Department of Agricultural and Biosystems Engineering

CONSERVATION TILLAGE - EFFECTS ON WATER QUALITY

ISU Extension Pub # AE-3051
Author: Mr. Vince McFadden, Field Specialist - Agricultural Engineering
Department of Agricultural and Biosystems Engineering, Iowa State University
Date: 11/90
Content reviewed 4/95 by Dr. Mark Hanna

Conservation tillage practices have a major impact on water quality in addition to reducing soil erosion. Sediment and chemicals(pesticides and plant nutrients) are the two main types of contaminants affected by conservation tillage.

Sediment is the largest pollutant by volume of surface water in Iowa. Most sediment comes from agricultural sources. Sediment increases the turbidity of water, thereby reducing light penetration, impairing photosynthesis, altering oxygen relationships, and reducing the available food supply for certain aquatic organisms. It can destroy fish populations in areas where sediment deposits cover spawning beds. Increased sediment also fills lakes and reservoirs.

The reduced soil erosion from conservation tillage systems, compared with moldboard plowing or conventional tillage systems, beneficially decreases the problems associated with sediment in water.

Conservation tillage practices also affect the chemical loses in surface runoff water and sediment. Surface drainage water and sediment carry dissolved nutrients and pesticides.

The largest quantities of total P and usually of total N lost from Iowa fields are associated with sediment in runoff water (see table 1). If chemical concentrations in sediment were constant, decreasing soil losses by one-half with conservation tillage practices would decrease chemical losses from sediment by one-half. However, the finer, more chemically active clay particles are more easily eroded than larger, less chemically active soil particles. Therefore, a reduction in soil erosion will reduce the losses of chemicals attached to sediment, but not quite proportionally.

Conservation tillage usually reduces the amount of runoff, but the amount of reduction is highly variable. Less runoff generally means less chemical loss, but this depends on the timing and duration of rainfall.

Conservation tillage practices sometimes have been falsely criticized for increasing chemical contamination of groundwater for two main reasons: increased herbicide use and increased leaching.

One theory is that more herbicide is required for weed control in conservation tillage compared with conventional tillage systems. But in Iowa, recommended herbicide rates are the same for all tillage systems. Sometimes a burndown herbicide is used in conservation tillage to control vegetation instead of mechanical tillage, but such herbicides usually are strongly adsorbed by the soil and therefore resist leaching.

Some no-till farmers have eliminated the need for a burndown herbicide by applying split applications of herbicides. The first application is put on before most weeds start to germinate and the second application is either a preemergence or a postemergence herbicide. This practice still requires spraying the field twice, but generally the total amount of herbicide used is the same as for one application.

Another criticism is that increased groundwater contamination will result from increased leaching through macropores, particularly for no-till. Macropores are small, open channels in soil that are created by earthworm activity, soil cracking, and root growth. Macropores are more prevalent in no-till fields because tillage mechanically mixes the upper soil profile, thus disturbing macropores. These macropores can allow larger quantities of water to infiltrate faster in the soil. This increased infiltration creates concern over increased leaching chemicals.

However, studies have shown that macropores can be beneficial if they allow water to bypass chemicals adsorbed within the top few inches of soil. This is highly dependent on how strongly the chemical adsorbs to the soil and the timing and duration of rainfall after chemical application. Macropores may allow highly soluble chemicals to infiltrate deeper and faster in the soil if a heavy rain occurs shortly after application.

One experiment near Ames compared the movement of NO3-N through no -till and moldboard plowed field plots after two simulated rainfalls. The first simulated rainfall was a 5-inch rain followed the next day by a 7.5-inch rain. Soil samples were taken after each rainfall to measure the NO3-N remaining in the soil at various depths. Table 2 shows that more NO3-N remained in the no-till field plots than in themoldboard plowed field plots. This test helps show that macropores do not always cause increased No3-N leaching.

There is increasing concern over traces of herbicides detected in water samples taken from wells and drainage tiles. Preserving the quality of groundwater is vital, but one must consider the magnitude of the herbicide concentrations detected in groundwater and subsurface runoff compared to those measured in surface runoff.

Surface water is a source of drinking water for some Iowans, so it is important to protect it. Herbicide concentrations in sediment from a treated field often exceed 1 part per million (ppm), particularly for the first rainfall after application. Herbicide concentrations in surface runoff water also can exceed 1 ppm for herbicides that are not strongly absorbed if rainfall occurs shortly after application.

By comparison, herbicide concentrations found in groundwater or in water collected from drainage tile flow usually are much lower than for surface runoff. These concentrations generally range from below the detection limit to 1 part per billion (ppb). Higher concentrations have been detected, but usually point sources of contamination were suspected in these situations. This means that, with normal agricultural practices, herbicide concentrations in surface runoff shortly after application are about 1,000 times greater than those detected in groundwater.

In conclusion, reducing the amount of tillage on Iowa's valuable soils will help maintain the quality of the state's land and water resources.

For more information on conservation tillage systems, see the following publications:

AE-3049 Conservation Tillage--Planning
AE-3950 Conservation Tillage--Effects on Soil Erosion
AE-3051 Conservation Tillage--Effects on Water Quality
AE-3052 Conservation Tillage--No-till Systems
AE-3053 Conservation Tillage--Ridge-till Systems
AE-3054 Conservation Tillage--Fertility Practices and Equipment for No-till and Ridge-till
AE-3055 Conservation Tillage--Cultivators for No-till and Ridge-till
AE-3056 Conservation Tillage--Planters for No-till
AE-3057 Conservation Tillage--Planters for Ridge-till

Author: Vincent J .McFadden, extension soil and water engineering specialist Dept. of Agricultural and Biosystems Engineering, Iowa State Univ.

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