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Determining Plant Available Soil Moisture for Flex-Crop Systems

Agronomy Technical Note Number MT-81

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Agronomy Technical Note Number MT-81 (PDF; 550 KB)

September 2001.
Richard A. Fasching, State Agronomist

Background

Historically, many producers in summer fallow areas of Montana followed a strict crop-fallow rotation regardless of soil moisture conditions. By effectively using precipitation and stored soil moisture, producers have the ability to improve their overall production as well as minimizing environmental hazards such as saline seep development, nutrient leaching, and soil erosion.

Introduction

Flexible cropping can be used in areas of Montana where precipitation is not adequate for or is not predictable enough to allow for annual cropping systems. Flexible cropping or Flex-crop is where the decision to recrop is based on the amount of stored soil moisture and rainfall probabilities that will attain a satisfactory yield. It is designed to accomplish several objectives including (1) increase grain production by using water more efficiently; (2) control saline seeps by cropping as often as possible to use stored soil moisture and precipitation; (3) reduce wind and water erosion by reducing time the land is in a fallow condition. This process is also effective for determining expected yield and associated nutrient applications.

Yield Expectations: Data collected across Montana show that recrop small grains generally average about 70 percent of yields attained on fallow acreage. However, considering the two-year total crop production of both systems, flex-cropping produces 130 to 180 percent of grain production on fallow land. Nonetheless, following a rigid annual cropping system can lead to low uneconomical yields or crop failures in dry years; thus the concept of "Flexible Cropping".

Successful use of the flexible cropping system is highly dependent on efficient soil water management, effective weed and volunteer grain control, adequate fertilization, and in general, good soil and crop management. Failure to take care of any of these will often result in lower yields.

Water for the Crop: Approximately 9 inches of available water is needed to produce, at a minimum, a small grain crop. Available water is stored soil moisture plus the growing season precipitation. Neither barley nor spring wheat will produce grain with less than 4 inches of total water use. With more than 5 inches of water use, barley, and spring wheat yields will generally increase by 7 and 4 bushels per acre for each extra inch of water, respectively. Grain yields may exceed these estimates in years of favorable climatic conditions but may be lower when there are adverse effects such as weeds, insects, diseases, inadequate fertility, and unfavorable weather.

Determining growing season precipitation needed to produce an acceptable yield is crucial. If the amount of precipitation needed to successfully produce a crop is high and the probability of receiving that amount of precipitation is low, the risk of crop failure is high. With a failure, not only will a producer lose that year's crop but may also risk reduced yields the following year due to excess soil moisture depletion.

Conversely, fallowing land that has adequate soil moisture in spring will aggravate salinity problems in areas that are susceptible.

Water Conservation: The chances of a successful flex-crop system will be improved if fall and winter water conservation measures are incorporated. Fall weed and volunteer grain growth use stored soil moisture that can be saved for the following year's crop. Controlling weeds and green growth will not only save moisture for the next crop but also will stop further weed seed production and reduce future weed problems.

Allowing standing stubble during winter months will trap snow and increase the water content of the soil in spring. Fields with standing stubble may gain 1 to 3 inches more stored soil water during the winter period than fields with tilled stubble. The additional soil water conserved by controlling green growth and weeds in the fall and by trapping snow in the winter will reduce the amount of growing season precipitation required to attain an acceptable yield. "Scalping" maximizes snow catch in stubble. Scalping is the process of cutting grain at various heights at each pass, that is, first pass grain is cut at 10 inches, 6 inches on the second pass, 12 inches on the third pass, and so forth.

Fertilization: Fertilization with the necessary plant nutrients at optimum rates is essential for producing good yields and high quality crops. If stored soil water and the probability of growing season precipitation justify a decision to crop, then fertilizer needs should be determined also. Soil samples should be collected and recommendations obtained based on Montana State University Guidelines or industry guidelines. See Field Office Technical Guide (FOTG), Section IV, Practice Standard 590 - Nutrient Management.

Nitrogen fertility will typically be lower in soils cropped in consecutive years. Soils accumulate nitrate-nitrogen (available to the plant) through microbial conversion of organic nitrogen forms. Annual cropping or recropping allows less time for conversion of organic nitrogen to nitrate than does a crop-fallow. Also, harvesting higher yields because of improved water management will require greater quantities of nitrogen and other nutrients.

Precipitation Probabilities: TABLE 2 shows the probability (percent %) of receiving at least the indicated amounts of growing season precipitation for three different time periods at 24 locations in Montana. For locations not covered in table, use the Precipitation Probability Map, FIGURE 1.

