By Jamie Patton
Farmers continue to struggle with wet and unplanted fields. It’s hard to focus on next year’s cropping season while contemplating today’s agronomic decisions. However, taking a proactive approach to managing soil health for next year’s crop is important to preserving future grain yields.
While leaving fields unplanted and controlling weeds is one potential prevented planting management option, it’s not the optimal strategy. Bare fields are more susceptible to soil erosion by wind and water, depressed microbial populations, reduced soil organic matter concentrations, and decreased crop growth the following season due to fallow syndrome.
Fallow syndrome, also known as post flooded syndrome or flooded soil syndrome, is characterized by slow, uneven early-season crop growth and visual phosphorus deficiency symptoms (older leaves becoming bluish-green or purple in color). Deficiency symptoms are often the result of poor plant nutrient uptake due to decreased arbuscular mycorrhizal fungi (AMF) infection of plant roots — not actual soil fertility limitations.
Fallow syndrome is more pronounced in corn than in soybeans planted into ground left bare the previous year. While research is extremely limited, fallow syndrome may depress corn yields up to 15% in the crop year following the unplanted season.
Beneficial AMF form symbiotic relationships with an estimated 70% to 80% of all plant species. The fungi produce threadlike structures, called hyphae, that invade the plant root system and extend into the surrounding soil. In some cases, the hyphae may collectively be more than 100 times longer than the plant’s actual root system.
This extensive fungal network grows throughout the soil matrix, accessing and sharing available water and essential plant nutrients — such as phosphorus, zinc and copper — with its host plant. In return, the plant provides the fungi sugar and carbohydrates to aid in its growth. Studies have shown that AMF may provide the plant with up to 80% of its phosphorus, 60% of its copper, 25% of its nitrogen and zinc, and 10% of its potassium. The benefits of this symbiotic relationship are often more pronounced when the plants are stressed by drought or low soil fertility situations.
In addition to their role in nutrient uptake, actively growing AMF also produce the beneficial “soil glue,” glomalin. Glomalin is key in binding surface soil particles together into soil aggregates, thereby creating pore spaces for root growth, water infiltration and air exchange, as well as protecting soil particles from erosion by wind and water. It is estimated that glomalin may be the primary glue for up to 90% of surface soil aggregates.
AMF require a plant host to survive. Most agricultural plants and weeds are hosts for AMF, except for species in the brassica family (kale, radishes, rape, turnips, etc.). When fields are left bare, the natural AMF population can decrease significantly, reducing the potential beneficial infection of the subsequent crop. When a host crop is planted, whether a cover crop or the next cash crop, AMF populations increase, with the time required for the population to rebound determined by the overall population decline. Typically, the impact of fallow syndrome remediates itself within two years of the fallow event.
As we move into late summer and fall, you can still reduce or eliminate the impact of fallow syndrome in unplanted fields by using cover crops. Planting non-brassica cover crops will promote AMF growth and reproduction, with the impact influenced by the amount of time the cover crop is actively growing — the longer the growing period, the greater the impact on AMF population. Additionally, cover crops will provide soil erosion, organic matter, microbiological, nutrient scavenging and compaction benefits.
Cover crop species should be selected based on planting date, desired goals, subsequent cash crop, seed availability and seed cost. Planting of brassica species should be limited when AMF population promotion is the primary goal. General guidance on selecting full- or late-season cover crops for Wisconsin prevented planting acres can be accessed online.
To ensure all Risk Management Agency rules are followed, it is imperative to consult your crop insurance agent regarding cover crop planting and any potential forage harvest if you are taking a prevented planting insurance indemnity.
Although the 2020 planting season is months away, planting cover crops this summer or fall will help to ensure your soils and soil microbes are ready to make the most of next year’s growing season. Avoid next year’s fallow slump by feeding the fungi.
Patton is a senior outreach Extension specialist for northeast Wisconsin.