How Much Does Diversification Improve Soil Water Holding Capacity?

Overview

Practical Farmers of Iowa and the Iowa Learning Farms initiated a long-term on-farm coordinated strip-trial where-in farmers committed to keeping an area of their farm in this research program for 10 years. The experiments consisted of two treatments: use of an over-wintering fall-planted rye cover crop, and a control with no cover crop. All other management was consistent between the plots, but was unique to each farmer's production system. Additionally, the USDA has been managing long-term cover crop trials for the past 20 years. By leveraging these long-term experiments, I had access to taking measurements in plots that would exhibit long-term effects of cover-cropping - a unique opportunity in this world of 3-year research grants. I am very thankful for this access, and consider management of long-term plots to be a public service.

Site Descriptions

In the spring of 2019 I visited two farmer fields (both in corn/soybean rotations) and two USDA managed fields (one corn/soybean rotation, the other a corn silage/soybean rotation). One farmer field was located in Jefferson County Iowa, the other in Washington County. The USDA fields were both in Boone County, and were immediately adjacent to each other.

Soil Sampling

Soils were sampled for both weed seed germination and water-holding capacity measurements. I chose to take soil samples immediately before or after planting to minimize the amount of root material in the soil samples. All fields were no-till, so the previous year’s planting rows were still apparent; samples were taken between planting rows in the middle of each plot.

For the weed seed measurements, we used PVC pipe. For the water-holding capacity measurements, we used specialized equipment used that is designed to sample bulk densities of soils. A hole 10 cm deep and ~1 foot wide was dug. A metal tube held a spacer, the sampling ring, another spacer, and was placed in the hole and capped with a lid. The sampling ring was struck with a heavy flat weight to ensure the ring entered the ground evenly. Once the top of the ring was flush with the hole surface, the surrounding soils was dug out and a spatula was used to scoop the entire metal tube out from the soil. The spacers and sampling ring were removed from the metal tube. The soil was ‘cut’ flush by using a knife to cut through the slot between the spacer and the sampling ring. Care was taken to ensure the ‘up’ and ‘down’ orientation of the soil sample was maintained. The soil was inspected to ensure it was free of rocks, worm, or other foreign material that would affect the pore space in an un-representative way.

Once the spacers were cut away, the metal ring containing the intact soil core was wrapped in aluminum foil, labeled, and stored in a cooler.

Determination of Soil Water Holding Capacity

The pressure cell method was used to determine the amount of water held by the soil at pressures ranging from atmospheric to 500 cm H2O.

Core preparation

All soil cores had 4 layers of cheese cloth taped to the bottom with electrical tape. The electrical tape completely covered the edges of the cheese cloth to ensure water could only enter the core from the bottom. Additionally, an empty sampling core was taped to the top using electrical tape to ensure water could not enter from the top of the core.

Core saturation

Prior to being placed in the pressure cells, the soils were saturated. The amount of water needed to saturate the soil is used to calculate pore space, therefore it was important to ensure there was minimal air trapped in the soil during saturation. To achieve this, the cores were placed in a vacuum chamber. A solution of 0.01 M CaCl was added until the level reached the bottom of the empty sampling core taped to the top of the soil sample. A vacuum was applied to remove air from the water. The samples were left in the vaccum chamber for 24 hours.

Pressure cells

After saturation, the empty soil core taped to the top of the sample was removed and the entire saturated sample (metal ring, soil, cheesecloth, electrical tape) was weighed in a tared sour-cream container. The sample was immediately moved to the pressure cell footprint with a previously-weighed cylinder placed beneath it. The weight of the water remaining in the sour cream container was recorded.

The samples were allowed to drain at atmospheric pressure (2.5 cm H2O) for 12-24 hours. The cylinder and water were weighed, the water was dumped into the sink, the remaining water blown out with compressed air, and the empty cylinder was returned to the funnel draining the pressure cell.

The samples were enclosed using heavy-duty plastic rings sealed to the pressure cell’s floor with vaccum grease. The top was sealed using a heavy-duty plastic plate that was connected to a pressurizing system with plastic tubing. The top was secured using wingnuts. Eight samples were processed at a time, and I chose to do 8 from a single site.

Air pressure was applied to the sealed chambers containing the soil samples in an increasing manner. After the indicated duration, the weight of the water released at that pressure was recorded, dumped, and the empty cylinder was returned to the pressure cell outlet funnel. The schedule of pressures is:

1. 10 cm H2O, 24 hours
2. 25 cm H2O, 24 hours
3. 50 cm H2O, 24 hours
4. 100 cm H2O, 24 hours
5. 200 cm H2O, 48 hours
6. 500 cm H2O, 48 hours

After the final pressure application, the sample was weighed then placed in an oven at 60 deg C for 1 week. The dried soil was removed from the sample ring and weighed. Additionally, the sample ring and the cheesecloth/tape were weighed and recorded.

Data processing

Data was processed to obtain a value for the volumetric water content of the soil sample at each pressure, together creating a soil-water-retention-curve for each sample. The calculations are available using an example dataset on a github repository (https://github.com/vanichols/ghproj_sare18) and will be more easily available on my research website once I build it.