AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1998, No. 2
Sometimes old concepts die hard. Potassium fertilization of cotton in the San Joaquin Valley of California is a case in point. Information has been accumulating for decades that potassium fertilization is essential for maximum yields in many fields. In fact, intensive university research in the 1980s proved without a doubt the benefit derived from potassium fertilization and suggested that nearly half of all the cotton acreage might benefit from such additions.
This research flies in the face of an old and certainly out-dated concept that fields in the arid western states are rich in plant available potassium and that supplemental fertilization with potassium is not necessary. Mostly true decades ago when yield levels were much lower and crops had not mined fields of potassium without replenishment, it is not true today. Many high value crops in California and the other western states produce more yield of better quality due to a properly managed potassium program.
The 1997 cropping season was especially bad for mid-season bronzing of cotton in the San Joaquin Valley. The symptom had previously been identified as potassium deficiency. It frequently occurs during boll development. There was a lot of speculation that the problem was something different...poor root vigor, disease (such as Verticillium wilt, fusarium, or agrobacterium), nutritional interactions, unidentified multiple factors, and even ozone. Pima seemed to be hit particularly hard. It is resistant to Verticillium wilt and pathologists rarely found vascular streaking or discoloration symptomatic of wilt in it indicating that this common disease was likely not the problem.
Let's consider potassium deficiency. There was a heavy boll load relative to a rooting system that may have been compromised by spring conditions. Bolls are a strong sink for potassium. That is to say, bolls need a lot of potassium, and they can obtain it preferentially from other plant parts. Additionally, potassium deficiency had been occurring on even well fertilized fields in previous years. This is clear from research results where fields that had been properly fertilized preplant or early in the season with potassium responded to in-season foliar applications. It appears that the cotton plant, even under the best of conditions, may have trouble getting all the potassium it needs...up to 3.0 pounds per acre per day during boll formation...to produce a high yielding crop.
If in doubt, let's test the new concept rather than waste years...again...debating the issue. Put out test strips in troublesome fields, not at 50 pounds of K2O per acre...California soils fix a lot of potassium...but at 400 pounds of K2O per acre or with foliar sprays at the very first sign of trouble. Let the results do the talking.
Mid-season bronzing in cotton fields during the 1980s and before was due primarily to potassium deficiency that was readily corrected by applying potassium fertilizers. If so then, why not now?
Old concepts die hard...even in the communication age.