Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

5/26/04

Contact:
Brian Lang, Extension Crop Specialist, bjlang@iastate.edu, (563) 382-2949
Mahdi Al-Kaisi, Extension Soil Specialist, malkaisi@iastate.edu, (515) 294-1923
David Gibney, NRCS District Conservationist, Clayton County, (563) 245-1048, ext. 3
Luann Rolling, NRCS District Conservationist, Allamakee County, (563) 568-2246

Torrential Rains and Erosion -- What's the Real Loss?

DECORAH, Iowa -- Recent heavy rains and erosion have raised concerns about the impact of bad weather on this year's crops, with replanting and lost growing days the commonly cited losses. But that erosion is a particularly serious problem for northeast Iowa, says Brian Lang, crop specialist, Iowa State University Extension

"We're very susceptible to erosion," says Lang, who works in Decorah. "Relative to the prairie soils across most of the rest of the state, we have a higher percentage of row crops on hilly terrain, with a high percentage of land farmed on 5 to 14 percent slopes with less organic matter. Compared to most row-crop production in the rest of the state at 2 to 9 percent -- our slopes run pretty steep."

"Also, our soils used for row crops are mostly 'wind-deposited' loess soils, which are more easily disturbed and moved by rainfall as opposed to the glacial-deposited loam soils located farther west and in central Iowa, "he said. "And while dairy cow numbers have remained fairly constant, more dairies use local land for corn silage and buy hay from elsewhere. So we've seen significant movement of erodible land out of forage production and into row-crop production."

"Now, in light of all that, our hilly terrain held up better than I expected," Lang said. "That can be attributed in part to the fact that many farmers have moved to conservation tillage systems. Those who didn't, lost a lot of soil, which is a shame -- those losses could have been minimized with conservation tillage systems."

Mahdi Al-Kaisi, soil specialist, Iowa State University Extension, says soil erosion seriously impairs soil productivity, and that no amount of 'management' (e.g.: nutrient inputs) can compensate for the lack of conservation practices and residue cover in rain events similar to the one we experienced this week.

" The vertical cross-section of soil (or, the soil profile) is divided into three parts: topsoil (A horizon), subsoil (B horizon), and parent material (C horizon)," says Al-Kaisi. "Topsoil is enriched with organic matter, which provides larger soil pores, reduces soil density and enhances water infiltration and aeration. When topsoil erodes, yield suffers due to organic matter and nutrient loss as well as the damage done to soil physical properties."

"Further, as crops mature, roots extend through the topsoil layer into the subsoil, seeking water and nutrients. Unfavorable subsoil properties like coarse sand and gravel, shallow depth to bedrock, and high soil densities and clay content in excess of 42 percent, can limit root elongation, development and yield."

Al-Kaisi says that while 'soil fertility' is vital to a productive soil, a 'fertilized soil' is not necessarily a productive soil. "Nearly all organic matter is located in the topsoil. Approximately 50 percent of plant-available phosphorus is concentrated in the topsoil as is nearly all of the plant-available potassium. Losing topsoil to erosion contributes to a loss of inherent soil fertility levels. While manure and fertilizer can supply needed crop nutrients and help offset some loss of inherent fertility caused by soil erosion, the productivity of eroded soils can only be restored by adding inputs only when favorable subsoil material is present."

But Al-Kaisi says that productivity lost by excessive soil erosion cannot be restored with additional inputs when subsoil material has unfavorable physical and chemical properties for plant root growth. "For soils that have unfavorable subsoils characterized by limited rooting depth, coarse sand and gravel, or high densities, there is little or no ability to recover yield losses with increased inputs. The loss of yield potential is devastating and final."

Lang says that soils in northeast Iowa evolved from deciduous forest growth. "We inherently have less topsoil development than soils formed under native grasses, so our soil is rather shallow when compared to that of most of Iowa's prairie soils. That makes conservation tillage strategies just that much more important in minimizing soil erosion in northeast Iowa."

" While some argue that 6-inch rainfalls will cause erosion no matter what, the key is to identify strategies that minimize the impact of these heavy rainfall events. Conservation tillage with waterways and contour buffer strips would certainly keep more soil on the land and less silt in the streams. Corn silage fields that leave little residue after harvest can be seeded down with a cover crop after the silage is harvested. Soybean fields rarely need tillage after harvest or before planting."

Natural Resource Conservation Service (NRCS) District Conservationists David Gibney (Clayton County) and Luann Rolling (Allamakee County) say there is financial assistance available to farmers through many federal and state conservation programs. Riparian buffers, filter strips and newly established waterways are all eligible for payments under the Conservation Reserve Program with other federal and state programs. Landowners or farmers who have not visited with a local NRCS conservationist about their farm's conservation plan should do so as soon as possible.

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