Management Considerations for Eroded Cropland

Roger Veseth

Chapter 1 Erosion Impacts, No. 7, Summer 1987

Eroded ridgetops and sideslopes on Northwest cropland typically have markedly lower crop yields than toeslopes and bottomland. A combination of factors are often involved, such as: lower soil fertility, less available soil water, restricting soil layers, lower organic matter content and poor soil structure. Identification of the yield-limiting factors is important in determining management options needed to optimize production potential in those areas.

The impact of lower soil fertility has received some research attention over the years. In the 1960’s, Washington State University soil scientist Fred Koehler noted that additions of phosphorus fertilizer significantly increased yields on eroded ridgetops. However, the increased yields were still not comparable to yields obtained on the lower slope positions. Other factors were also limiting yields,

Landscape-Tillage System Study

In 1985, field plots were established near Pullman, WA, with the objective of characterizing surface and subsoil phosphorus levels and to identify yield-limiting factors across a Palouse landscape from the ridgetop to the toeslope. The study was conducted by STEEP researcher Bill Pan, WSU soil scientist, and Al Hopkins, graduate research assistant.

Karniak winter barley was seeded under tilled and no-till systems after spring barley on three landscape positions: ridgetop, sideslope and toeslope. The research plots were located on a southwest aspect with an average slope of 20 percent. The tilled treatment consisted of two tandem disk operations followed by two field cultivator harrow operations, A research no-till drill, with coulters and hoe seed-fertilizer openers on 12-inch spacings, was used to seed both the tilled and no-till plots. Fertilizer was banded 2 inches below the seed with the drill at the following rates per acre: 100 pounds nitrogen (N), 60 pounds phosphorus (P₂O₅ and 10 pounds sulfur (S).

Although the seedling top weight was lower under no-till than the tilled treatments (Table 1), no significant differences were found later at anthesis (flowering). No-till plots on the ridgetop had significantly greater grain yield than on the tilled plots. The researchers point out that improved water conservation was probably an important factor in the yield increase. Yields on the other two landscape positions were very similar under the two tillage systems. Yields with both tillage treatments consistently decreased from the toeslope to the ridgetop.

In an attempt to explain the reduced yields on the ridge tops and sideslope, the researchers conducted several investigations into possible yield-limiting factors. These included soil phosphorus availability, root depth limitations and soil water content.

Soil Phosphorus Availability

Soil test phosphorus availability (NaOAc extractable) showed a sharp difference from the toeslope to the ridgetop (Fig. 1). Phosphorus levels decreased with soil depth in all positions. The toeslope contained the highest phosphorus level with 14 ppm in the surface layer, decreasing to 4 ppm in the subsoil. In contrast, the sideslope and ridgetop positions contained 5 to 6 and 2 to 3 ppm phosphorus in the surface soils, respectively. Phosphorus content declined to less than 1 ppm in the subsoils on both landscape positions.

Analysis of plant samples at harvest revealed that percent phosphorus content in the residue (stems, leaves and
chaff) was not significantly different between any slope position or tillage system. Phosphorus content in the grain was significantly lower on the ridgetop and sideslope positions, however. Total grain phosphorus content on the ridgetop and sideslope positions was only about 40 and 70 percent, respectively, of the phosphorus content from the toeslope.

The lower yields on the ridgetop and sideslope seemed to be associated with the lower soil phosphorus availability and lower plant phosphorus uptake in the grain by harvest. This was despite the 60 pounds/acre phosphorus (P₂0J fertilizer application at seeding. The researchers theorize that when the phosphorus in the surface soil became unavailable as the soil dried during grain filling, a late-season phosphorus deficiency may have occurred because of the phosphorus-deficient subsoils.

Table 1. Comparison of plant weights and yields of Kamiak winter barley under tilled and no-till systems on three Palouse landscape positions near Pullman, WA, 1986 (Pan and Hopkins, WSU).

Yield Components

Low yields on the sideslope, even lower yields on the ridgetop, were associated with reductions in numbers of tillers, heads and kernels per head and kernel weight (Table 2). Little difference was noted between tillage systems. From the field study, the researchers concluded that subsoil phosphorus was severely limiting on the ridgetop and sideslope, and that water stress in the surface soil layer between anthesis and harvest may have accentuated a late season phosphorus deficiency stress.

Growth Chamber

The researchers conducted growth chamber studies on phosphorus fertilizer applications to soils taken from the three slope positions in the 1986 field study. The results help reaffirm the field research findings indicating phosphorus deficiency as a contributing factor in reduced winter barley yields on the ridgetop and sideslope, On soils from these two slope positions, the researchers determined that with the addition of phosphorus fertilizer: (1) top dry weight of seedlings increased when the initial soil phosphorus levels werebelow 2 ppm; (2) number of tillers perplant increased when initial soil phosphorus levels were below 3 ppm; and (3) rootlength increased when initial soil phosphorus levels werebelow 1 ppm, On soils from the toeslope, there was no or avariable response tophosphorus fertilizer. This would be expected because of the high soil testlevel.