AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Summer 1999, No. 8

Potassium is more easily absorbed by grass roots than by legume roots. Thus, in grass-legume stands, a shortage of potassium will first result in a decline in legumes from the stand. Later, undesirable weeds will begin to appear.

Potassium need increases with harvest frequency. Younger plants are higher in potassium content and protein level. With more frequent harvests, the higher quality feed is removing more potassium from the field. In general, this will be between 50 to 60 pounds of potash per ton of forage harvested.

Potassium improves winter hardiness and disease resistance for many forage crops. Plants low in potassium are similar to a tractor radiator with no antifreeze. The low electrolyte content of plant cells makes them more easily injured by sudden temperature changes. The loss of desirable plants from the stand creates the need for more frequent re-establishment.

Potassium needs are best established by combining soil analysis information with crop needs as determined by yield goal and harvest frequency. Soil test levels should be in the high range prior to crop establishment and then maintained through a topdress fertilization program based on returning the potassium removed from the field in the forage crop. In general, a six ton hay crop will remove at least 300 pounds of potash per acre.

Potassium is usually best applied according to plant growth stage and/or the harvest schedule. It is usually not necessary to fertilize after each harvest. Good yields of quality forage have been attained by splitting potassium needs and applying half after the first harvest in the spring and the remainder following the next to last harvest in the Fall. Multiple applications assure potassium availability throughout the season for optimum forage yield and quality. Also, they can minimize the risk of tetany in the spring, promote rapid regrowth following harvest, and build root reserves in the fall for better winter hardiness.

Potassium interactions with nutrients such as nitrogen and with certain harvest practices are well documented in research studies. Phosphorus and boron deficiencies can restrict potassium activities since phosphorus is vital for active root growth while boron is needed for new growth of both roots and shoots. Each participates in its special way in photosynthesis, the translocation of leaf sugars, and vital energy relations within the plant.

Potassium promotes nodule development in legume crops and nitrogen utilization by grasses. This allows forage crops to produce and/or utilize nitrogen for the formation of high quantities of quality plant protein.

Potassium’s contribution to forage yield and quality influences animal performance. Palatability, digestibility, and animal performance are measures of the value of insuring that potassium is not a limiting factor in the production of forages for animal consumption.