Moisture and Target Yields

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)
Depth of Moist Soil (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
Percent of Available Soil Moisture	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)
Total Available Soil Moisture (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)
Total Available Soil Moisture (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)
Total Available Soil Moisture (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)
Total Available Soil Moisture (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
Total Potential Available Moisture (inches)	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

Manitoba Agriculture, Food and Rural Initiatives