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 2002, No. 3

IS YOUR CORN FINDING SOIL PHOSPHORUS AND POTASSIUM?

How can you tell? During the season, there are two primary ways: looking and measuring. Looking involves scouting fields and areas within fields. Both potassium- and phosphorus-deficient corn can be stunted and poor growing. Unfortunately, many other factors can make corn look this way, and stunting is extremely difficult to diagnose without having healthy plants nearby for comparison.

What other visual symptoms may help? Phosphorus-deficient corn may exhibit some purpling on the lower leaves. Corn lacking potassium may have lower leaves with yellow or brown margins. These more diagnostic symptoms are best seen earlier in the season. The most scientific assessment of deficiencies involves taking tissue samples of the corn plant and sending them to a reliable laboratory for analysis. The laboratory will compare the nutrient levels of the sample to those known to be sufficient. The laboratory will guide you on what plant parts to send in. One thing to keep in mind: If you find a nutrient deficiency, yields have likely been irreversibly reduced for the season. Realizing this leads to the next question.

Where are sufficient supplies located? To answer this, you’ll need to rely on a few pieces of information: 1) soil test results, preferably taken from a few locations and depths, 2) where nutrients have been and will be placed in the soil, and 3) the tillage system and how long it has been in place. Soil sample results provide a relative index of plant-available phosphorus and potassium levels. Evaluating these levels both across the field as well as at incremental depths within the soil help identify nutrient distribution problems. Nutrient placement drastically affects how phosphorus and potassium are distributed. Applied at the same overall rate, broadcast applications provide better spatial distribution but less concentrated supplies. Banding produces localized zones with higher nutrient concentrations. Tillage systems also affect the distribution of nutrients. The longer reduced-tillage systems are implemented, the more likely it is that phosphorus and potassium are concentrated near the soil surface. Once you have a good idea where supplies
are located, you can move on to the next question.

Where are corn roots located? Early in the season, more of the root system is located shallower in the soil. As the plant matures, roots proliferate deeper and extend out further from the plant. The location of phosphorus affects root distribution. When corn roots find an area of higher phosphorus concentration in the soil, they proliferate there. Potassium does not have this effect. Rather, it tends to hasten the overall growth rate of plant roots. The implication is that potassium can help roots explore a greater soil volume more quickly and consequently can hasten the proliferation of roots in areas where phosphorus is concentrated. Once you can anticipate where the roots are, you can tie this to knowledge about the location of soil supplies.

Matching root growth to soil supplies. In reduced tillage systems, the highest concentration of nutrients at the soil surface may not match uptake requirements of roots deeper in the soil, particularly in drier conditions later in the season. Consequently, yield responses to deep band applications of phosphorus and potassium have been observed in reduced tillage systems. When overall soil supplies of these nutrients are low, concentrated bands alone may not be adequate to supply all of the crop needs. Increasing the overall fertility of soils allows exploring roots greater access to nutrients throughout the season.
—TSM—

For more information, contact Dr. T. Scott Murrell, Northcentral Director, PPI, 3579 Commonwealth Road, Woodbury, MN 55125. Phone: (651) 264-1936. E-mail: smurrell@ppi-far.org
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