Target yield is the yield a crop can be expected to produce based on the
amount of spring soil moisture and expected growing season precipitation. Heat determines
whether a crop will mature but moisture establishes its yield potential. Armed with this
information, a producer is able to make decisions before seeding regarding crop choice,
herbicide use and fertilizer application.
Each crop requires a minimum amount of
moisture to produce the first bushel. For wheat, this amount is about four inches. Each
additional inch of water increases the target yield about four bushels per acre. The
amount of water available to a crop during the growing season is equal to the amount of
soil moisture available at seeding time plus the amount of precipitation received over the
growing season.
Soil moisture can be assessed fairly easily in the spring by using the "feel and
appearance" method or a soil moisture probe. Expected growing season rainfall is
determined by referring to historical rainfall data for the area.
Soil Moisture and Texture
To estimate the probability of obtaining a given yield, the amount of soil moisture
available must be determined. There are physical limits dictating the amount of soil water
that is available to a plant. The upper limit is call field capacity. Amounts of soil
water in excess of the field capacity result in saturated soils and are harmful to the
crop. The lower limit is called the permanent wilting point. Soil moisture levels below
the permanent wilting point are too low to be available to the plant.
The amount of water available for plant use is the difference between the field
capacity and the permanent wilting point.
Soil texture affects the amount of water available to the plant. The amount of
available water for various soil textures is shown in Table 1. Obviously, there is a
sizeable variation in available water between sandy and clay textured soils.
Table 1: Soil Texture and Available Moisture1
Depth of Moist Soil (inches) |
Available Moisture (inches) |
Sand |
Loamy Sand |
Sandy Loam |
Clay Loam |
Clay |
6 |
0.51 |
0.7 |
0.9 |
1.2 |
1.4 |
12 |
1.0 |
1.4 |
1.9 |
2.4 |
2.7 |
18 |
1.5 |
2.1 |
2.8 |
3.7 |
4.1 |
24 |
2.1 |
2.8 |
3.7 |
5.0 |
5.4 |
30 |
2.6 |
3.5 |
4.6 |
6.1 |
6.8 |
36 |
3.1 |
4.3 |
5.6 |
7.4 |
8.1 |
42 |
3.6 |
5.0 |
6.5 |
8.6 |
9.5 |
48 |
4.1 |
5.7 |
7.5 |
9.8 |
10.8 |
*Soil Survey Data for the
Black Soil Zone in Manitoba. |
Example One
If a field of clay loam soil had an average depth of moist soil of
24", the amount of soil moisture available for plant use would be 5.0 inches.
Determining Spring Soil Moisture
Spring soil moisture should be determined on an individual field basis due
to variation in soil texture, precipitation, previous crop and tillage practices. Always
sample soil moisture at four or five sites in an 80 acre area. Use more sites if the
topography or soil texture is variable throughout the field.
Exact available soil moisture can be measured and calculated, but this is
time-consuming. There are two other less complicated methods that are suitable for most
practical purposes.
Feel and Appearance Method
Available soil moisture can be estimated in the field by examining the
feel and appearance of the soil. An auger or shovel is required to dig holes for sampling
the soil at various depths. Different soils and varying moisture levels will create
different reactions to hand pressure and touch. Soil appearance may also vary. This simple
method is relatively accurate once experience has been gained.
Brown Soil Moisture Probe
The depth of moisture in a soil profile can also be used to calculate the
amount of available soil water for plant use. This moist soil depth can be determined with
a Brown soil moisture probe.
The 3.5 foot long probe is pushed vertically into the soil without turning
or twisting. The probe will not penetrate dry soil, so the depth at which the probe stops
will determine the depth of soil moisture. The probe will not penetrate rock, gravel or
frozen soil either, but distinguishing these obstructions from dry soil is not difficult.
On the end of the probe is a section of wood bit. To gather a sample of
soil anywhere along the vertical path of the probe, simply twist the probe clockwise and
withdraw it from the hole. Samples gathered in this way can be examined for texture and
soil moisture content by using the "feel and appearance method".
