner K.M., and Mack M.C. 2007. Mycorrhizal responses to nitrogen fertilization in boreal ecosystems: potential consequences for soil carbon storage. Global Change Biology. 13, 78-88.

Riederer, M. and C. Muller. 2006. Biology of the Plant Cuticle. Blackwell Pub., Oxford, Ames, Iowa.

Schönherr, J. 2006. Characterization of aqueous pores in plant cuticles and permeation of ionic solutes. Journal of Experimental Botany 57:2471-2491.

Totten, F.W. 2006. Long-term evaluation of liquid vs. granular nitrogen fertilization in creeping bentgrass. Ph.D. Dissertation. Clemson University, Clemson, SC.

Totten, F. W., H. Liu, L. B. McCarty, J. E. Toler, and C. M, Baldwin. 2008. Efficiency of foliar versus granular fertilization: a field study of creeping bentgrass performance. J. Plant Nutri. 31(5): 972-982.

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