Soils that test “low” in potassium negatively affect weed control in soybeans.
When soils test low for potassium, soybean plants will express deficiency symptoms that reduce canopy closure and allow more weeds to germinate and compete with the soybean crop for other resources.
In the spring of 2014, Dr. François Tardif and I began a long-term study of cropping systems that aims to quantify the contributions of “cultural practices” (e.g. fertility, cover crops, tillage, plant populations and rotation) on weed control. When we collected soil tests for each replication of the trial, we were surprised to find a range of potassium levels between replications that could be characterized as low (ranging from 50-90 ppm).
To address this limitation, potash was broadcast on the site to correct this deficiency and build the soil potassium level. Unfortunately, we only had the opportunity to apply potash in the spring. The significance of this is that fall applications of potash would have provided a greater opportunity for the potash to move deeper into the root zone for plant uptake the following year, particularly since some of our trial treatments would be in a no-till system.
What was observed during the growing season as we collected data was that plots that fell in a region with very low-testing potassium (50 ppm) showed deficiency symptoms, even after potash was broadcast in the spring, whereas plots that fell in a region with better levels of potassium (80 ppm), showed little to no symptoms. The significance of this is that when we evaluated canopy closure by quantifying the amount of light that reached the soil, there was noticeably more light reaching the soil surface in the plots showing potash deficiency (Figure 1 below).
When we evaluated weed control and seed production prior to soybean harvest, there were more weeds and weed seeds in the plots that expressed potassium deficiency symptoms (Figure 2 and 3 below).
This serves as a nice reminder that if soil fertility issues aren’t addressed adequately, your investment in other management, like weed control, will never be maximized.