Table 2: Interpretation Chart for Feel and Appearance Method of
Determining Soil Moisture2
Percent of Available
Soil Moisture |
Feel or Appearance of
Soils |
Sand |
Loamy Sand-
Sandy Loam |
Loam-Clay Loam |
Clay |
0 |
dry, loose, single-grained, flows through fingers |
dry, loose, flows through fingers |
powdery, dry, sometimes slightly crusted but easily breaks
down into powdery condition |
hard, baked, cracked, sometimes has loose crumbs on surface |
50 or less |
still appears to be dry; will not form a ball with pressure* |
still appears to be dry; will not form a ball* |
somewhat crumbly but will hold together from pressure |
somewhat pliable, with ball under pressure* |
50 to 75 |
same as coarse texture under 50 or less |
tends to ball under pressure but seldom will hold together |
forms a ball* somewhat plastic, will sometimes slick slightly
with pressure |
forms a ball, will ribbon out between thumb and forefinger |
75 to field capacity |
tends to stick together slightly, sometimes forms a very weak
ball |
forms weak ball, breaks easily, will not slick |
forms a ball and is very pliable, slicks readily if
relatively high in clay |
easily ribbons out between fingers; has a slick feeling |
At field capacity |
upon squeezing, no free water appears on soil but wet outline
of ball is left on hand |
same as coarse |
same as coarse |
same as coarse |
*Ball is formed by squeezing a handful of soil
very firmly with fingers. |
Available Moisture Probabilities
On average, growing season rainfall in Manitoba will provide about two-thirds of
moisture needs for wheat. The likelihood, or probability, of having adequate moisture to
reach a target yield is based on combination of spring soil moisture and rainfall
probability.
Rainfall probability is a statistical term that expresses the likelihood of receiving a
certain amount of rainfall. A probability expressed as 1.0 is a sure thing, a 100 percent
probability. A 0.33 probability describes an event that, on the average, would occur once
every three years.
Knowing the probability of reaching a certain moisture level can allow a producer to
make decisions in advance about crops to plant and the level of input such as herbicides
and fertilizers. Tables indicating the probabilities of various total available moisture
situations for different seeding dates for Brandon and Pierson are provided. Later seeding
dates than those indicated reduces the probability of receiving adequate moisture, and in
turn, the attainable target yield. (Tables 3A & 3B and 4A & 4B)
Although the average growing season for wheat in southern Manitoba is 100 to 120 days,
the "usefulness"of precipitation diminishes toward the end of the growing season
and, in fact, can reduce yield and quality in the last week or so. It is more important to
have sufficient moisture during crop establishment and crop filling stages than during
ripening. Therefore, the probability tables take into account precipitation from seeding
until Zadoks' growth stage 85 (GDD 2130), or the soft dough stage.
Table 3A: Probability of Potential Total Available Moisture
for Brandon - Seeding Date May 73
Total Available Soil
Moisture (inches) |
Potential Available
Moisture (Soil Moisture Plus Rainfall in Inches) |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
10 |
1.0 |
0.99 |
0.96 |
0.92 |
0.84 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
9 |
0.99 |
0.96 |
0.92 |
0.84 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
8 |
0.96 |
0.92 |
0.84 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
7 |
0.92 |
0.84 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
6 |
0.84 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
5 |
0.75 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
4 |
0.58 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
3 |
0.43 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
2 |
0.28 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
1 |
0.16 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0 |
0.08 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
Adapted from
Environment Canada historical data.
Note: If available soil moisture values fall between those
provided in the table the associated probability would also be adjusted. Example: The
probability of receiving 13 inches of total moisture when soil moisture is 7.5 inches is
between 0.75 and 0.58, or about 0.67. |
Table 3B: Probability of Potential Total Available Moisture for
Brandon - Seeding Date May 213
Total Available Soil
Moisture (inches) |
Potential Available
Moisture (Soil Moisture Plus Rainfall in Inches) |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
10 |
1.0 |
0.98 |
0.96 |
0.91 |
0.82 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
9 |
0.98 |
0.96 |
0.91 |
0.82 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
8 |
0.96 |
0.91 |
0.82 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
7 |
0.91 |
0.82 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
6 |
0.82 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
5 |
0.70 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
4 |
0.56 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
3 |
0.40 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
2 |
0.26 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
1 |
0.15 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0 |
0.08 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
Adapted from
Environment Canada historical data. |
Example Two
Use the data in example one. Refer to Table 3A
(Probability of total available moisture for Brandon, seeding date May 7). With 5.0
inches of available spring soil moisture, the probability of ending up with a total of 13
inches of available moisture (an additional 8 inches of moisture through rainfall is 0.28,
or a chance of occurring approximately once in four years. If 13 inches is required to
achieve the target yield, the producer will have to weigh the risks of additional inputs
against the probability of NOT receiving 13 inches (three out of four years).
