No till (NT) agriculture presents some unique opportunities for reducing wind and water erosion and for improving crop productivity and economic returns. Higher surface residue in NT hinders planting, yet is essential for control of wind and water erosion. Heavy NT drills may compact the soil. Crop yield potential can increase through more efficient use of soil moisture, and thus can expand cropping options to the high and intermediate rainfall areas by allowing for more spring-grown crops. Reduction in tillage can increase soil organic matter and thereby increase soil quality. Many producers and researchers believe that soil quality improves under NT, but long-term documentation is needed. Our goal is to document changes in soil quality attributed to NT agriculture. We have evaluated soil physical, chemical, and biological properties in cooperating grower fields in NT operation for 10 to 25 years in comparison to adjacent conventional practices.
Findings and discussion:
Soil quality attributed to no-till (NT) agriculture in three agroclimatic zones in eastern Washington and northern Idaho was evaluated by assessing soil physical, chemical, and biological properties of long-term NT fields. Fields in this study have been in NT from 10-25 years. With the exception of the Pullman site, all fields are managed by the grower using field-sized equipment and conditions. Two sites were sampled and analyzed in 1996. In spring of 1997, four additional sites were identified and sampled. All NT sites were compared to adjacent land managed conventionally. Characteristics of the sites are listed in Table 1.
|Rainfall zone||Location||Production system/comparison|
|High||Palouse, WA||20+ y NT wheat-barley-pea vs. first year NT|
|Pullman, WA||16 y NT continuous wheat vs conventional|
|Colfax, WA||10 y NT mostly wheat, some lentil vs. conventional|
|Intermediate||Lewiston, ID||NT winter wheat (1980-92), 3 y rotation since vs. conventional|
|St. John, WA||15 y NT wheat-fallow vs. conventional|
|Low||Touchet, WA||14 y continuous NT spring wheat vs. wheat-fallow|
Soil physical properties:
Soil impedance (SI), the force (lb) required to push a probe of known area (in2) into the soil, was measured with a penetrometer to a depth of 20 inches. Bulk density (BD) of the surface soil layer (0-4 inch depth) was measured with a surface density gauge. At all sites, SI was greater in the no-till (NT) compared to the conventional tillage (CT) system, except at the Touchet location where a strong tillage pan in the conventional tillage field (wheat-fallow) was present (Fig. 1). The highest SI values were between the 4 and 8 inch depths in the NT systems, and are a reflection of heavy machinery pressures without any disruptive tillage. The SI values in this zone were greater than 250 psi and are at levels that can limit or restrict root growth. While SI values within the NT systems were greater, no significant differences in surface BD existed between tillage systems (data not shown).
Ponded infiltration was measured at the Palouse and Colfax sites at four locations within each tillage system. Infiltration rates on the NT exceeded CT infiltration rates by 20 to 40%. While the level of compaction (bulk density) may not be greater in the NT compared to the CT systems, higher NT infiltration rates indicate that larger pores and better pore continuity are present in the NT systems compared to the CT systems. Larger and more continuous pores within the NT systems may result from root channels and insect/earthworm activity that are not disrupted by tillage as in CT.
Soil Chemical Properties:
To determine the incremental changes in soil properties with depth, soils at the six sites were sampled from 0-2, 2-4, and 4-10 inches. Soil organic matter (SOM) at the surface 0-2 inch increment was statistically higher at five of six sites under NT as compared to adjacent conventional sites (Fig. 2). An increase of 50% SOM was noted at the Palouse site under NT for 25 years. Both the total SOM and the response to NT were highest at the higher rainfall sites in the annual crop areas. Only modest increases in SOM were noted at the Touchet site, which reflects the lower rainfall and lower crop residues produced. In some instances, the higher SOM observed for NT was evident only in the surface 2", whereas in the Palouse site under 25 years of NT, higher SOM was found down to 10 inches.
