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.

Jodi Boe

Fallow Syndrome: Preventing Phosphorus Problems

in , , , , , , /by

Some crops that do not support mycorrhizal fungi (left to right: sugar beet, canola, radish).

Producers in the northern Great Plains and upper Midwest need to consider the risk of fallow syndrome in their crop nutrition plans. You are probably asking, what is “fallow syndrome” and why should I care? After all, summer fallow is not that common anymore! But the greater number of Prevented Planting acres in 2019 and 2020 meant that we have had many unintended fallow fields, making fallow syndrome a serious and widespread concern for the next year.

Fallow syndrome is an induced phosphorus deficiency caused by a lack of mycorrhizal fungi in soil. Some plant species, like corn and wheat, rely heavily on mycorrhizae to colonize the plant root system and help acquire important nutrients like phosphorus and zinc. If soil is lacking sufficient mycorrhizae to colonize plant roots, a case of fallow syndrome will increase phosphorus fertilizer needs and even cost crop yield potential.

Understanding mycorrhizae

Mycorrhizae fungi occur naturally in soils and readily colonize plant roots. Upon root colonization, mycorrhizae fungal filaments act as extensions of the root system and increase the soil volume available for plant water and nutrient uptake. The combined root-mycorrhizae surface area can be up to 10-fold greater than roots without mycorrhizae. Mycorrhizae depend on living plant roots to support stable mycorrhizae populations. However, not all plant species host and support mycorrhizae growth. Some common field crops are non-host species and their planting results in rapid drops in mycorrhizae populations.

Summer fallow or unplanted cropland, such as Prevented Planting in 2020, is a classic example of providing no or few living plant roots in soil to maintain mycorrhizae populations. In addition, some crop species do not support mycorrhizae, such as those in the goosefoot family (sugar beet) and mustard family (canola, radish, turnip). Following a classic case of summer fallow or a non-mycorrhizae supporting crop, the mycorrhizae population in soil will quickly drop. A cover crop mix that included a grass species (e.g. barley, rye) should still support mycorrhizae and prevent fallow syndrome concerns.

Preventing fallow syndrome

The easiest prevention strategy after fallow is planting a crop species without fallow syndrome risk like soybean, canola, or sugar beet. Avoid planting susceptible crops like corn and wheat. These crops are highly dependent on mycorrhizae to acquire phosphorus, and extra starter phosphorus will be required if fallow syndrome risk is present.

To reduce fallow syndrome risk in corn or wheat, extra phosphorus fertilizer must be placed with or near the seed. Applying more broadcast phosphorus or relying on high soil test P will not prevent fallow syndrome. The starter phosphorus rate should be 20 to 40 lb/acre P2O5. In some university research trials, up to 60 lb/acre P2O5 with 2×2-band placement near the seed was needed to prevent corn yield loss to fallow syndrome.

For wheat, these phosphorus rates are typically seed safe with monoammonium phosphate (MAP, 11-52-0). Most corn planters can safely apply 20 lb/acre P2O5 (5 gal/acre ammonium polyphosphate, APP, 10-34-0) in the furrow. For medium/fine-textured soils with good soil moisture at planting, you can generally apply up to 10 gal/acre 10-34-0 (40 lb/acre P2O5) safely in the furrow at 30-inch row spacing. Higher 10-34-0 rates may exceed seed safety limits on dry soils or coarse-textured soils and require 2×2-band placement to maintain seed safety.

Complete liquid fertilizers, such as 6-24-6 or 9-18-9, are not suggested for preventing fallow syndrome. Compared to 10-34-0, the products have lower P concentration that result in less applied phosphorus, even if used at maximum seed safe rates. The extra N + K2O in “complete” liquid fertilizers increases the salt index and lowers the seed safe rate.

John Breker

Prevented Planting Acres in 2020: Maximizing Cover Crop Effectiveness

in , , , , , , /by

In 2020, there are again widespread acres of Prevented Planting (PP) in North Dakota and northwest Minnesota. Farmers are now making plans to plant cover crops on unplanted cropland in the next few weeks. It is important to establish cover crops on PP fields because growing plants help reduce the chance these fields will be PP fields again next year.

