Saturation. Denitrification. Ruts. Saturated soils need special attention to come back to normal production
Excess water can deplete soils. During years of excess water, plants will experience extra stress. In the years after land has been saturated, it’s important for farmers to re-evaluate their fertility programs.
“Soil testing is especially important since nitrogen levels may be depleted due to denitrification and the potential for salinity may increase,” says John Heard, crop nutrition specialist with Manitoba Agriculture, Food, and Rural Initiatives (MAFRI). “Supplemental nitrogen fertilizer can offset a portion of the yield losses due to excess water.”
Excess water can reduce root respiration and other critical plant processes. It can also cause the production and accumulation of phytotoxic compounds, for example ethylene in plant roots and soil.
Ethylene is a root growth inhibitor with varying effects on different crops. Barley is especially sensitive to ethylene, barley roots will die at quite low concentrations. Several other crops will respond to a ethylene build-up with survival mechanisms, like producing secondary roots and upward growth of roots in search of oxygen. Potatoes respond to ethylene by increasing the size of their tuber pores, to increase air exchange. Legumes are indirectly affected by ethylene —it inhibits the formation and function of nitrogen-fixing rhizobium.
“Saturated soil conditions change the soil’s redox potential, favouring loss of nitrogen and production of ions (toxic under certain soil conditions),” said Curtis Cavers, potato agronomist at the Canada-Manitoba Crop Diversification Centre (CMCDC).
This hinders healthy plant growth, causing significant yield loss.
“Crop tolerance and adaptation to water-logging is plant species dependent,” said Cavers. “Plant roots and shoots can adapt to short-term reductions in oxygen by lowering respiration rates and slowing growth of shoots. Under conditions of excess water, it’s the lack of oxygen that changes the soil and crop environment.
“Oxygen diffuses in water 10,000 times slower than it does in air, resulting in changes in nutrient availability and microbial activity, reduced plant respiration, and energy production and the accumulation of compounds in roots and soil that may become toxic to plants. Prolonged exposure to excess water creates symptoms similar to those experienced by crops under drought conditions.”
According to Heard, mid-season excess moisture can be much more damaging than early-season moisture excess. “There are several factors that influence the magnitude of impact excess water stress has on growing crops, including soil type, plant species, plant growth stage, temperature, day length and duration of the stress,” says Heard.
The stress from excess water is greatest when there is rapid respiration. This usually occurs in July on soils with less drainable pore space, such as clay soils. On some soils, excessive May rainfall could result in crop yield increases.
“Like oxygen, carbon dioxide and ethylene gases diffuse more slowly through water than through air, accumulating around plant roots.”
“Reduction of nitrate to nitrous oxide (N2O) and nitrogen gas (N2) can result in nitrogen fertilizer losses of two to four pounds per acre per day when soil temperatures are greater than 5 C,” said Cavers.
“Different plant species are able to tolerate low oxygen levels depending on several mechanisms of plant adaptation, including lowered respiration rates of roots and changes in metabolic pathways to produce less toxic end products, such as malic acid rather than the more toxic ethanol.
“Producing replacement roots, which grow in well-aerated surface soil, doesn’t guarantee plant survival, in that roots developed this way still need oxygen and tend to grow horizontal rather than vertical — so these plants are shallow-rooted and more susceptible to later-season drought.”
Plant growth in excess water
Plant shoots slow overall growth to adapt to excess water stress. Although stem elongation still occurs, plants do not fill in. “The senescence and abscission of older leaves takes place, often remobilizing mobile nutrients, such as nitrogen to younger tissue,” said Cavers.
“Most annual crops can tolerate three to seven days of water stress, while forage legumes tolerate nine to 14 days, and forage grasses can tolerate 10 to 49 days. However, it’s important to remember other factors influencing the tolerance period, such as soil type, plant species, soil temperature, etc.”
Crop tolerance to excess water stress depends on the plant species. “In cereal crops, oats are the most tolerant of excess water stress, followed by wheat, and then barley,” said Cavers. “Barley plants are particularly vulnerable in the five- to eight-leaf stage, as head formation is initiated at the fifth leaf stage and water-logging decreases pollen viability.”
“Plants at the seedling and early vegetative growth stages haven’t consumed as much soil water, so soils may waterlog sooner with less rainfall. In addition, due to the lower oxygen demands from plants at this growth stage, soil oxygen depletion is slower, soils are cooler, and root biomass is smaller.
“Larger plants at flowering stages have used up much more soil moisture so the soil can absorb more water before approaching water-logging. These plants are photosynthesizing and respiring at their greatest rates and have the greatest need for water — which can’t be moved to the shoots under water-logged conditions. Furthermore, soil oxygen depletion is at a rapid rate. Large root biomass and warm soil temperatures encourage microbial respiration.
“As a result, high water use crops may be more buffered against the negative impacts of late-season rainfall events than low water use crops. However, once the soil becomes saturated, high water use crops will be negatively impacted more quickly and at a larger magnitude that low water use crops.” †