Jan de Regt, MSPB Chairman, sent me some data from results of studies conducted in Kansas and Nebraska in the late 1980's/early 2000's that were reported by Dr. Norman Klocke. The following is my summary of some of those results.

According to Dr. Klocke, the greatest soil water evaporation rates occur when the soil surface is wet (following rain or irrigation) when rates are controlled by radiant energy that reaches the soil surface. Crop residues will modify the amount of radiant energy that reaches this wet soil surface, and thus they have the potential to reduce soil water evaporation from a wet soil.

Data in Table 1 are from a study conducted with sprinkler-irrigated corn at North Platte, Nebraska using canister-type lysimeters. Half of the lysimeter treatments were bare soil and half were covered with flat wheat straw mulch that was the equivalent to that resulting from a 60 bu/acre wheat crop. The other variable was irrigation frequency–dryland, limited irrigation, and full irrigation.

Evaporation measurements were not taken for days of irrigation or rainfall. Only six rainfall events of more than 0.4 inch occurred.

Major results from this study follow (Table 1).

• Neither the straw mulch nor the corn canopy had any appreciable effect (<1 inch) on soil water evaporation in the dryland system.


• In the limited irrigation treatment (three irrigation events of 2.0, 2.0, and 1.75 in), the straw mulch resulted in a 2 to 2.5 in. reduction in soil water evaporation in both the unshaded and corn-shaded treatments. The corn canopy reduced soil water evaporation by almost 3 in. compared to the unshaded treatment when both had no wheat straw mulch, and by about 3.5 in when both had wheat straw mulch.


• In the full irrigation treatment (nine irrigation events), the straw mulch resulted in a large reduction in soil water evaporation from both the unshaded and corn-shaded treatments. The corn canopy reduced soil water evaporation by about 6-7.5 in. compared to the unshaded treatment when both had no wheat straw mulch, and by over 4.5 in. when both had the wheat straw mulch.


• With limited or deficit irrigation of corn, the straw mulch reduced soil moisture evaporation by an average of 2.6 in. With full irrigation, the reduction was 3.8 in.

 

 

Data in Table 2 are from a study conducted with sprinkler-irrigated corn and soybeans at Garden City, Kansas using PVC cylinder-lysimeters. The experimental variables were soil mulch cover (bare soil, corn stover, and wheat stubble) and irrigation frequency (once or twice weekly).

Across both irrigation frequencies, corn stover decreased soil water evaporation from 2.70 to 4.24 in. in the soybean crop, and from 2.68 to 2.82 in. in the corn crop. Wheat stubble decreased soil water evaporation from 3.13 to 3.83 in. in the soybean crop and from 2.85 to 3.06 in. in the corn crop.

 

 

The results from the research at both locations lead to the same conclusions; i.e., crop residue cover can significantly reduce or suppress soil water evaporation, especially where irrigation is applied and even with soil shading by the crop. The potential water savings in these studies suggest that this suppression can be equivalent to one or more furrow irrigations on a cracking clay soil.

I recognize that these results from the westernmost Corn Belt may not be applicable to growing conditions and weather in the humid, subtropical environment of the Midsouth. However, they do indicate that crop residues can play an important role in water conservation for growing crops, especially in irrigated systems.

These data and results also indicate that this important dynamic associated with irrigated crop production is worth investigating in the soybean, corn, and wheat production systems that are common in the Midsouth.

Composed by Larry G. Heatherly, July 2014, larryheatherly@bellsouth.net