Hybrid grass roots out soil salinity.

Hybrid grass roots out soil salinity

Soil salinity, the bane of irrigated agriculture, is poisoning an increasing number of croplands throughout the world (SN: 11/10/84, p. 298). But an Agricultural Research Service soil scientist has stumbled onto a low-cost reclamation scheme for affected soils: Plant them with a special hybrid forage grass.

Though salt can usually be leached out of the soil and washed away by applying enough water onto the top of an affected field, that may not be sufficient to restore crop productivity, explains Charles Robbins of the Agriculture Department's Snake River Conservation Research Center in Kimberly, Idaho. "If you remove the bulk salts from the soil without replacing the sodium,' he points out, "you could destroy the soil structure.' Such compacted soils lose their permeability to air and water, making it tough growing for roots.

One way to prevent soil collapse is to substitute calcium for sodium in the soil. And while measuring the respiration of crop roots last year, Robbins and his colleagues identified a commercially available forage grass that will aggressively promote such a calcium substitution because of the unusually high carbon dioxide (CO2) output of its roots.

Roots take in oxygen and give off CO2. In moist soil, the CO2 will combine with water to form carbonic acid (H2CO3). Robbins points out that the acid dissolves any lime (calcium carbonate) in the soil, making its calcium available for sodium substitution. Though he studied a brandname forage-grass hybrid of sorghum and sudangrass, Robbins says any hybrid of those grasses should yield a similar CO2 production rate from its roots.

When growing vigorously in moist soil, the hybrid produced roughly two to three times as much CO2 as did cotton, barley or alfalfa. More important, there was twice as much sodium in drainage exiting the instrumented growing pots in which the hybrid had been raised as from those in which either alfalfa or barley had been grown, and three to four times more sodium from the hybrid than from cotton.

To reclaim saline soils in some areas, farmers now add between 10 and 50 tons per acre of gypsum, a mineral form of calcium sulfate. The $65 to $70 per ton it costs farmers to buy this soil amendment doesn't account for the costs of transporting or applying it, Robbins says. Moreover, he notes that with gypsum, a growing season might also be lost while farmers wait for it to become adequately dispersed through the root zone of the soil. In contrast, Robbins says, seeds for this hybrid grass might cost only $3 or $4 per acre, the fertilizer another $20 to $30. Yet at the end of the growing season, farmers could have a harvestable crop worth $300 to $500 per acre, he says.

In a limited "worst-case' trial in south central Idaho last year, the hybrid grass was planted in soil so saline it would support neither corn nor barley. "And we got 25 tons an acre' of the grass, Robbins reports. This year, he says, sodium removal tests are being conducted under more typical field conditions.

In the laboratory, he plans to delve deeper into the operant chemistry. It is possible, he says, that formic and acetic acids--which can also form when the grass grows--are even more effective than carbonic acid at liberating calcium for sodium substitution, and hence at helping reclaim saline soils.