Table 4A: Probability of Potential Total Available Moisture for Pierson -
Seeding Date May 13
Total Available Soil
Moisture (inches) |
Potential Available
Moisture (Soil Moisture Plus Rainfall in Inches) |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
10 |
1.0 |
0.98 |
0.95 |
0.90 |
0.81 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
9 |
0.98 |
0.95 |
0.90 |
0.81 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
8 |
0.95 |
0.90 |
0.81 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
7 |
0.90 |
0.81 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
6 |
0.81 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
5 |
0.69 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
4 |
0.54 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
3 |
0.38 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
2 |
0.25 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
1 |
0.14 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0 |
0.07 |
0.03 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
Adapted from
Environment Canada historical data. |
Table 4B: Probability of Potential Total Available Moisture for
Pierson - Seeding Date May 153
Total Available Soil
Moisture (inches) |
Potential Available
Moisture (Soil Moisture Plus Rainfall in Inches) |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
10 |
1.0 |
0.98 |
0.94 |
0.87 |
0.77 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
9 |
0.98 |
0.94 |
0.87 |
0.77 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
8 |
0.94 |
0.87 |
0.77 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
7 |
0.87 |
0.77 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
6 |
0.77 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
5 |
0.63 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
4 |
0.46 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
3 |
0.31 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
2 |
0.18 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
1 |
0.09 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0 |
0.04 |
0.02 |
0.01 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
Adapted from
Environment Canada historical data. |
Adjusting Nitrogen Fertilizer Inputs
Expected soil moisture available for crop growth may be used to predict potential or
target yields assuming no weed, insect or disease pressures apply and average temperature
conditions exist. A formula has been developed to predict target yields for varying
amounts of total available moisture based on Manitoba climactic and cropping conditions (Table 5).
In order to achieve the target yields, not only does the weather have to cooperate, it
may also be necessary to add nitrogen. Soil testing will reveal the difference between the
existing soil nitrogen and the required amount to achieve the target yield. Remember that
only one-half of applied fertilizer nitrogen actually becomes available to the plant, so
apply twice the amount required to bring the available nitrogen up to par.
Example 3
Soil testing before seeding reveals two things - the amount of soil moisture and the
amount of soil nitrogen. This fact sheet lists the probability of receiving certain
amounts of rainfall, and therefore the probability of reaching certain levels of total
available moisture.
With this information, a producer must decide how much risk to take, choose a target
yield for each field and manage for that result. Achieving the target yield will depend
not only on the weather, but on the amount of fertilizer that must be applied.
Low risk scenario
Producer "A" has five inches of available spring soil moisture and 42 lbs/ac
of soil nitrogen. Near Brandon, the probability of receiving at least 10 inches of
available water over the growing season when starting with five inches of spring soil
moisture is 0.75, or three out of four years. The expected yield for hard red spring wheat
under those conditions would be 24 bu/ac.
A 24 bushel crop of HRS wheat requires 42 lbs/ac of nitrogen. Therefore, this field
does not require additional applied nitrogen.
In this case, the producer who has set a target yield of 24 bu/ac will seed the crop,
apply no extra fertilizer and have a 0.75 probability of harvesting at least 24 bu/ac.
The risk taker
Producer B begins the season with the same conditions as Producer A; five inches of
spring soil moisture and 42 lbs/ac of soil nitrogen.
But Producer B has chosen to take the risk that the total available soil water
accumulated over the growing season will be 13 inches. The probability of this occurring
when starting out with five inches of spring soil moisture is 0.28.