Particulate organic matter (POM) carbon, the partially decomposed sand-sized organic C fraction, was also higher under NT in the surface 2 inches which reflects the accumulation of residue at and just below the soil surface (Fig. 2). The POM fraction represents a slowly available reserve of soil nutrients such as N and P which may be released during the growing season and replenished from seasonal surface residue. Total soil organic N followed the same general trends as for total SOM, although the differences were less dramatic (Fig.2).
Fig. 2. Soil organic matter, particulate organic matter carbon, and total soil N at all sites. Graphs are arranged according to annual precipitation. Touchet receives 7-9 inches of rainfall and Palouse receives approximately 22 inches.
At three of the sites, soil pH was statistically lower under NT as compared to CT in the 2-4 inch zone which may reflect the depth of fertilizer placement (Fig.3). Soil pH was as low as 4.6 at the Touchet site and 4.9 at Palouse. Ammonium based fertilizers acidify soil pH through the process of nitrification to nitrate. Since the NT drills used at these sites places the fertilizer roughly in this zone, the pH effect may reflect the local area of fertilizer placement. At the Lewiston and St. John sites, however, soil pH was generally higher under NT, which suggests that different soil types and management play a role in determining soil pH.
Soil test P was statistically higher in three of the NT sites compared to CT (Fig.3). At most sites, the higher P levels were evident to a depth of 10 inches. The three growers at these sites use the Yielder drill and routinely band starter P. Soil K was statistically higher at three sites and likely reflects the placement of K fertilizer. While some relationships between soil test P, soil pH, and soil organic matter might be possible, the higher levels of P under NT at three of the sites and the higher P value for CT at Lewiston, likely represent differences in fertilizer management between growers.
Fig. 3. Soil pH, phosphate-P, and microbial biomass C at six sites. Graphs are arranged according to increasing annual precipitation. Touchet is the driest and Palouse is the wettest.
Soil biological properties:
Since soil microbial activity is often related to levels of SOM, higher activity might be expected where management practices increase SOM. Microbial biomass carbon, which is the weight of carbon in organisms such as bacterial and fungi, was statistically higher at the Colfax and Pullman NT sites at 0-2 inches (Fig.3). However, at the lower depths, microbial biomass was lower under NT as compared to CT for 5 of the sites. This trend, also observed for SOM and POM C at some of the sites, is probably due to the inversion of crop residues to lower depths under CT. Under NT, most of the residues remain at or near the soil surface.
Soil enzyme activity was estimated for two enzymes, phosphatase and b -glucosidase, which provide a general estimate of microbial activity. The enzyme activities were often higher at the surface under NT, but higher for conventional management at lower depths (data not shown). These results suggest the same trend as for soil microbial biomass where the amount of SOM in the soil profile determines microbial activity. In NT, the residues and SOM accumulate near the soil surface and support higher levels of microbial activity. At lower depths in NT, microbial activity may be higher under CT because of higher levels of SOM.
Most of the physical, chemical, and biological soil quality attributes were improved under NT. While penetration resistance increased under NT from heavy equipment, water infiltration increased, suggesting that larger pores and/or worm channels may be maintained under NT. Soil organic matter and most biological measures were enhanced under NT at all of the sites. Soil nutrients P and K were often higher under NT. Soil pH was significantly decreased at some of the sites, which may be due either to larger quantities of N applied or to the continued placement of fertilizers at the 2-4 inch soil depth.
We appreciate the cooperation of the following growers who provided land for the study: Tom Cocking, Mike Cronk, Duane Kjack, Frank Lange, Dick Lloyd, Roger Pennell, Bob Rea, John Rea, Rick Repp, and Garry Schwank. We also acknowledge the following for their input in site selection and design and interpretation of data: Jim Cook, USDA-ARS, Pullman; Tim Fiez, WSU Cooperative Extension; Roger Veseth, WSU and UI Cooperative
Extension; David Huggins, USDA-ARS, Pullman: and Steve Albrecht, USDA-ARS, Pendleton.