Farmers could make better cropping decisions if they had a more complete picture of fall moisture levels
This year’s growing season provides an example of how crops can use existing soil moisture when growing season precipitation is disappointing.
Fertilizer and soil science researcher Tom Jensen said there were decent crops this year in areas that had terrible growing season moisture.
“They shouldn’t have had as good of crops as they did, but when you thought back to the fall of 2014, they got some pretty good rains during harvest that soaked into the ground, and that’s what helped make the 2015 crop,” he said.
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If soil moisture levels before seeding are such an important ingredient to growing a good crop, why are they often not included in planting and fertilizing planning?
Some agronomic services include soil moisture in their modeling and fertilizer prescriptions, but many producers still stick to their fertilizing formula regardless of soil moisture content.
Some farmers rely on the precipitation they received to determine soil moisture levels, but this can be unreliable because the amount of plant-available water can be less or greater than precipitation levels would indicate.
This is because of multiple variables that can affect soil moisture levels, such as the varying amounts of water that plants pull out of the soil, different kinds of snow melt in the spring and the fact that different soil types retain moisture differently.
What really matters is the amount of water that is available to plants in the spring and summer for spring seeded crops, so is it worth bothering to check soil moisture levels in the fall?
For people applying fertilizer in the fall, it could make a significant difference.
Producers may not hesitate to apply full fertilizer rate in soil that is fully charged with water, but it might make sense to hold back nutrients on parched soil.
Fall moisture levels may also have a bearing on the winter fertilizer purchases that producers make.
Rigas Karamanos, a soil and fertilizer scientist, said having a better idea of fall moisture levels may help producers with their crop planning while they have more time to spend on it during the winter.
“Conceivably what somebody can do is utilize the fall moisture and the winter precipitation,” he said. “If you combine it with soil texture, you should be able to come up with an estimate of what your soil moisture is.”
For example, the Saskatoon area has dark brown soil, and every 25 millimetres of water will provide about 4.5 bushels of wheat and 3.5 bu. of canola in an average year.
“If your soil is fully charged with moisture, it means it has about six inches (150 cm) of water. That means you’ve got darn near 28 to 30 bu. of wheat already in the soil. Don’t you want to know that and plan accordingly?” Karamanos said.
“Saskatoon means about nine to 10 inches (225 to 250 mm) of rain in summer months. Nine inches of rain is about 38 bu. of wheat. Wouldn’t you like to know there is another 28 or 30 in the soil, so you can fertilize accordingly?”
Jensen said producers have adjusted their fertilizer rates based on moisture for many years, and the management strategy continues to be useful because it doesn’t make sense to put on a lot of nitrogen that’s not going to be used.
“I grew up in a dry area down in southern Alberta, and the dry land farmers would cut back on their fertilizer rates depending on moisture conditions,” Jensen said.
“If we had a dry winter, dry spring, they might cut back 15, 20 percent. On the other hand, if it was above normal moisture conditions, they would bump up their rates.”
A brown soil moisture probe and a small soil auger are common tools to test soil moisture in dry areas.
Farmers who use a small soil auger such as a Dutch auger will auger down and feel the soil moisture level by hand.
The brown soil moisture probe is a 3 1/2 foot long, 3/8 inch steel shaft with a 1/2 inch ball bearing on the tip that penetrates moist soil easily. The bottom level of moisture in the soil is found when the probe can’t be pushed in anymore.
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- Select a representative area of the field. Avoid saline areas, potholes and other problem areas. Depression areas, slopes, and knolls can be measured separately for site specific crop planning.
- Push the probe into the ground in a single motion. It will stop penetrating when it hits dry soil. A rock or frozen ground in the spring will also stop the probe. Always push and pull the probe using your knees while keeping your back straight.
- Each field should be sampled separately because rainfall amounts can vary over short distances, crops and varieties differ in water use and crops mature at different times because of seeding date and differing days to maturity.
- Sample a minimum of 15 to 20 sites per field and record the average depth of moist soil. More sampling will be required for site specific management. Spring sampling may require more sites within a field because of increased variability caused by snow trapping, snow drifting, water runoff, moisture migration within the soil and variations in ground frost.
- Soil texture and average depth of moist soil are used to calculate the amount of plant-available water.
