Evaluating Dry Bean Nodulation

DIGGING UP ROOTS… is this part of your routine crop scouting? You could be counting nodules on soybean roots or checking for clubroot in canola, but what about other crops and conditions?

Digging up roots and inspecting them can be just as valuable as observing the crop above ground. Plant roots form an extensive network with soil, interacting with microbes, water and nutrients to produce biomass and yield.

We should ask ourselves — how are they functioning? Can our management system improve them?

In the soybean and pulse agronomy research lab, we are studying nitrogen, preceding crop and residue management in dry beans at Carman and Portage. Digging up roots is standard protocol for collecting data on nodulation and root rot to help explain research results.

Here’s how you can make observations about dry beans in your fields.

RESEARCH BACKGROUND

Nitrogen fertilization at an average rate of 60 lbs N/ac is standard practice for dry beans in Manitoba. Dry beans are managed like a non-legume crop in most production regions because they are inefficient at producing their own nitrogen (N).

On average, they acquire <45% of their N requirements through biological fixation (1), which is a particularly intricate process in dry beans and dependent on factors such as environment, market class, variety and inoculant. Our current N recommendations are based on research from the early 2000s — what are N dynamics and requirements in dry beans today?

In 2017, a dry bean nitrogen fertility study was established at Carman and Portage to determine the optimum N rate for Windbreaker pinto beans and T9905 navy beans. To date, three site-years have evaluated rates of 0, 35, 70, 105 and 140 lbs of applied N/ac.

Interestingly, the only statistically significant yield response is to the highest rate of N and when return to N ($/ac) is calculated, no N fertilizer addition is economical.

Another experiment evaluating the effect of preceding crop (wheat, canola, corn or pinto beans) and residue management (tilled vs. direct seed) on pinto bean production was also established. Both of these studies have contributed to interesting observations that will advance our dry bean production systems in Manitoba and western Canada.

ROOT NODULES

Pinto and navy beans start flowering 45–60 days after planting which usually falls during the second week of July. This is also the time you are likely weighing the decision of a fungicide application, which can be aided by scouting just prior to flowering for observations on the plant canopy and soil moisture conditions.

Digging up roots and checking for nodulation should be done after that — during flowering through early pod formation. The goal is to provide a general indication of whether or not functional nodulation is occurring and the severity of root rot. Identifying nodule presence and function is a starting point for future nitrogen management decisions.

The majority of dry bean farmers are not scouting for or accounting for nodulation, and this makes sense since they are a fertilized crop and inoculants are not used (previous research has supported these management decisions). However, based on our current research evaluating nitrogen management and studying dry beans in general, we should re-visit this.

In dry beans, nodulation and root rot rating scales are not correlated to productivity or yield but provide a relative indication of performance. For example, higher nodulation scores and good colour equals greater potential for biological nitrogen fixation (Figure 1).