Shallow Ground Water-Tables Influence Corn and Soybean Root Growth in Iowa.

Quantitative measurements of below ground root traits can improve our understanding of how crops respond to soil-weather conditions. However, such data are rare. The objective of our study was to quantify two important root traits: a) root front velocity over time and b) maximum root depth in corn and soybean crops growing in Iowa. The study included six Iowa locations spanning northwest to southeast gradients of precipitation, two crops (corn and soybean), and different management practices (e.g., early vs. late planting and soil with and without subsurface tile drainage systems). In total, 20 replicated combinations of crops, locations and management practices were studied in 2016. Root depth was measured with conventional soil probes every 10 days in the field. Measurements were made on and between rows. Corn root depth was 5% greater than soybean. Maximum corn root depth ranged from 89±11 cm between rows to 150±5 cm on the row for corn and from 78±12 cm between rows to 145±9 on the row for soybean. Correlation analysis between root length and explanatory variables such as leaf area index (LAI), biomass, plant height, days after planting, and thermal time revealed that thermal time since planting is the best predictor of root growth (r²=0.85). Both corn and soybean roots required about 420 ^oC-days from planting to cover the entire soil surface, while the maximum root front velocity was 4 cm/day.