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SWEEP Report #29

The Effect of Organic Mulches on Soil Moisture
and Crop Growth

T.J. Gillespie, C.J. Swanton, C.S. Wagner Riddle, University of Guelph, Guelph, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

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Associated SWEEP/LSP Research


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Completed: August, 1991

Key Words:

cover crop, soybeans, winter rye, soil water, soil temperature, timing, spring growth, fall, model, SOYGRO

Executive Summary

Many of the advantages of the conservation tillage systems currently in use derive from the presence of crop residue on the soil surface. Crop residues reduce soil erosion losses, therefore increase soil water infiltration, and decrease soil water loss by evaporation (Griffith et al., 1986).

Additionally, crops can be grown specifically for the purpose of covering the soil during periods between main crops, to then be killed and left as a mulch on the soil surface before planting of the main crop. The management practices used in the spring will then influence the value and effectiveness of cover crops (Triplett, 1986).

The timing of killing of the cover crop is a critical decision dependent on climate. Adequate amounts of mulch have to be present after killing, to assure that the soil water conserving properties of the mulch are maximized. On the other hand, delay in the timing of killing can have adverse effects on subsequent crop growth if soil water is limiting, such as during a dry spring or on soils with low water holding capacity. During a wet spring, the possibility of removal of excessive moisture by the growing cover crop suggests that a late killing could allow the timely planting of the main crop.

In addition to the soil water conserving properties of a mulch, effects such as a decrease in soil temperature variations can be an advantage for the growth of the subsequent crop, and should be considered in the management practices of the cover crop.

Winter rye is among the cover crops extensively used due to its winter hardiness, low fertility tolerance, early spring growth and persistence of the mulch produced (Moschler et al., 1967). In this study, soybeans were chosen as a standardized main crop and winter rye was used as the preceding cover crop on a sandy, a loam and a clay soil.

The use of a model linking climate to the effect of a rye mulch on soybean growth would be very useful in determining optimum management of the cover crop for various weather scenarios.

Therefore, the main research objectives of this project were:

  1. to study the timing of rye killing in a rye/soybean system, where the rye is planted in the fall, chemically killed and mowed in the spring, and soybeans are no-tilled through the rye mulch;

  2. to investigate the effect of a rye mulch on soybean growth and yield;

  3. to measure the effect of rye mulch on soil water and soil temperature.

  4. to model the effect of a rye mulch on soybean growth under various weather scenarios.

The rye mulch/soybean system studied proved satisfactory for soybean production on a sandy and a loam soil. On a clay soil, difficulties associated with excess fall moisture and winter kill interfered with cover crop establishment, while planter problems and slug damage in the spring decreased soybean yield on mulched plots. More research would be necessary if the system is to be adapted to clay soils, but our experience was not encouraging.

On the sandy and the loam soil site, rye mulch amount was significantly larger for the late killing date as opposed to an early killing date. Differences between soil drying rates under early killed rye mulch and late killed rye mulch were observed in 1990 at Delhi, but generally soil water content between the two mulch treatments was not different. Soil water conservation under mulch, although significant for short periods early in the season during one year, did not affect early soybean growth due to timely rainfalls.

For rye mulch amounts larger than 2000 kg/ha, no significant increases in percentage ground cover were gained by delaying rye cover crop killing. Also, mulch decomposition proceeded at a faster rate for large mulch amounts.

Although the rye mulch had a significant effect on the soil surface drying rate early in the soybean growing season, shading of the soil surface by the soybean canopy plus a substantial decrease in the amount of mulch present on the soil surface, resulted in little difference between mulch and no mulch conditions later in the season.

Analysis of hourly soil temperatures measured under mulch and no mulch treatments revealed that extreme temperatures are more frequent under the bare soil condition. Over the whole growing season these differences were limited to a small percentage of hours, but the lower variability of temperature under the mulch may have benefited microbial activity and root growth slightly.

The change in the moisture and temperature regimes of the soils studied, induced by the presence of a rye mulch on the soil surface, did not result in significant soybean yield increases or decreases. Delays or decreases in soybean emergence observed for some treatment-years were compensated by the plants as the seasons progressed. If other beneficial aspects of the no-tillage system are considered, such as reduced soil erosion (Phillips et al., 1980) and weed suppression (Weston, 1990), rye mulch is recommended to be used in no-till soybeans.

A rye mulch/soybean computer simulation model was constructed, using the CERES-Wheat and SOYGRO as building blocks. The water balance subroutine (WATBAL) used in SOYGRO was modified to include the effect of rye mulch on soil evaporation and rainfall interception. Good agreement between simulated and observed soil water content, rye biomass, and soybean pod yield was obtained even with data that was not used to calibrate the models. Based on the simulation results obtained, killing the rye cover crop approximately one week before soybean planting resulted in the best soybean yields under a variety of simulated rainfall conditions, on both medium and sandy textured soils.

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)

The study examined no-till soybeans and winter rye cover crop on clay, loam and sandy soils. The objective was to study early versus late killing of fall planted rye; the effect of rye mulch on soybean growth and yield; soil water content and temperature. The use of modelling to predict the effect of rye mulch on soybean growth under various spring weather conditions was also examined.

The rye mulch/soybean system proved satisfactory for sandy and loam soils. Excess fall moisture and winter kill, on the clay soil site, interfered with crop establishment. Planter problems, and slug damage decreased soybean yield on mulched plots. The results on the clay site were not encouraging.

Rye mulch amounts larger than 2000 kg/ha did not significantly increase the percentage of ground cover, and little is gained by delaying kill to obtain larger amounts. Soil drying rates generally showed no difference between early and late killing dates. Extreme temperatures were measured more frequently under bare soil conditions. However, over the entire growing season, the temperature difference was limited to a small percentage of hours. The change in soil water content and temperature due to the rye mulch did not significantly increase or decrease soybean yield.

When other benefits of no-till are considered, for example reduced erosion and weed suppression, the study recommended that rye mulch be used in no-till soybeans.

The model, when given the variables obtained from the field study, performed well as a predictor of soybean growth and soil moisture for bare and covered soil.


The title of the study may be misleading. The study furthers research on growth models, however, there seems to be little in the way of practical advice for farmers. The field study was used to compare the accuracy of the model predictions and not to determine the benefits or effectiveness of using residue management for soybeans. No effort was made to determine the feasibility of producing high amounts of mulch prior to spring planting or planting into excessive levels of residue.

The use of a rye-soybean rotation, or part of a rotation, is not representative of rotations in the Norfolk area. It is also questionable as to whether this type of rotation is representative of Ontario.

Associated SWEEP/LSP Research:

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(C) Should soil moisture conditions (excess or deficiency) in various tillage regions become an important issue, then future research should be directed at those soil and crop rotation combinations of concern. These combinations should, however, be representative of Ontario conditions and current practices.




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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 02:42:50 PM