AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1996, No. 1
PULSE CROPS NEED POTASSIUM
Pulse crops, such as peas, faba beans, beans and lentils, need plenty of potassium. In fact, they often remove twice as much potassium as cereals. A 40-bushel crop of peas removes about 110 pounds of K2O, while 40 bushels of faba beans removes over 200 pounds. In contrast, 40 bushels of spring wheat removes only about 90 pounds of K2O.
Pulse crops are legumes and, like other legumes, are very responsive to potassium fertilization. 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.
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 swelling, or nodules 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 which can limit nodulation and nitrogen fixation. These factors include:
Potassium increases nitrogen fixation in legumes. Whether it’s soybeans, broad beans, alfalfa or clover... potassium has a positive effect on nitrogen fixation and nitrogen accumulation in leguminous plants. Pulse crops can fix from 60 to more than 250 pounds of nitrogen per acre annually, if conditions are favorable. Potassium plays a key role in ensuring the process is efficient and effective.
- Root growth—potassium increases rooting numbers and volumes, increasing the chances for root hairs to intercept the soil bacteria which 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, thus providing a sink for the nitrogen fixed.
- Nodule formation—potassium has been shown to increase nodule number, size and productivity.
For pulse crops...pay particular attention to potassium.
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