Features Agronomy Fertility and Nutrients
Nutrient uptake and timing guides fertilizer practices

Knowing when nutrients are taken up and byhow much helps target application timing.

Long-season crops like corn or candidates for post-emergence N applications. Photos by Bruce Barker

Are the old ways still the right ways? New technologies are prompting farmers to take a look at their fertilizer management practices. For example, controlled release fertilizers bring the hope of better fertilizer use efficiency by timing release with crop demand. And farmers still rely on balancing nutrient application with removal, but is that still appropriate given today’s environmental guidelines?

John Heard, a fertility specialist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI) at Carman, recently reviewed a series of research trials from MAFRI and Agriculture and Agri-Food Canada (AAFC) to help agronomists and farmers understand nutrient content and uptake patterns. He wanted to provide guidelines for using new technologies, and to answer questions that commonly come up when looking at nutrient uptake and removal. Understanding nutrient uptake patterns can guide farmers on when to apply a specific nutrient, and whether post-emergence applications of a nutrient could be successful.

Heard says the nutrient most often timed to match nutrient uptake is nitrogen because nitrogen losses can occur if applied too early. For example, if nitrogen is applied in the fall on land subject to excessive moisture, part of it can be lost in the spring either before or after planting. This type of application can pose a risk to the environment largely through nitrate leaching. On the other hand, N applied too late in crop development can compromise yield potential.

Since nitrogen is an expensive crop production input, one approach for growers is to apply part of the N fertilizer supply before or at seeding, withholding the rest of the nitrogen until sufficient moisture has been received after the crop has emerged. This may be a good strategy for some crops, while for others, timing is crucial for optimum yields.

In the case of cool season crops such as wheat and canola, approximately 50 percent of the nitrogen is taken up by mid to late June. For longer season crops such as corn, potatoes, soybeans and sunflowers, 50 percent of the nitrogen is not taken up until mid July to early August.

“In-season nitrogen is commonly applied to crops such as potatoes and corn because of their longer growing season and the reduced risk from splitting the N application,” explains Heard.

Heard points out that there is approximately a three week lag between the period of greatest N uptake by cool or short season crops (cereals, canola) and warmer or long season crops. He says longer season crops have a greater ability to utilize nitrogen that may be mineralized during the growing season or respond to slow release nitrogen fertilizers or manure. Heard says that in some jurisdictions, nitrogen credits from previous legume and pulse crops are doubled for long season crops compared to short season crops such as cereals and canola.

To aid in the top dress application, a number of crop diagnostic or precision farming tools have been tested to quantify nitrogen sufficiency. These include the use of a SPAD or chlorophyll meters, GreenSeeker sensors, pre-side dress nitrate tests and plant tissue/petiole nitrate analysis.

If nitrogen fertilizer is to be used in a ‘just-in-time’ approach, Heard says it will still need to be applied some two to three weeks before the time of high uptake.

Surface applications under dryland (non-irrigated) production systems are vulnerable to N losses (volatilization of urea forms) or surface stranding if rainfall is not received. This stranding has been observed in research done by Guy Lafond with AAFC at Indian Head, Saskatchewan. Production systems allowing in-soil placement, such as side dressing in row crops, or irrigation provide more opportunity to successfully meet in-season crop needs.

Nutrients such as phosphorus and potassium are not as mobile in the soil and therefore do not lend themselves for post-seeding application due to their low ability to move to the plant root zone. As a result, these nutrients should be banded into the soil prior to, or during, seeding.

Nutrient uptake and removal values
“The goal of balancing nutrient inputs with crop removals is twofold. It reduces the buildup of nutrients and addresses environmental concerns while keeping fertility costs to a minimum,” explains Heard. “However, this balancing philosophy may limit yields when used on low test soils.”

The Canadian Fertilizer Institute publishes charts for nutrient uptake and removal. The most recent for western Canada was published in 2001. ‘Uptake’ refers to total nutrient uptake in harvested (grain) and residue (straw) portion, whereas ‘removal’ is the harvested portion only.

For example, CFI reports that spring wheat takes up between 1.9 and 2.32 pounds per bushel of N, while 1.35 to 1.65 pounds of N per bushel are removed with the seed – the remainder is returned to the field.

To see if these figures correlated to other studies, Heard looked at research trials in western Canada. In general, values were similar to the CFI guidelines, but exceptions were not uncommon. This may be expected since some nutrients such as potassium are known to be taken up in amounts greater than those required for growth when there is ample soil supply, known as ‘luxury consumption’. Similarly, nitrogen content of crops and grain can be manipulated by fertilizer management.

Concern about phosphorus in the environment is driving much of the current interest in ‘nutrient balance’, with some crops being touted as high P accumulators with a good fit for high P testing soils. In Manitoba studies, Heard reports phosphorus was removed by the high yielding crops at the following rates: 61lb/ac P2O5 for corn (156bu/ac), 54lb/ac P2O5 for soybeans (46bu/ac), and 59lb/ac P2O5 for potatoes (376cwt/ac). Despite these seemingly high removal rates, there would be little short-term impact expected on soil P levels since estimates from Ontario and North Dakota show that 35lb/ac P2O5 and 20lb/ac P2O5 respectively is required to elicit a change of 1.0ppm in Olsen soil P.

“This suggests that the greatest decrease one might expect to see in soil test phosphorus values would be 1.0ppm to 3.0ppm Olsen P per year assuming very high yield values. This also illustrates why soils build very slowly in soil P when phosphorus is applied at or slightly greater than removal values,” explains Heard.

Generally, Heard says the data from several prairie studies on nutrient uptake and removal provide a valuable baseline similar to the CFI publications, but considerable variability may occur. Some nutrients are accumulated by crops in luxury amount – beyond that required for plant function. Similarly, crop nutrientcontent, especially nitrogen, reflects the fertility regime the crop is grown under. Nevertheless, the published charts from the Canadian Fertilizer Institute are a good starting place when doing nutrient budgeting.