Potassium fertilizer and roots?

from TurfNet Monthly November 2004
by Micah Woods

Turfgrass professors tell us that more roots are obtained at higher potassium levels, but plant science researchers tell a different story…

Earlier this year I presented  some preliminary results from  the soil testing studies I have been  conducting at Cornell University to a meeting of the Central NY GCSA. Many in the audience were interested  to know how potassium fertilizer applications, or the withholding of potassium fertilizer from the turf, had affected the L-93 creeping bentgrass roots in the study. I did not have any final data to share at that time, but the audience’s pointed questions about rooting certainly underscored in my mind the widespread, traditional assumption that potassium plays a critical role in turfgrass rooting.

In Turfgrass  Science and Culture (Beard, 1973), it is clearly stated that higher soil potassium levels yield increased root development and branching. The recently published Turfgrass Soil Fertility and Chemical Problems (Carrow et al., 2001) emphasizes the role of potassium in sand rootzones, where potassium encourages “a more extensive fibrous or branched root system.”  The theory of root stimulation by potassium is propagated and promulgated anew whenever superintendents or turfgrass students are taught about turfgrass nutrition. I recently participated in an online seminar about fall fertility strategies for cool-season grasses, and sure enough, we were reminded by the instructor that a primary plant response to potassium is deeper roots with more branching.

Turfgrass professors tell us that  more roots are obtained at higher potassium levels, but plant science researchers tell a different story. The definitive plant nutrition reference is Mineral Nutrition of Higher Plants (Marschner, 1995), in which Marschner wrote that mineral nutrient supply can strongly influence root growth, with nitrogen having a particularly marked effect, less so for phosphorus,  and usually no effect for other nutrients.

A classic study of the effect of variable ammonium, nitrate, phosphate, and potassium supply on barley roots [Drew, 1975] found that the number and length of lateral roots were stimulated by all nutrients except for potassium. In fact, some plants have more roots when no potassium is applied [Cherney et al., 2004], perhaps because the roots explore a larger volume of soil in order to obtain the potassium that they need.

So why, I wondered, are we taught that potassium increases roots, even though the consensus among plant scientists is that potassium has little effect on roots?

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