Response of Weeds to Soil Phosphorus and Potassium

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Abstract
Ten warm-season and seven cool-season weed species were grown in the glasshouse on Hartsells fine sandy loam taken from field plots that had been in long-term fertility studies. The soil pH was 5.9 and phosphorus (P) levels ranged from 8 to 95 kg/ha and the potassium (K) levels from 40 to 213 kg/ha. Growth response to P and K levels was measured by dry herbage yield. Redroot pigweed (Amaranthus retroflexus L.), jimsonweed (Datura stramonium L.), and Florida beggarweed [Desmodium tortuosum (Sw.) DC] were the most responsive warm-season weed species to P, whereas chickweed [Stellaria media (L.) Cyrillo] was the most responsive cool-season weed to P. Extreme P deficiency symptoms were evident on species giving a high response to P. P-deficient plants were severely stunted and exhibited a characteristic reddish-purple color. Showy crotalaria (Crotalaria spectabilis Roth), tall morningglory [Ipomoea purpurea (L.) Roth], sicklepod (Cassia obtusifolia L.), Carolina geranium (Geranium carolinianum L.), and coffee senna (Cassia occidentalis L.) were the most tolerant to low soil P. Redroot pigweed, jimsonweed, and Florida beggarweed were the most responsive warm-season weeds to K. Wild mustard [Brassica kaber (DC.) L.C. Wheeler var. Pinnatifida (Stokes) L.C. Wheeler] and annual bluegrass (Poa annua L.) were the most responsive cool-season weeds to K. Potassium deficiency symptoms were characterized primarily by severe stunting. Buckhorn plantain (Plantago lanceolata L.), Carolina geranium, and curly dock (Rumex crispus L.) were the most tolerant cool-season weeds to low soil K. Generally, weeds were more sensitive to low soil-test P than K.