Are there fields that you were unable to plant by the last insurable planting date? Considering allowing the field to go fallow this year? Crops planted after a fallow period often struggle to yield, an effect referred to as “Fallow Syndrome.”
The problem is the result of breaking critical relationships between plants, microorganisms, and nutrient cycles. It may require months to rebuild these relationships once they are destroyed. If the weather has kept you from planting cash crops in a field this year, consider using a cover crop during the summer and fall to maintain a healthy growing environment that is ready to perform next year.
Most discussions of “Fallow Syndrome” focus on soil phosphorus. While total soil phosphorus may be high, available phosphorus may be limited. There are 2 major causes.
First, it may be out of reach. Phosphorus moves very short distances in the soil. Thus, plants have symbiotic relationships with fungi called mycorrhizae (literally “fungal root”). In exchange for sugars, mycorrhizae interact with plants to increase root surface area, enabling them to interact with more soil and find necessary nutrients like phosphorus. If these mycorrhizae are absent or at low populations, plants have difficulty intercepting tucked-away nutrients.
Second, phosphorous can cycle through several chemical forms, most of which are not available for plant uptake (figure 1). Ortho-phosphorus is the chemical form that is plant-available. For example, orthophosphorus interacts with aluminum or calcium to form insoluble minerals and is converted to an organic form when used by living organisms (lower right figure 1). Other soil microorganisms produce specific enzymes can once again make these nutrients available for plant uptake. Similar to mycorrhizae fungi, these microorganisms likely require living plants for part or all of their life cycle, though the connection might not be direct and physical.
During drought, flood, or lengthy fallow periods, soil microbial populations crash due to a lack of services provided
by living plants. In turn, nutrients are not being cycled because of low or slow microbial populations. Crops planted the following year face a 2-pronged, uphill battle to obtain nutrients: 1. There are few mycorrizae to help scavenge the soil for available nutrients. 2. The nutrient cycle is moving very slowly. The effect is lower crop yield after a fallow period. Losses in corn have been documented between 7-32 bu/acre the year after a fallow (Table 1 remade from Ellis 2013, Journal of Production Agriculture).
Instead of a fallow period, consider planting cover crops. Summer annual crops with fibrous roots like buckwheat, phacelia, sorghum-sudangrass, or sunflowers will keep soil microbes healthy and maintain a vigorous phosphorus cycle for the following year.