Understanding the Role of NPK in Promoting N Fixation in Legume Crops

The process of biological N fixation involves the symbiotic relationship between plants and soil-dwelling bacteria.In this relationship, plant roots are infected by a specific bacterium, which are able to convert atmospheric N2 gas into a more biologically useful form, NH4+. This N can now be used to fuel plant growth and yield (Better Crops 1998). For their service, the bacteria are ‘paid’ with sugars derived from plant photosynthesis, which drives the biological activity of the bacteria cell. In general, plant N fixation capacity is highly correlated to overall legume crop yields (Sulas et al. 2016). Here, I review how management of nitrogen (N), phosphorus (P) and potassium (K) can help improve N fixation rates and crop yield.

Macronutrient Factors

Nitrogen

Due to the relationship between the crop and N-fixing bacteria, one might assume the plant does not need any additional N fertilizer to fuel crop growth.  However, this assumption is not valid as it does not recognize the temporal dynamics inherent to the development of optimal rates of N fixation (Marschner 2012). Legume crops are in need of N fertilizer inputs to drive growth at a few key stages (Figure 1):

• Seedlings must rely upon energy from the seed for initial growth. Nutrients and limited photosynthesis are expected to nourish the plant for the next 10 days to two weeks after germination. Pre-plant NPK and micronutrients can help seedlings develop their root system and optimize N fixation for future growth (Better Crops 1998).
• Fields that have not had a legume crop planted for >2 years will have depleted plant specific Rhizobiaceae populations, which increases the time needed for proper nodule infection and formation. N fertilizer will be needed to drive plant growth (Marschner 2012).
• Crop growth rate is demanding N at a rate that N fixation cannot keep up (Peoples et al. 1989).

Phosphorus

The role of P in crop production is well established, and it also plays an important role in promoting N fixation:

• P is a crucial component for converting solar energy into food, fiber and other plant products via photosynthesis (Wyant et al. 2013).
• P also plays a key role in the metabolism of sugars, energy storage, cell growth and the transfer of genetic information (Wyant et al. 2013).
• Under low P supply conditions, P deficiency limits plant root growth and the creation of ATP (biological currency) used to build sugars. Remember the critical relationship between plants and Rhizobiaceae bacteria: No sugar = no carbohydrates to pay for N fixation. In a study, Cassman et al. (1980), show that by increasing the P supply to a soybean crop, they were able to increase both root and nodule weight. This, in turn, drives an increase in above-ground yield as measured by shoot dry weight (Table 1).

Potassium

K increases crop yield, improves crop quality and is considered one of the primary plant macronutrients, along with N and P. Except for N, plants often require more K than any other nutrient, as it plays vital roles in crop growth and development. However, unlike N and P, K is not permanently assimilated into the chemical structure of the plant and is readily translocated between plant structures. Thus, its mobility in the plant allows K to influence most plant growth stages (e.g., germination, vegetative growth, etc.) and plant structures (e.g., roots, grain kernels, and fruits). In general, K has been shown to increase rates of N fixation and overall yields via the following mechanisms (Better Crops 1998):

• K contributes to good root growth and has been shown to improve the number and size of nodules on roots.
• K serves as a co-factor for the action of an enzyme needed to transport carbohydrates across cell membranes and into the phloem.Remember the critical relationship between plants and Rhizobiaceae bacteria: No sugar = no carbohydrates to pay for N fixation. In a study, Better Crops (1998), showed that by increasing the K2O supply to a soybean crop, they were able to increase both nodule number and nodule weight. This, subsequently, led to an increase in above-ground yield and seed protein quality (Table 2).

How to check for N fixation in the field

To check for effective nodulation beginning two to four weeks after germination, perform the following steps (Peoples et al. 1989):

1. Carefully dig around and remove several plants.
2. Wash the roots in a bucket of water.
3. Look for nodules and examine their distribution. Effective nodules generally are clustered around the taproot.  Roots with few nodules have not developed or are operating at reduced N fixation capacity.
4. Slice and observe the interior of several nodules. Nodules that have been actively fixing nitrogen have a red or pink interior. Nodules with white or pale-green interiors are not currently active.
5. If poor nodulation is found, examine soil test results for possible issues with your NPK program.

A more detailed assessment procedure can be found here: http://proof.saskpulse.com/files/general/150521_Nodulation_and_Nitrogen_Fixation_Field_Assessment_Guide.pdf

Understanding and improving N fixation capacity in legume crops requires an in depth understanding of the impact of current soil fertility status of the field, knowledge of the cropping history and the interplay between plant host and crop specific bacteria that form nodules. Below is a list of tools that can be used to organize and optimize N fixation in your crop.

Tools Used to Better Understand N Fixation Capacity in Legume Crops

• Soil and Plant Tissue Test Results
• Nutrient Uptake Curves
• In-field observation of root infection

Suggested Reading

Better Crops/Vol. 82 (1998, No. 3) - https://tinyurl.com/y9r59dq6

Better Crops/Vol. 93 (1999, No. 3) - https://tinyurl.com/ycppr3x3

- Dr. Karl Wyant, Western Division Agronomist