Let’s look at the major reasons why cover crops are valuable tools on Prevented Planting acres.

Soil Water Use

A field without any growing plants is a fallow field. Before no-till, summer fallow was a widespread soil water conservation strategy in dryland agriculture. Actively growing plants transpire (use) a lot more water than evaporation from the soil surface alone does. Cover crops help fill the water-use void by transpiring a lot of water, helping to dry the soil surface and lower the water table before the following year. This also opens space in the soil profile for summer and fall rains to leach soluble salts from the soil surface and reduce salinity in the root zone.

Soil Erosion Control

Tillage is a popular weed control tool, but it also destroys crop residue and leaves soil exposed and vulnerable to water and wind erosion. Planting cover crops protects the soil surface from rain and wind, keeping soil firmly in place. Just because you cannot grow a cash crop on the field this year, you should not let your soil blow into the next field, letting your neighbor farm it next year.

Weed Control

An established cover crop can compete with weeds, helping suppress weed growth and weed seed production. For fields with problematic broadleaf weed histories, a cover crop mix containing only grass species is preferred. In grass cover crops, you can still use selective broadleaf weed herbicides to control the problematic broadleaf weeds of conventional or no-till systems such as Canada thistle, common ragweed, kochia, volunteer canola, and waterhemp while not killing the grass cover crop. For fields with low weed pressure, a cover crop mix containing grasses, brassicas, and legumes will provide more soil health benefits.

Soil Biological Activity

Have you heard about “fallow syndrome” before? Fallow syndrome is an induced nutrient deficiency, often seen in corn following fallow, when the population of mycorrhiza fungi is insufficient to colonize plant roots and help them acquire water and nutrients. Mycorrhizae are especially important in plant uptake of phosphorus, so plants with fallow syndrome often show phosphorus deficiency symptoms. Fallow syndrome is a major concern in corn following summer fallow or Prevented Planting without cover crop.

During the Prevented Planting year, it is important to include grass species in the cover crop mix to support and maintain the mycorrhiza population through next year. Brassica species, like radish and turnip, are often included in cover crop mixes for their deep taproot architecture and high forage value for grazing livestock, but brassicas do not support mycorrhizae. You do not want a cover crop mix consisting of brassica species alone because fallow syndrome might occur next year.

 

In June 2020, excessive rainfall slammed some parts of the upper Midwest and northern Great Plains, drenching soils with 3 to 15 inches of rain over a couple days. On summer-flooded fields, cereal rye is an attractive soil management tool. You can plant or fly on cereal rye well into August or mid-September, and it will continue to use soil water through late summer and fall. Next spring, the overwintered rye will grow again, using more soil water and maintaining soil structure, providing you with a much better chance to plant the field. If soybean is the next crop, you can plant glyphosate-tolerant soybean into green cereal rye then terminate the cereal rye with glyphosate later. This practice has become more and more popular on difficult fields.

Do not forget about soil fertility and plant nutrition for cover crops. A modest application of nitrogen will help cover crop establishment, plant water use, and competition with weeds, as cover crops with adequate nitrogen will grow faster and larger than those without nitrogen. Around 46 lb/acre nitrogen (100 lb/acre urea, 46-0-0) should be enough to establish nice cover crop growth. Prevented Planting fields, being wetter than those successfully planted in spring, lost some, if not most, soil nitrogen via nitrate leaching or denitrification. Although additional nitrogen may have mineralized from soil organic matter during May and June, excess precipitation in June may have caused additional soil nitrogen loss. The best way to know is collecting 0-12 or 0-24 inch soil samples for nitrate-nitrogen analysis.

As you choose the appropriate cover crop mix on Prevented Planting fields, you must consider the pros and cons of each cover crop species and how each will help accomplish your goals. These are some helpful resources that will provide additional information on what cover crop options will work best on your fields.