Stephen D. Merrill¹, Donald L. Tanaka, Joseph M. Krupinsky, Mark A. Liebig, John R. Hendrickson, Jonathan D. Hanson, and Ronald E. Ries²

Northern Great Plains Research Laboratory, USDA-Agricultural Research Service³, P.O. Box 459, Mandan, ND 58554.

1. To whom inquires may be addressed: 701-667-3016, merrills@mandan.ars.usda.gov

2. USDA-ARS retired.

3. The U. S. Department of Agriculture, Agricultural Research Service is an equal opportunity/ affirmative action employer and all agency services are available without discrimination.

Conservation tillage has allowed producers in the semiarid and subhumid Northern Great Plains to move away from small grain – fallow rotations to continuous cropping. The use of minimum-till and no-till weed control, combined with chemical weed control reduces evaporative losses of soil water and increases precipitation storage (Tanaka and Anderson, 1997). Continuous crop rotations with a diversity of species are essential for improving soil health and decreasing the impact of disease, weeds and insects. Diverse cropping systems will improve chances for economic survival of farm operators beset by input costs and market dynamics over which they have no control in the current political-economic system.

The design of diverse cropping systems requires information about the comparative water use of different crop species. Study of the timing and soil depth patterns of crop root growth helps us to understand where in the soil different crop plants can extract water and nutrients. Differences in water use and precipitation storage are associated with the amounts and structures of soil-covering residues generated by crop species and left on the soil surface after harvest. Soil coverage by crop residues is a prime determinant of wind and water erosion hazard, and is an immediate indicator of potential soil health outcome.

The objectives of this research were to determine comparative soil water use and conservation by a diverse array of crop species, examine correlations between observed crop root growth and water use, and observe soil surface coverage by crop residues.

Root growth was measured in a field study conducted during 1995-1997 that consisted of seven alternative crops grown in spring wheat – winter wheat – alternative crop rotations under minimum-till management (Study No. 1). Root growth was observed with a minirhizotron system (Taylor, 1987). Minirhizotrons are clear, plastic tubes installed in the field. Tube diameters were 2.0 and 3.8 inches. Roots growing against minirhizotron walls were observed with a microvideo camera fitted with viewing optics that magnify 10-fold or more (Upchurch and Ritchie, 1984).

Soil water content and soil surface coverage by crop residues were studied in a crop sequence experiment which is discussed in Krupinsky et al. (2002, this proceedings). The experiment was set up as crop by crop residue matrices consisting of 100 plots, which were obtained by growing a set of ten crops in strips in one direction, and then in a perpendicular direction the following year. Elementary plot size was 30 x 30 ft and each matrix block was replicated four times. Production-scale farm equipment was used under no-till management. Soil water content was measured with a neutron moisture meter in 1-ft depth increments to a depth of 8.5 ft. Soil coverage by crop residues was measured with a line transect technique.

All work described here has been carried out at the Area IV SCD – ARS Cooperative Research Farm near Mandan, ND. Soil at the site is classified as Wilton silt loam (fine-silty, mixed, superactive, frigid Typic Haplustolls).

Full season soil water depletion is a useful measurement for comparing the relative use of the stored soil water resource by alternative crop species. Full season soil water depletion values for 1999 and 2000 (Fig. 1) indicated that the oilseed crops safflower and sunflower depleted the most soil water. Dry pea depleted the least amount of water, and barley and crambe were the next lowest depleters of water on average. Precipitation in 1999 was above normal, while precipitation in 2000 was near normal. This is reflected in greater soil water depletions in 2000 than in 1999, as crop plants used less soil water in 1999. The relative pattern of soil water depletions was similar to the results with crop water use (Fig. 2), where crop water use is defined as soil water depletion plus precipitation during the growing season.

During the growing season there is a net depletion of soil water by crops in our semiarid/subhumid region. During the fall, winter, and early spring, there is a net recharge of soil water. Snow capture by crop residues and subsequent snowmelt is an important process. The amount of snow held varies considerably from year to year and among crop types, with no-tillage greatly enhancing snow capture. When snow depths were measured in Feb. 2001, residues of legume pulse crops soybean, dry bean, and dry pea were observed to have held less snow than the other crops because of the relative lack of standing crop residue (Fig. 3).

When soil water in the profile was measured after snowmelt in April 2001 (Fig. 4), the amounts of water were greater than amounts measured in fall 2000 (Table 1). Differences caused by different water usages by the crops over the 2000 season persisted in the spring-measured amounts. This persistence of differences in seasonal water depletion the next spring is shown by data in Table 1 comparing soil water use and recharge among sunflower, spring wheat, and dry pea.

The oilseed crops sunflower and safflower had the lowest amounts of spring soil water (Fig. 4). The greatest amount of soil water was measured in dry pea, which had 3.5 inches more water than sunflower. Thus, residual effects of seasonal soil water were apparently not overcome by differences in snow capture, especially for more the more heavy water-users, such as sunflower.

