Field Specific Fertility Planning

In recent years, we have seen significantly different in-season precipitation across the 4.5 mile north to south spread of our acres – which has resulted in variable yields across same crop types. This makes it more difficult to predict residual nutrients left in our soil without the use of annual soil testing.

Fertilizing with a flat rate across all your fields of one crop type will certainly result in either under applying (which will short your crop of required nutrients) or overapplying (inefficient for your wallet). On Pitura Seed Farms, we soil test all our fields every year and will adjust our nitrogen fertility plan based on what residual levels are remaining in each field. We find this is even more necessary during the last 5 years where we have been seeing extremely variable weather patterns year to year and within a season.

To quickly review the lessons learned in the last drought season – Summer/Fall 2021:

• 2021 crop season: Roughly 125mm of seasonal rainfall and starting out with very low levels of available moisture in the spring produced some of the weakest crops seen on the farm and left our fields with a range of 20-64 lbs N in top 6” and 34-168 lbs N in 0-18” in Fall 2021.

• Re-sampled some fields in the spring of 2022 and saw nitrate leaching in spring 2021, unusual for heavy clay soils. Having these samples gave us the information needed to customize our fertility plan for the needs of each field.

This year across Manitoba we again saw the effects of what Cal calls “the Thunderstorm Lottery.” Within our acres we saw a difference of up to 1” of rain from our north fields to south fields.  Below is a chart reporting data from May 20th to Aug 11th from our soil probes (installed after the first ½” (approximately) of rain we received during seeding).  Spring soil moisture was higher than we had experienced in 2021.

Going back to field-specific fertility plans… according to the Manitoba Soil Fertility Guide, a spring wheat crop will uptake 2.3 lbs of nitrogen per bushel and will remove 1.65 lbs of N per bushel. Using those removal ratios, consider how much N is being taken up across fields that are showing variable yields based on environmental conditions.

Below is an example for one of our harvested wheat fields. This is a demonstration of yield variability within a field but it is also important to note the range of yields between fields on your farm and document it, especially if you are choosing not to soil test. Documenting these yields can still allow you to calculate a nutrient budget and estimate nutrient levels based on nutrient removal rates and any previous soil test.

Example: Field 13 AAC Hodge VB

-          Fall 2022 soil test = 32 lbs N remaining in soil.
-          Applied 110 lbs Urea in the fall. + 40 lbs N with the seed.
-          Total of 182 lbs N available to the crop which should allow for a yield goal of 75-80 bu.
-           (These numbers do not account for losses via various N cycle pathways).

Field 13 2023 Yield Map

*Red blocks on the maps were a system glitch

In order to accomplish field-specific fertility, I would recommend soil testing at least one field of each crop type every 1-2 years on your farm. If your farm is more spread out, I would add another field of each crop type in a few other areas – wherever you are aware of variability from one field to the next. Alternatively, you could soil test in the first year and then calculate a nutrient budget based on yields to make a fertility plan for each soil tested field in the second year.

~Katie Meggison

For an additional read, check out our article on Lessons Learned From 2021 Left Over Nitrogen