Verdure: Dive into that microbial pool

A 15-year study reveals valuable insights on the long-term effects of N rate and source on several measures of turf soil health.


It has long been known that carbon and/or nitrogen (N) stimulate macro- and microbial activity in the soil. It’s a key part of discussions about the microbial pool, decomposition of plant residues and management of organic matter. With that discussion comes the idea that certain materials promote microbial activity over others, including organic fertilizers. But it is interesting that little research examines the effect of fertilizers on microbial activity, and research that does tends to be short-term (two years or so).

To really examine organic matter and the role of microbes in organic matter, one needs to look at things for a long time, as the systems often change slowly. That’s why this paper from Ohio State University scientists is so useful — it evaluated changes in nutrient pools over 15 years, using a Kentucky bluegrass home lawn as the research site.

There were nine “management programs” in this experiment, with N source, N rate, and additions of various commonly applied homeowner post-emergent and pre-emergent herbicides, insecticides and fungicides as treatments.

Within these nine treatments, three basic N fertilizer regimes were applied: 1) untreated control; 2) application of an organic fertilizer (commercial sources from Richlawn for the first nine years, then Ringer) to apply 4.4 pounds N/1,000 square feet/year (216 kilograms N/hectare) plus phosphorus (P) and potassium (K); or 3) application of a mineral fertilizer (urea) at a high (4.5 pounds N/1,000 square feet/year; 219 kilograms N/hectare), medium (3.5 pounds N/1,000 square feet/year; 171 kilograms N/hectare) or low (2.0 pounds N/1,000 square feet/year; 98 kilograms N/hectare) N rate, plus P and K. While the basic N source was typically urea, in some treatments it was applied as a liquid, and sulfur-coated urea was included in others.

Soil samples were collected (6 inches [15 centimeters] deep) three times during the 14th and 15th years of the study. One of the main objectives of the work was to examine nematode species and populations and their effects on the soil food web. However, for turfgrass managers who regularly want/battle organic matter (thatch) and carbon, the effects of the fertilizer treatments on these variables may be of greater interest. To quantify these, the scientists collected soil ammonium-N, soil nitrate-N, soil organic matter content and microbial biomass nitrogen.

So, which variables were not affected by 15 years of fertilization with organic fertilizers? Well, total nematodes, free-living nematodes and plant-parasitic nematodes were all unaffected by N rate (low, medium or high) or nitrogen type (none, organic or mineral). The inclusion or exclusion of herbicides, fungicides or insecticides never had an effect on any of the studied variables either. The authors noted that nematode populations (especially free-living) often increase in response to N, which was not seen in this long-term study. However, white clover invaded the control plots, and the nematodes may have simply used that nitrogen to maintain populations.

What was affected by years of organic fertilizer application? Both N rate and fertilizer type affected several measures of soil health, including microbial biomass N, dissolved organic N and soil organic matter.

The treatment with greatest impact was the fertilizer source. For example, at the final sampling in October 2004, any treatment that had received 15 years of N as the organic source had greater microbial biomass N than that measured in any treatment that had received the same amount of N as urea. That same treatment also had the highest soil organic matter (5.0%, as compared with an average of 4.5% in the other treatments). As an environmental concern, soil nitrate-N was unaffected by N rate or N source.

This research shows that long-term application of organic fertilizers did indeed promote soil microbial activity, and organic matter and soil microbial biomass N increased as a result.

Source: Cheng, Z., P.S. Grewal, B.R. Stinner, K.A. Hurto and H.B. Hamza. 2008. Effects of long-term turfgrass management practices on soil nematode community and nutrient pools. Applied Soil Ecology 38:174-184 (

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 22-year member of GCSAA.