Water Quality Benefits from Soil Health

Dr. Vinayak Shedekar, Ohio State Drainage Specialist shared additional information on water quality. Dr Shedekar showed improved soil health, cover crops, and no-till had both direct and indirect effects on water quality. A direct effect is increased soil organic matter (SOM) over time. The gain is slow, but on long term research plots, no-till plus cover crops increased SOM from 2.3% (conventional tillage for 75-100 years) transitioned to no-till and cover crops with 4.3% SOM after multiple years.

A research question was: Does no-till and cover crops improve water quality? Dr. Kevin King has 41 paired water quality sites with over 200 years of site data. He measures both surface flow and subsurface (tile) flow on these paired plots, comparing conventional tillage to no-till fields. Tile discharge was similar however less nitrogen (N) leaves the no-till fields and on soluble reactive phosphorus (SRP), there was no difference. No-till by itself had some gains for N but no difference for SRP.

Dr Shedekar published his own data (2021) for two test plots. One was a paired site between a cover crop and no cover crop. On no-cover (conventional), the tile discharge or water runoff was much higher and both both N and SRP was much higher. On surface runoff, there was not much difference in water discharge, N or SRP losses. At another site same design, there was a large differences with much higher tile discharge and slightly higher SRP losses on the conventional no-cover plots. The conclusion was that soil type plays a factor in the amount of nutrient runoff. It should be noted that this research was done in last 3-4 years and precipitation has been more subdued. Also, the size and planting date of the cover crop was a major factor in preventing nutrient runoff. Early planting and good growth give higher benefits.

As a comparison, Smith & Warnemuende-Pappas, 2015 reported similar information during a period of greater precipitation. They compared no-till plus a cover crop to vertical tillage. Their data showed 3X higher surface runoff of SRP and total phosphorus and 5X higher runoff of sediment (soil erosion) from vertical tillage compared to no-till plus a cover crop. No-till, cover crops, soil health, and water quality generally improve when these practices are used together.

Dr Shedekar recently started research at two sites in the last 2-3 years. One site is the David Brandt farm, a noted soil health farmer, recently deceased. The Brandt farm has a paired site with conventional tillage compared to no-till plus a cover crop on a clay loam soil. The Brandt site had higher SOM. Another NW Ohio site had the same comparisons on a sandy soil with lower SOM. Brandt farm had some different results in different years. In 2021, slightly more discharge of water but much less in 2022. However, the concentration of N (nitrates) and the total pounds or load of N lost was less on the Brandt farm with good soil health.

On the NW Ohio site, the conventional had the largest water discharge but slighter lower concentration of N(nitrates). However, total loads of N lost was much greater on the conventional plots. It appears no-till and cover crops help improve total N losses or load. Load is measured in pound of N leaving the farm and is the concentration of N times the water discharge.

On SRP, both sites had higher SRP concentrations on the soil health plots but both the total discharge of water and the total loads where much lower. Typically, under a soil health system, the solubilizing bacteria increase the concentration of SRP. Also, iron in the soil may react with the SOM to make SRP more available. Perhaps the good news is that on healthy soils using no-till and cover crops, less P fertilizer is needed? Also remember, the concentration may be higher but total loads or SRP lost is much lower. So overall, this may be a good thing and even save farmers money on fertilizer.

From all the research, soil erosion losses are much less with soil health practices. NRCS uses a theoretical number called T which is defined as a tolerable rate of soil erosion per year , about 4-5 ton per acre. Conventional highly tilled soils, especially on slopes, may lose 5 to even 20 tons of nutrient laden top soil per year. On no-till fields with a good growing early planted cover crop, the soil erosion rate is measured in pounds, 100 to 1,000 pounds per acre. The natural rate of new soil regeneration is about 1000 pounds or 0.5 tons per acre, so cover crops plus no-till build new soil.