This two year long soil health and grazing project has confirmed that in the Midwest region on diversified pastures of cool season grasses and forbs the act of grazing by sheep or cattle will stimulate an increase in microbial life in the soil as indicated by CO2 levels measured with the basal Solvita test. This increase of microbial life occurs within 12 to 48 hours of the grazing event and the production of CO2 returns to neutral or even slightly negative by 7 days post grazing.
Our 2014 trial was focused on comparing the impacts on soil health of grazing immature (before flowering) vs late maturity (after seed head formation) forages. In addition, we repeated our soil health impact on a mid- summer grazing of 2nd growth pastures and a fall/ September grazing. We had three plots in each experiments; control, (nothing done), a mechanically clipped plot, and our grazed plot and did 3 replications, all soil sampling done on day 7 post grazing. We did both Haney and PLFA soil testing on all soil samples. The only consistent finding was that on the average, the grazed plot showed lower scores on the various test results indicating soil health, Haney soil health score, Solvita CO3 levels, and PLFA total biomass.
The results were not expected and further literature reviews suggests that the soil life will respond to grazing simulated event in 12 to 72 hours. We only tested at 7 days post grazing and may have missed changes that occurred sooner.
While most of our SARE funds were used in the 2014 activities, we did have about $700 left over from unused labor funds and decided to do a trial that looked at the impacts of soil heath post grazing over time. While we had hoped to look at impacts on soil health of various grazing practices, it appears that we first need to validate a positive impact of grazing on soil health indicators.
Our 2015 trial design was to do a basal Solvita test and nitrate level test 0, 12 hrs, 24 hrs, 48 hrs, 3 day, 5 day and 7 day post grazing on both a control and grazed plot. We also did a PLFA on the 48 hr sample. We did 3 different time periods: an immature first grazing with sheep, an immature 2nd grazing and a mature 2nd grazing with cattle. While we had the testing funds to do three replications in the 2014 trial we only did one test per time period in 2015. We also limited the kind of tests we did to both save funds and focus on identifying soil health changes due to grazing activity. We did a basal Solivta test instead of a burst Solivita test. The burst test as done in 2014 is done in a laboratory (as part of Haney test) with the soil sample being dried and then re-wetted and incubated at fixed temperature. This standardizes the results and takes out environmental impacts. The basal test is done on farm and is a representation of what is actually happening in the soil. We added a nitrate test as it was very inexpensive, about $5 per sample, and should indicate the positive outcome of increased soil life activity. In our first trial we did not do a 0 hr sampling but realized that since the trial plot had been grazed for 24 to 48 hrs before we removed the animals, a 0 hr sample could really be 24 hours or more since the plants had been grazed and added the 0 hr sampling in the 2nd and 3rd replications.
Comments on Solvita results: The standard procedure to do in “in field” basal Solvita test is to collect about ½ a jar of soil, put in an indicator paddle, and then do a color change evaluation of the paddle at 24 hrs. We were able to borrow a digital reader to actually put numerical values to these color changes. We started the trial 10 days before our first completed sampling sequence because in our first attempts to measure differences between grazed and control we were getting both control and grazed samples maxed out in color change. After discussion with Wood’s Laboratory personnel and trial and error, we decreased the sample size and also did our color change readings at 5 hrs post sampling. Our soil is very healthy with abundant soil life and it took less soil and less time to generate the required amount of CO2 to activate the Solvita test paddle. While there is some variation within each sampling period (more replications per period would have been useful) there is a fairly clear trend on the 3 trial average to see increased CO2 production via the Solvita test that would indicate increase soil life/activity in the 12 and 24 hr period post grazing. In addition, these results are also consistent with the 2014 trial that showed less soil life activity at the 7 day mark. The 7 day test window for the 2014 trial was determined by visiting with various “experts” and I think emphasizes the general lack of knowledge of how the soil life responds to grazing. These results also reinforce the principle that to capture the impacts of grazing on the short term indicators of soil health, activity of life in the soil and their by-products, i.e., CO2 in this case, it is very important that all sampling be done at the same time to compare treatment vs control. A before and after sampling routine will be subject to variations in temperature, soil moisture, etc.
