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

Spring 1999, No. 1


Crop scientists from around the world gathered recently to discuss the reasons for past and future yield increases. Surprisingly, many agreed that increased stress tolerance, more than greater yield potential, has increased yields in the past and will continue to do so in the future. Today’s genetics confer a stress tolerance which contributes to higher crop yields. Nutrient management, particularly with potassium, could enhance this contribution.

Every year, the U.S. National Corn Growers Association sponsors a maximum yield competition among farmers. Over time, a very interesting phenomenon is emerging. While the maximum yields of rainfed corn have increased in some states, the highest yields for irrigated corn have not. Also, the (unofficial) record yield of 370 bushels per acre, harvested by Herman Warsaw in 1985, has not been topped after 13 years.

During this time, the average yield of corn in North America increased 1.7 percent per year. If the same trend applied to maximum yields, we would expect a record yield over 450 bushels per acre by now. Are we approaching a maximum limit on corn yield? Has the genetic potential for higher yield stopped improving?

Older corn hybrids yield as much as newer hybrids when grown in low-stress environments, but under stress they don’t hold up as well. Examples of stresses in which new hybrids show their advantages are increased plant populations, drought stress, and weed competition.

Potassium has long been known as the nutrient most associated with stress tolerance in plants. It can reduce the amount of water lost through the leaves. Plants have tiny openings in their leaves called stomates through which water transpires to the atmosphere. Closing the stomates is a defense mechanism to conserve water. Plants with ample potassium close their stomates more quickly. For example, a Montana experiment showed that barley plants exposed to hot, windy conditions were able to slow water loss within 5 minutes when they had adequate potassium. But with lower levels of potassium, it took about 45 minutes.

Additionally, potassium keeps photosynthesis going under stress. Plant cells that lose too much water slow down in photosynthesis because of internal distortion. Potassium within plant cells has an osmotic effect that helps retain water. Plant scientists in Connecticut found that leaf potassium concentrations above optimum for normal conditions can be beneficial for stress conditions. When wheat plants were nourished with a solution three times richer in potassium than optimal for normal conditions, their leaves sustained rates of photosynthesis 67 to 114 percent higher after an 8-day water stress period. What is often called "luxury consumption" may help plants resist stress.

High crop yields always occur under stress. The highest yield occurs when plants are crowded together closely enough to use as much as possible of the available resources for growth… water, sunlight and nutrients. In order to produce high yields, the crop needs the genetics and the nutrition to overcome stress. Look for the
optimum combination of new genetics and potassium nutrition to boost your yields with greater stress tolerance.


For more information, contact Dr. Tom Bruulsema, Eastern Canada and Northeast U.S. Director, PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, CANADA.   Phone: (519) 821-5519; E-mail:
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