John Breker

Soil sampling on drown-out, unplanted, and Prevented Planting acres

in , , /by

Across the northern Great Plains and Canadian Prairies, weather patterns have ranged from too dry to too wet. For the too wet parts, excessive spring and summer rainfall has resulted in extensive stretches of unplanted (Prevented Planting) acres or drown-out acres. As people think about the fall soil sampling season ahead, we are starting to get questions about these unplanted or drown-out fields: When can I start soil sampling? What kind of residual soil nitrate-nitrogen amounts can I expect in the fall?

Extremely wet soil conditions can cause soil nitrogen losses to leaching or denitrification. Warmer soil temperatures and good soil moisture can promote more nitrogen mineralization from soil organic matter. Fallow fields without growing crops (or weeds) can accumulate nitrogen in the soil profile. There are a lot of variables in the equation, and soil testing is the only way to know how much nitrate-N is actually present in the soil profile. Sorry, no points for guessing! The soil nitrate-N level will depend on numerous management and environmental factors, which vary from field to field and zone to zone.

Management Factors

  • Did you apply nitrogen with intent to plant the field? What was the nitrogen fertilizer rate and application timing? Was it applied last fall?
  • Did you do any summer tillage? More tillage promotes nitrogen mineralization.
  • How was your weed control? Did the weeds get large and acquire a lot of nitrogen from the soil profile?
  • Did you plant a cover crop to take up excess water (and nitrogen)?

Environmental Factors

  • Did excessive rainfall cause nitrate leaching on well drained soils?
  • Did excessive rainfall cause denitrification on poorly drained soils?
  • Were summer temperatures warm? Warm temperatures promote nitrogen mineralization.

For immobile soil nutrients (e.g., P, K, Zn), you could start soil sampling anytime, as soon as you can collect good quality soil cores (not too muddy). If these nutrients were applied the previous fall or spring, a soil test will reflect their current availability in soil, following any fixation reactions and nutrient uptake from cover crop or weed growth. For soil nitrate-N, however, the timing will depend on tillage, nitrogen mineralization, and nitrogen uptake from cover crops and weeds.

For “clean” fallow fields (no cover crop or weeds), soil testing may begin in mid-August. It is important to prioritize soil sampling on fallow fields while you can still drive across them. Since these fallow fields have no plant growth to use excess water through fall, the field trafficability might become challenging if excess precipitation continues into fall. To help ensure you can collect good quality soil samples on fallow fields, start soil sampling in August and early September.

For fields with cover crops, soil testing should be delayed until the cover crop is terminated or growth has slowed and nitrogen uptake has stopped. A healthy cover crop can take up a lot of nitrogen through the fall, so you do not want to collect soil samples for nitrate-N too early. In NDSU cover crop projects, fall-planted cover crop mixes can contain 100 to 150 lb/acre N in the plant biomass, which is a sizeable amount of nitrogen that would not be measured as soil nitrate-N.

AGVISE has also performed fallow and cover crop comparison projects; we have seen 35 to 90 lb/acre nitrate-N differences in the 0-24 inch soil profile between fallow and cover crop areas of the same field (Figure 1). To best reflect the amount of residual soil nitrate-N available for next year, it is suggested to wait until cover crop nitrogen uptake has slowed or stopped in October. If more precipitation arrives in fall, the cover crop will continue to use excess soil water and also provide a nice plant residue surface to drive on.

Figure 1. Soil nitrate-N following fallow or cover crop. Cover crop planted in August; soil samples collected in October. AGVISE Laboratories, Northwood, ND. 2020.

We also recommend splitting fields into management zones for soil testing. The unplanted or drown-out parts of the field can very considerably from the rest of the field, which will skew the field-average soil test result and resulting nitrogen fertilizer rate for next year. Often, the unplanted or drown-out parts will have higher soil nitrate-N (no  nitrogen uptake), but sometimes the situation is oddly reversed for no good reason (Figure 2). This data highlights the importance of collecting separate soil samples for the planted and unplanted/drown-out parts of the field.

Figure 2. Soil nitrate-N variability in fields with unplanted or drown-out areas. Paired soil samples in close proximity from the cropped and unplanted/drown-out area in the same field. AGVISE Laboratories, Northwood, ND. 2014.

John Breker

Soil Testing Behind the Combine

in , /by

As harvest gets underway, savvy soil samplers are following right behind the combine and starting to collect soil samples. These soil samplers understand the many reasons why taking soil samples right behind the combine gives them the best quality soil samples and data.

In the past, the reasoning to wait until later in the fall to start soil sampling was that there may be additional nitrogen that would be converted to nitrate through the fall as small grain straw and crop residue start to decompose. However, we now know that small grain straw has a high carbon content, and it takes a long time for wheat straw nitrogen to convert to nitrate-N in soil for future crops. Research has shown that soil nitrate-N levels after small grain harvest are quite stable with small changes (up or down) through the fall. Soil sampling right after harvest provides actionable soil nitrate-N data for making fertilizer decisions for next year.

Soil testing behind the combine has several other advantages. If you sample right behind the combine, you beat chisel plows and disk rippers to the field. Taking soil samples before fall tillage allows you to obtain clean and consistent soil cores with your soil probe; this is important for high-quality soil samples. If you sample after tillage, you will be dealing with soil clods that do not feed smoothly into the soil probe. Soil sampling after tillage can also lead to inconsistent sample depths, which will affect soil test levels for P, K, Zn, etc.

Here are some comments by Dr. Dave Franzen, NDSU Extension Soils Specialist (retired) about soil testing right after harvest:

“It is more the rule than the exception that soil sampling begins in mid-September, rather than starting immediately following small grain harvest. However, many producers miss an excellent window for soil testing by waiting too long. The reason for waiting is the hope that additional nitrogen will be made available through mineralization (decomposition of crop residue and organic matter). A review of research has shown that soil nitrate levels change very little, up or down, following small grain harvest.”

Soil sampling right after harvest is recommended and has numerous advantages

  1. Producers are more likely to use the actual soil test results for deciding fall nitrogen fertilizer rates if the soil test results are in their hands before fall fieldwork begins.
  2. Soil sampling before fall tillage provides more consistent 0-6 inch soil cores, which gives the best soil sample quality for phosphorus, potassium, zinc, organic matter, and other non-mobile soil nutrients tested on topsoil.
  3. Soil sampling right after harvest guarantees that fields will be soil sampled on time and not missed due to weather problems that could happen later in the fall.