PLFA Total Biomass results
PLFA discussion: Due to cost, about $40 per sample, we did a limited number of PLFA tests but wanted to have some numbers to compare with our Solvita results. The grazed average is 4% greater than the control but the lack of replications is a serious limiting factor to making any conclusion. Two items are worth noting: one is the trend to increased microbial biomass which follows changes in soil temperature. On the day we collected soil for the PLFA tests the soil temperatures were: 1st sample- 55 degrees; 2nd sample 59.5 degrees; and 3rd sample was 67 degrees. The second is that all samples were from average to above excellent in amounts of “total living microbial biomass”. This is consistent with the results from last year and is another indication of the general “very healthy” condition of our pasture soils.
Nitrate discussion: The one very interesting finding with the nitrate tests is the consistently low results. While both Solvita and the PLFA tests would indicate a “healthy” soil, the amount of nitrate available is very low. Dr. Manske suggested he would like to see 100 pounds of nitrate in July in his grazing lands. In discussion with Wood’s laboratory, their only suggestion is that any free nitrate that is produced by the microbial action is immediately taken up by the pasture plants. The other two trends in the nitrate results are that the grazed samples do have consistently more nitrate than the control samples. There was no correlation with the Solvita test in this set of data but a larger sampling may show an increase in the nitrate with a lag time to the Solvita results as this data set does show more nitrate in grazed vs control at the 3, 5, and 7 day points. The other interesting observation is that the differences are greater in the earlier and decrease with time. Is this due to differences in soil temps, or stage of plant maturity or other? There are always more questions, just as the questions from our results in 2014 led us to a continuation of this project in 2015.
The results of our 2014 portion of this project, complete with tables and discussion are found in the file “2014 Results”
Impact of Results/Outcomes
I think this 2 year project of field research and literature review has been very useful in finding out how some of the soil health testing procedures can be used to monitor changes in soil health. We started out wanting to verify that grazing immature plants would stimulate more soil activity than grazing mature plants but we ended up just trying to verify a way to measure the impact of grazing on soil health. It is important to note that we were looking at ways to monitor short term changes in soil health due to specific management events, and not long term changes like % organic matter which could have a many contributing factors.
Educational & Outreach Activities
In addition to the five presentations in 2015, the results and subsequent questions from our 2014 project were shared in two consecutive monthly articles with the readership of the GRAZE publication which covers mainly the midwest but has subscribers across the U.S. Our results from the amended project in 2015 were summarized and presented in a 90-minute presentation at the GrassFed Xchange national conference in September of 2015. In addition, those results will be presented in an hour long session at the GrassWorks Conference in WI January 16. During a trip to Africa in February/March of 2016 a day-long meeting with local farmers in South Africa and a conference in Kenya will provide an opportunity to share these findings beyond the US.
While grazing management is often looked on as an “art” as much as it is a science, the more we can find numerical unbiased tests to evaluate our grazing management practices, the better we will be able to select and recommend various grazing practices. Traditional grazing productivity measurements are usually animal gain or forage production and soil health is often measured as long term changes in organic matter, however, these measurements either take years to show results or have many potential conflicting contributing factors. If we can assume that soil that is “healthier”, has increased microbial activity and is therefore more productive, then using the Solvita test can help us sort out the value of various grazing practices. For example – when to graze a plant, how much residue to leave, is mob grazing really more beneficial and if yes, why?
Our Farmer and Rancher SARE grant has given us considerable grazing and soil health information and stimulated a lot of literature review but we have just barely scratched the surface of applying current knowledge and the new soil health testing methodologies available to the very complex interaction of grazing, plant life, and soil life. We would recommend that university researchers do more grazing projects that utilize two different grazing management practices and monitor the impact on soil health. We also need more information on the tangible benefits of “increased soil health”, i.e., will “better soil health” grow more grass with the same amount of rain and temperatures? What is “better soil health” and how does it vary by geographical region, soil type, etc.