If 13 inches of available water was received, the expected yield according to expected
yield according to long term data is 40 bu/ac.
An expected yield, or target yield in this case, of 40 bu/ac requires a total of 67
lbs/ac of nitrogen. Available nitrogen before seeding is 42 lbs/ac, leaving a shortfall of
25 lbs/ac. To achieve the target yield, Producer B must apply 50 lbs/ac of nitrogen (only
50 percent or 25 lbs/ac will become available to the crop).
Producer B has a 28 percent probability of achieving the target yield and has made
additional investments toward that aim. But this is not an all-or-nothing situation if the
target yield is not reached. Some yield will be received whether or not the target yield
is achieved. The additional expense for fertilizer may turn out to be an unneeded cost if
the target yield is not reached.
Table 5: Total Potential Available Moisture, Target
Yields and Required "N"
Calculation of Potential Total Target Yield and Nitrogen Fertilizer to Achieve that
Yield
Total Potential
Available Moisture (inches) |
HRS Wheat |
CPS Wheat |
Barley |
Yield
(bu/ac) |
Total N Required
(lb/ac) |
Yield
(bu/ac) |
Total N Required
(lb/ac) |
Yield
(bu/ac) |
Total N Required
(lb/ac) |
10 |
24 |
42 |
30 |
44 |
53 |
57 |
11 |
29 |
51 |
37 |
55 |
59 |
61 |
12 |
35 |
60 |
44 |
63 |
66 |
69 |
13 |
40 |
67 |
59 |
68 |
72 |
76 |
14 |
45 |
73 |
58 |
77 |
79 |
85 |
15 |
51 |
82 |
64 |
86 |
85 |
92 |
16 |
57 |
93 |
72 |
98 |
91 |
108 |
17 |
62 |
102 |
78 |
107 |
98 |
108 |
18 |
68 |
112 |
86 |
119 |
104 |
115 |
Another Approach to Using the Tables
The tables can also be used to calculate the probability of achieving a certain yield,
given a known amount of soil moisture. For example, if a producer was hoping for a 60
bu/acre wheat crop, about 17 inches of total available moisture would be required
according to Table 5. Given 5 inches of soil moisture at a seeding date of May 7, the
probability of receiving 17 inches of moisture (hence the 60 bushel crop) would only be
0.02 or one chance in 50. This would be an unrealistic yield goal to set, based on
anticipated moisture conditions.
Using the information available, each producer makes a well-informed choice
based on probabilities calculated using long-term data. The level of risk to take is
purely an individual decision.
References
1 Based on data provided by Canada-Manitoba Soil Survey,
Winnipeg, Manitoba
2 Lesson 2 - Moisture and Climate.
Soils '84, Manitoba Agriculture, Food and Rural Initiatives
A Manitoba Home Study Course
3 Canadian Climate Normals. Volume 3 - Precipitation 1951-1980.
Environment Canada, Atmosphere Environmental Services
Winnipeg, Manitoba
4 Target/Yield/Moisture Utility Software - User's Guide, Version 1.0
Saskatchewan Soil Testing Laboratory, 1992
5 D.R.S. Rourke, M. Adaran, M. Empry, A. Hargrave, Dr. Anne Hinchalwood.
Risk Management Guide for Wheat Production, 1993
Published by Canada Grains Council
Edited by D.R.S. Rourke
Other Reading
1 Brown, P.L., A.L. Black and C.M. Smith, 1981
Soil Water Guidelines and Precipitation Probabilities.
Bulletin 356
Montana State University Extension Service
2 Brown, P.L. and G.R. Carlson, 1990
Grain Yields Related to Stored Soil Water and Growing Season Rainfall.
Special Report 35
Montana State University
3 Ash, G.H.B., C.F. Shaykewich and R.L. Raddatz,
1992 Agricultural Climate of the Eastern Canadian Prairies
Winnipeg Climate Centre, Environment Canada
For more information, contact your local Manitoba Agriculture, Food and Rural Initiatives Representative.
Funding for the preparation of this factsheet provided by:
Farming for Tomorrow - Southwest Region, Manitoba |