Table 1. Full season water depletion, fall soil water, snow capture, and spring soil water of

sunflower, spring wheat, and dry pea compared.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

May – Oct. 2000 Oct. 2000 Feb. 2001 April 2001

Crop Water depletion Fall soil water Snow capture Spring soil water

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

- - - - - - inches water - - - - - - inches snow inches water

Sunflower 5.9 25.0 9.1 28.0

Spring wheat 2.7 28.2 12.0 30.1

Dry pea 1.8 29.4 3.3 31.5

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Summary data for root length growth measured in Study No. 1 (Merrill et al., 2002) are shown in Fig. 5. The maximum and median rooting depths are shown for those dates when the root growth of the crops had reached greatest development. Median rooting depth is that depth at which half of the root length growth was above the depth and half was below it. Median rooting depths were approximately half of the maximum depths.

The results show that, although rooting depths varied somewhat from year-to-year, the relative differences in rooting depth among the crops did not vary greatly. The ten crops can be classified by rooting depths (Merrill et al., 2002); the oilseeds, sunflower and safflower root the deepest, whereas rooting patterns for pulse legumes, soybean, dry bean, and dry pea, are more shallow, less than 4 feet. Spring wheat and crops in the mustard family, crambe and canola, are intermediate in rooting depth. The most deeply rooted crops tend to deplete the most soil water (Figs. 1, 2, and 5; also Merrill et al., 2001a, 2001b).

Soil surface coverage by crop residues is important for conserving soil water from evaporative losses and for protecting the soil from wind and water erosion hazards. Soil coverage values were measured immediately after spring seeding following two previous seasons of spring wheat – alternative crop sequences Percent of soil coverage values were over 50%, reflecting the generally good residue conservation achieved in no-till systems. The lowest values on average were measured when the spring wheat was followed by legume pulse crops or sunflower. Sequences with dry pea and sunflower as the second crop had the lowest coverage values in spring 2000 and 2001 (Figs. 6A and 6B).

Low residue production by pulse legume or sunflower crops can be a soil health concern if these crops are grown consecutively. For example, the sequences dry pea-sunflower and sunflower-sunflower yielded next-spring soil coverage values in the range of 30% to 40% (Fig. 6C), which were somewhat marginal levels of soil protection. However, these results were observed under conditions of average to above average precipitation levels. Drought conditions coupled with back-to-back sequences of lower residue-covering crops will generate unacceptably low levels of soil protection. Crop growth is attenuated under drought; wind erosion hazard can occur, even under no-till management as used in our study. When climatic push comes to environmental shove, the only thing that will give some protection against significant wind erosion hazards encountered with lower residue-covering crop sequences would be moderate levels of soil roughness generated by hoe-type seed drills compared to very low disturbance levels of some disc- and coulter-type seeders (Merrill et al, 1999).

Krupinsky, J. M. , D. L. Tanaka, S. D. Merrill, J. R. Hendrickson, M. A. Liebig, R. E. Ries, R. L. Anderson, and J. D. Hanson. 2002. Crop sequences influence crop seed production and plant diseases. In Proc. 2002 Zero Till Association Workshop, Jan. 29-30, 2002. Manitoba – North Dakota Zero Tillage Farmers Association, Minot, ND.

Merrill, S. D., D. L. Tanaka, J. M. Krupinsky, and R.E. Ries. 2001a. Sunflower root growth and water use in comparison with other crops. p. 148-156. In Proc. of the 23rd Sunflower Research Workshop. Jan. 17-18, 2001. National Sunflower Association, Bismarck, ND.

Merrill, S. D., D. L. Tanaka, J. M. Krupinsky, and R. E. Ries. 2001b. Safflower root growth and water use in comparison with other crops. p. 227-231. In Proc. Vth International Safflower Conference. July 23-27, 2001. Montana and North Dakota State Universities and USDA-ARS, Williston, ND and Sidney, MT.

Tanaka, D. L., and R. L. Anderson. 1997. Soil water storage and precipitation storage efficiency of conservation tillage systems. J. Soil and Water Cons. 52:363-367.

Taylor, H. M. (ed.) 1987. Minirhizotron observation tubes: Methods and applications for measuring rhizosphere dynamics. Spec. Publ. 50, ASA, CSSA, and SSSA, Madison, WI.

Upchurch, D. R. and J. T. Ritchie. 1984. Battery-operated video camera for root observations in minirhizotrons. Agron. J. 76:1015-1017.

We thank D. Wetch, J. Hartel, C. Flacker, M. Hatzenbuhler, D. Schlenker, M. Tokach, C. Klein, J. Bullinger, and L. Renner for technical assistance.