Ernest W. French

Superintendent

Williston Experiment Station

North Dakota State University

Williston, North Dakota

INTRODUCTION

Crop production in this semi-arid Great Plains area is limited by low and variable precipitation amounts. Seasonal rainfall is perhaps the most important factor in determining crop yields but stored soil water is critical to achieving good crop stand and stabilizing crop production. Potential evapotranspiration normally exceeds the growing season rainfall and a water shortage can occur unless a reserve of soil water is stored before the crop is planted.

The source of this reserve is precipitation that occurs during the non growing season. In Western North Dakota approximately 25% of the annual precipitation occurs during the non growing season (October-March). In small grain production) April and September are also non growing season months. In considering these two additional months, approximately 40% of the annual precipitation occurs during the non growing season with about half of that total occurring as snow. Some of this is lost through evaporation, runoff) or snow blown off the fields but the extent of the loss can be affected by tillage management decisions.

To achieve maximum storage and crop utilization of this non-growing season precipitation) it is necessary to reduce rainfall and snowmelt runoff, reduce evaporation losses and minimize weed growth. Residue management is a key to conservation of non growing season precipitation. Residues reduce the water evaporation rate and help to keep precipitation in place where it falls. As any form of tillage will reduce residue cover, it becomes a management practice as to what tillage implement to use, how many operations to perform and how to properly operate the implement so that the residue loss will be minimized. Recent developments in conservation tillage practices and potential snow management techniques have improved the potential for greater water conservation and higher crop yields in present cropping systems.

SOIL WATER STORAGE AND RETENTION

The amount of precipitation that is stored and utilized for plant growth under an annual cropping systems is about 35% of the precipitation received during the 9 months from harvest to seeding. In a 21 month summerfallow period, the amount stored is less than 20% of the total precipitation received.

Water is lost by evaporation) runoff, snow blow-off, and weed growth. Evaporation accounts for the greatest loss and is difficult to control but soil management practices can help to minimize the loss. Evaporation is affected by weather factors, mainly, temperature, wind speed, and humidity. High temperatures and low humidity increase evaporation losses from the soil. High wind speeds also increase evaporation by moving air with different moisture contents and temperatures over the soil surface. Soil water losses from evaporation can occur to depths of two feet if soil conditions are favorable and if no water has been added for a long period of time. Runoff and high evaporation rates are main factors in reducing the storage of rainfall during the summer months of May through August. Rainfall is normally of higher intensity than early spring or late fall rains and unless there are some barriers to water movement such as crop residues or surface roughness, runoff amounts can be high.

Retaining snow where it falls greatly improves the potential for water conservation. The snow must be trapped) distributed and held on the field if the soil water is to be increased. The success in storing more water will depend on the soil permeability but generally if soil is dry before the snow covers the ground, the chance to increase soil water is good.

Weed growth must be controlled to save stored moisture for crop use. A full stand of weeds can consume from .25 to .30 inches/day and can rapidly reduce the amount of stored moisture. It is particularly important on fallow land to control weeds early in the spring to preserve the water stored during previous fall and to insure storage of early spring rains.

METHODS TO INCREASE AND MAINTAIN SOIL WATER RESERVES

SUMMERFALLOW MANAGEMENT

The practice of summerfallowing has long been used in this small grain production area as a means of increasing soil water. It is not an efficient means of storing water, but in the drier areas, it has helped to increase yields and stabilize production. Perhaps more than anything, it shows the value and effect a small amount of stored moisture has on crop yields.

Most of the water in fallow is stored during the first 9 months after harvest. Very little precipitation is stored during the summer of the fallow period or during the following fall and winter. To protect and maintain this water supply, it is necessary during the fallow period to minimize losses from evaporation and weed growth.

Crop residues are important in conserving the soil water because they reduce evaporation losses and help to keep precipitation in place where it falls. Residues on the soil surface reduce soil temperatures and wind velocity and thereby reduce evaporation. Standing stubble is very effective at trapping snow which helps to increase soil water.

Weed growth on fallow can be controlled by tillage or herbicides. Tillage for weed control increases evaporation losses and depending on the implement use, reduces residue cover but if the right implements are used, these losses can be minimized. The soil surface roughness will be increased by tillage helping to retard wind velocities and increase soil permeability. Herbicides can be an effective means to control weeds without some of the disadvantages of tillage.

MANAGEMENT PRACTICES

Success in achieving the maximum amount of soil water reserve from a given amount of precipitation will depend on developing a management system to minimize losses. There are a number of practices that can be used to enhance water storage. Two of the most cost effective residue management practices are to eliminate fall tillage and to use a late fall or early spring application of 2,4D herbicide to control winter annual weeds. These practices reduce the number of tillage operations performed and thereby conserve crop residues. The herbicide application can substitute for one or two tillage operations and control winter annual weeds which can consume large quantities seasonal rainfall to produce satisfactory yields than land planted to shorter season crops. Winter wheat has an advantage of longer grower season and greater rooting depth than spring wheat and is a good choice to follow a spring wheat or barley crop. There are several possible crops that could be used in sequences to more fully utilize the rooting depths and water use patterns of these crops.

SUMMARY

As water is a limiting factor in crop production, plant and soil management practices must be concerned with utilizing all the precipitation received as effectively and efficiently to achieve good crop yields and stabilize production.

Stored soil water is essential for maintaining good yields in this area where evapotranspiration greatly exceeds the precipitation received during the growing season. The source of this soil water reserve is from non-growing season precipitation and the amount stored can be affected by tillage and management decisions.

Water is lost by evaporation, runoff, snow blow-off and weed growth. Crop residues are important factor in reducing water losses by evaporation, runoff and snow blow-off. Weed growth is controlled by cultivation or herbicides and how this control is accomplished can greatly effect residue cover and surface roughness and consequently evaporation and runoff. Retaining snow and rainfall where it falls is also affected by residues.

Tillage operations and equipment can effect the amount of soil water available for crop production. Poor weed control and excessive tillage operations, can reduce the amount of stored water in summerfallow.

In years of average precipitation with normal monthly distribution, stored soil water will supply only a small part of the moisture needed to produce a "Bumper" crop; it supplies a larger part of the moisture needed for an "Average" crop. In years of low rainfall, abnormally high temperatures, and poor rainfall distribution, it may help to produce a "Harvestable" crop or it may have little effect on crop yield. These uncertainties in the weather and their effect on crop yields, make more important the value of good soil and plant management practices such as proper fertilization, adequate crop stands and populations, good weed control and timely harvest. Water is critical for crop production, but good management practices will insure top yields when weather conditions are favorable.