Figure 1. Precipitation Probability Map (PDF; 554 KB)

Using the Soil Probe: Using a soil probe (such as, Brown soil moisture probe) is a convenient tool for determining depth of moist soil. The probe is used by vigorously pushing, without turning, it into the soil. The probe will penetrate moist soil but will stop at dry soil depths. The depth of penetration is the moist soil depth measured in feet (such as, 1.0, 1.5, 2.5, and so forth).

TABLE 1 shows inches of plant available water per foot of moist soil according to textural class. To determine stored soil moisture, multiply depth (in feet) of moist soil X the estimated Average Water holding Capacity (AWC) for the appropriate textural class. EXAMPLE: 3 feet of moist soil were probed in spring on a fine sandy loam field. 3 X 1.5 = 4.5 inches of stored soil water.

To acquire accurate stored soil moisture estimates, each quarter section should be probed at 6 to 10 representative locations or at least once per 20-acre block. Atypical areas such as wet spots, saline seeps, and rock outcrops should be avoided or recorded separately. For each management unit, average the moist soil depth readings, then multiply by the AWC from TABLE 1.

Determining Plant Available Soil Moisture

1. Determine the soil texture in the upper 4 feet of soil profile by field inspection or from a soil survey.

2. Determine the average depth of stored soil moisture as close to planting as possible using the soil probe or soil auger. Convert the depth of soil moisture to plant available moisture using TABLE 1 - Plant Available Water Capacities for Textural Classes in Montana. Note that recropping with spring grains is not recommended if stored soil moisture is less than 3 inches.

3. Use TABLE 2 - Growing Season Precipitation Probabilities, to estimate the probable amount of growing season precipitation.

4. Determine expected yields based on stored soil moisture plus probable growing season precipitation. TABLE 3 - Estimated Barley Yields; TABLE 4 - Estimated Spring Wheat Yields; or TABLE 5 - Estimated Winter Wheat Yields may be used for estimating yields of small grains.

5. Based on yield estimates, decide to crop or fallow (as a general rule, a recrop yield of 60 to 75% of the average yield on fallow should be economical).

a. Spring Planting Decisions for Flex-Cropping.
If the available soil moisture plus the estimated growing season precipitation at the 70% probability level is less than 9 inches, do not plant a small grain crop for grain harvest. If calculated available water will be greater than 9 inches, plant a crop (the decision to plant a crop is ultimately the producer's and should be based on economics as well as resource considerations). The decision process is the same for non-small grain crops except water requirements are different.

b. Fall Planting Decisions for Flex-Cropping.
Plant a winter wheat crop when there is 1 inch or more of plant available moisture in the top 1 foot of soil and the average winter-stored soil moisture plus growing season precipitation is equal to 8 inches or more.
For winter wheat, growing season precipitation is assumed to equal average precipitation from fall seeding until June 30 with the following adjustments:

  • Under fallow conditions, when the soil profile is at field capacity (4 feet), no additional moisture credit is given for precipitation received from time of planting to April 1.
  • Under clean-tilled fallow conditions, with the soil profile at less than field capacity to a 4-foot depth, 12 percent of the average precipitation from date of planting to April 1 is assumed.
  • 33% credit is given for winter precipitation when there is standing stubble and 25% for worked stubble.
  • Use 48% credit for a herbaceous barrier system.

c. Planting Decisions for Summer Fallowed Land for Flex Cropping Rotation.

  • Plant a crop on summer fallowed land regardless of stored soil moisture. Tilled double summer fallow is not recommended due to potential for erosion and salinity problems.

d. Planting Decisions for Flexible Legume-cereal Rotation.

  • Plant a leguminous crop as early as possible when it is determined that available water makes a small grain (or other grain or root) crop unfeasible.

References

Soil Water Guidelines and Precipitation Probabilities. Montana State university Extension Service Bulletin 356. Reprinted May 1990.

USDA-NRCS Field Office Technical Guide, Section IV, Practice Standard 328 - Conservation Crop Rotation

USDA-NRCS Field Office Technical Guide, Section IV, Practice Standard 590 - Nutrient Management

If you have any questions, please contact:

Patrick Hensleigh, State Agronomist
Phone: (406) 587-6837
Email: Patrick.Hensleigh

Jerry Schaefer, State Resource Conservationist
Phone: (406) 587-6998
Email: Jerry.Schaefer

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Last Modified: 11/10/2011