Pulse crops such as peas, faba beans, beans and lentils need plenty of potassium during plant growth. In fact, they often take up twice as much potassium as cereals.
A 40 bushel crop of peas requires about 110 pounds of potassium, while a similar yield of faba beans needs more than 200 lb. In contrast, 40 bu. of spring wheat only takes up about 90 lb. of potassium.
Pulse crops are legumes and as such are responsive to potassium nutrition. Potassium has a big impact on legume yield and may have an even bigger effect on biological nitrogen fixation. But the effect is not a direct one.
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For nitrogen fixation to occur, legumes must enter into a mutually beneficial partnership with certain soil bacteria called rhizobia.
The rhizobia invade plant roots and multiply rapidly, causing a nodular swelling to form.
Nitrogen in the soil air surrounding the nodules is converted by the bacteria in the nodules to a form the plant can use.
The rhizobia obtain food from the plant and the plant obtains nitrogen from the rhizobia.
Both benefit and potassium helps.
Potassium fertility influences several plant factors that can limit nodulation and nitrogen fixation. These factors include:
- Root growth. Potassium increases root numbers and volumes, increasing the chances for root hairs to intercept the soil bacteria that initiate nodulation.
- Photosynthesis. Nitrogen fixation requires tremendous amounts of energy and potassium is known to increase photosynthetic
activity.
- Carbohydrate supply. Potassium increases carbohydrate accumulation in plants. A good supply of carbohydrates to the nodules is essential to complete the nitrogen fixation process.
- Translocation. Potassium accelerates the translocation of plant metabolites to and from the nodules, making the nitrogen fixation process more efficient.
- Protein formation. Potassium promotes the conversion of plant metabolites into proteins and amino acids.
- Nodule formation. Potassium has been shown to increase nodule number, size and productivity.
Whether grain or forage legumes, potassium has a positive effect on nitrogen fixation and nitrogen accumulation in the plant.
Grain legume crops can fix from 60 to more than 250 lb. of nitrogen per acre annually, if conditions are favourable.
Potassium plays a key role in ensuring the process is efficient and effective.
