Drought Could Boost Soil Nutrients

Tuesday, April 23, 2024

Last year's dryness could provide higher residual nutrients in 2024.

By Richard Kamchen

Parched Prairie conditions last year may boost available soil nutrients for the 2024 growing season.

In many cases, crops with yields hindered by extreme dryness removed fewer nutrients from the soils they grew in, potentially leaving higher levels of nutrients available in the following year.

Residual nutrients

Jeff Schoenau, a soil science professor at the University of Saskatchewan, says many fields from drought-stricken areas do show high residual available nutrients from unused applied fertilizer nutrient.

"Depending on conditions, the residual nutrient present in the fall can carry over into the following spring and be available for crop use," he says.

Edgar Hammermeister, manager of professional agronomy with Western Ag Professional Agronomy adds that the timing of crop stress has an impact on residual fertility.

During good growing conditions early in the season, crops will take up nutrients aggressively. If severe stress follows this nutrient uptake, conversion into yield can be limited. This results in little nutrient removal from the field as yield, but also low residual soil nutrients as the nutrients are tied up in plant biomass, Hammermeister explains.

But if the drought stress starts early and limits vegetative growth, then there should be higher residual fertility, he says.

Other factors

But crop uptake of nutrients isn't the only way nutrients are removed from soil, notes Bryce Geisel, senior agronomist, agent of Koch Agronomic Services in Canada.

"Take nitrogen for example; depending on environmental conditions and the soils of the field, there can be losses from the environment, microbes in the soil, and uptake from weed species," Geisel says.

Hammermeister adds that autumn rains, followed by good growing conditions, could result in volunteer crops and weeds germinating and growing.

"If these plants are not controlled, but are allowed to grow quite large, there will be a considerable amount of nutrient taken up into the plants, and there will be reduced nutrients available to the following crop," he says.

Hammermeister notes that following the 2021 drought, swathed or harvested fields of canola started to regrow, even to the point of flowering and podding, and some farmers experienced similar phenomena in 2023. He found a strong correlation between nitrogen supply potential and stubble type.

"Cereal stubbles tended to have higher N supply power because they completed their life cycle and, with the use of pre-harvest glyphosate, less weed growth occurred after harvest. The canola fields started to regrow and N supply rates were depressed," Hammermeister says.

He says that, in theory, farmers should be able to use less nitrogen fertilizer in 2024 due to last season's growing season and fall weather conditions, but notes there can be a high degree of variability between fields.

"As an example, for one of my customers having sampled 35 fields, there was a range of N supply potential from 35 lb to 200 lb/ac. It will pay big dividends to tailor fertilizer application rates field by field."

Nutrient variability can be affected by soil texture and soil moisture-holding capacity. Crops succumb to stress earlier on lighter textured or sandy soils, which hold less moisture, than on loam and clay soils.

"Depending on the degree of crop growth, there will be an influence on residual nutrients," Hammermeister says. "The impact of the variability on fertility planning is determined by how much of the field has which type of soil texture."

Dry winter

A dry winter in a thirsty Prairie region complicates matters.

"Below-normal winter precipitation may be more of a problem given that it is the main source of water for soil recharge in the spring and supports seed germination and nutrient availability until the summer rain comes," says Derek MacKenzie, an associate professor at the University of Alberta's Soil-Plant Relations Lab.

Geisel adds that early and timely rains can offset that dryness, providing "tremendous results when it comes to our crops."

Farmers and agronomists should discuss their expectations and develop a solid plan for their crops, he says.

"Contingency plans in case of those timely precipitation events could include tissue testing and top dressing to provide extra crop nutrition if yield potential increases from that of the spring plan," Geisel says. "Using proven products that provide the nutrients the crops need and reduce losses is key to making the most of these applications."

Schoenau sees significant value in checking the depth of soil moisture in the spring within the rooting zone.

"The presence or absence of stored soil moisture, especially in the subsoil, is good to know as it is an important contributor to the total available water, yield potential, and nutrient demand of crops here on the Prairies," says Schoenau.

Mario Tenuta, a University of Manitoba professor of applied soil ecology, says that without replenished soil moisture from snowpack and spring rains, farmers ought to adjust their yield expectations and N requirements.

"If a field is going into 2024 planting with lower moisture than usual, consider also adjusting yield goals and N addition."

He adds that if normal precipitation moistens soils this year, mineralization during the growing season will increase.

"I think of it as the microbes are woken up by the moisture and feast on the dead bodies of their mates and crop roots and residues of the previous year, mineralizing nitrogen."

Soil testing

In some cases, farmers could benefit from both fall and spring soil testing.

"Fall soil testing gives an indication of the amount of nutrition in the soil after removing crops, and is used by agronomists to come up with fertilizer prescriptions for next year," says MacKenzie.

Schoenau urges farmers to sample carefully to account for greater microscale and macroscale variability, explaining that spatial variability in available nutrients can be considerable due to residual fertilizer bands, and patchy regrowth and weed infestation from late-season rains.

Farmers may also wish to re-check a few fall-sampled fields in the spring to assess the efficiency of nutrient carryover.

"Residual phosphorus and potassium should carry over effectively as there is no potential for significant leaching or gaseous losses of these nutrients. Residual nitrate, however, is susceptible to loss by leaching and denitrification in spring, especially if wet. Sulfate could be leached, but no gaseous losses," Schoenau says.

Tenuta says farmers with fields stressed by drought in 2023 ought to expect slightly higher soil test nitrate and more mineralization in 2024. He adds that when a fall and spring are very wet, fall soil nitrate test accuracy can be off.

Geisel further explains that depending on when and where fall soil tests are taken, findings can change due to mineralization of unavailable nutrients increasing levels, or losses of nutrients reducing levels.

"Fall soil samples are an important step in helping farmers and agronomists get an idea of end-of-season nutrient levels to help with winter planning. However, depending on the length of fall and winter conditions, these numbers can change by spring," he says.

Plan for success by utilizing historic applications and yield results, crop nutrient uptake needs, and making adjustments based on either fall or spring soil testing, Geisel says.

For farmers having spring soil samples taken to verify what their fall tests indicated, Hammermeister recommends samples be collected in the vicinity of the original sample points, in order to limit the impact of natural variability that occurs across a landscape. BF