INTRODUCTION

Pritchard et al. 1988 produced a study titled: ‘Landforming a red-brown earth - effects of topsoil removal and redistribution on irrigated crop production’

The summary is shown here:

Landforming is the use of earthmoving machinery and laser controlled land planes to improve the efficiency of irrigation layouts.  Duplex soils with shallow A horizons are vulnerable to the exporsure of subsoil in this process, and production from areas where topsoil has been removed is substantially less than from normal soil.

A landformed area with a large range of depths of cut (soil removed) and fill (extra soil deposited) gave linear decrease in fry matter (DM) yield/ha of maize (Zea mays) from 23.7 to 11.2 t/ha, and oats (Avena sativa) from 13.9 to 7.5 t/ha with increasing depths of topsoil removal, but no progressive yield increase with the added soil, so that overall production was estimated to be decreased by 16%.  Whole plant N concentration was 24% lower, plant Zn 33% lower and soil Zn 43% lower from areas where topsoil was removed.

Ameliorating the exposed subsoil with ripping, gypsum and compensatory N and P fertiliser produced only 71% of maize DM compared with normal soil (16.5 v 11.7t/ha).  However, the replacement of 75 mm of topsoil completely restored the yield.

In a pot experiment which removed many of the physical limitations of the field, maize yields from subsoil and topsoil were similar when Zn was added to the subsoil in addition to N, P and K.  Although Zn deficiency can limit maize growth on exposed subsoil, soil physical characteristics are a major cause of reduced yield and these were not eliminated in the field by amelioration.  The cost of topsoil replacement during landforming may be less, in the long term, than amelioration plus cumulative production losses.  The importance of topsoil in maintaining the yield potential of cropping land should not be underestimated in landforming designs for irrigation.

Download and read the whole article here.

The results can be summarized graphically in Figure 2 of the paper copied below.
From this figure, we can calculate the reduction in crop yield for different depths of cut in this situation for both crops and metric and imperial units.

So for a 10cm or 4 inch cut, you can expect and 15.3% reduction in oats yield and a 17.6% reduction in corn yield. This yield reduction is obviously different for different crops and soil types but it does provide some numbers to show the potential impact of reducing the cuts using advanced 3D landforming technologies such as OptiSurface (www.optisurface.com). Below is a case study to examine the benefits in reducing yield loss using the OptiSurface technology.

CASE STUDY

A 135 acre field in Arkansas was being upgraded from dry land production of corn and soybeans to promote improved yields through furrow irrigation. Below is the existing topography. We will assume the soil types are similar to Pritchard et al. 1988 which is not the case but it does allow a hypothetical comparison of traditional planar style laser grading and OptiSurface 3D landforming.

Figure 1.  Existing Topography

METHOD

Two landform designs were calculated: Single Plane of Best Fit and the OptiSurface 1Way (3D variable grade draining in one direction). OptiSurface Designer V2.3 was used to design the field with the following parameters:

Plane of Best Fit:
Main Slope
- Bearing: 90.1 degrees

OptiSurface 1Way:
Main Slope
- Bearing: 90.1 degrees
 - Minimum Slope: 0.05%
- Maximum Slope:  0.50%
- Smoothing Distance:  700 ft/%
Cross Slope
- Minimum Slope: -1.00%
- Maximum Slope: 1.00%
- Smoothing Distance:  700 ft/%

RESULTS

Figure 2. Proposed Topography and Cut/Fill Map for Plane of Best Fit and OptiSurface

Table 2. Summary of Earthworks Results

Assuming potential yields of 300 bushels/acre (18.8 t/ha) for corn and 120 bushels/acre (4.5 t/ha) for oats with prices of $5/bushel for corn and $3/bushel for oats, we can calculate the theoretical yield loss costs as shown in the table below.

Table 3.  Proposed Cut/Fill Table and Corresponding Yield Loss

CONCLUSIONS

Table 4.  Calculated Yield Loss and Cost

OptiSurface greatly reduces the topsoil disturbance compared to a traditional single plane of best fit. Single Plane design yield loss is up to 13.19% while OptiSurface 1Way design only lost 3.65%. This results in about $20,000 difference in yield loss per corn crop in favor of OptiSurface 1Way design.

REFERENCES

K. E. Pritchard, W. K. Mason and S. P. Byrne, 1988, Landforming a red-brown earth - effects of topsoil removal and redistribution on irrigated crop production, Australian Journal of Experimental Agriculture, 1988, 28, 599-605