| Abstrakt: |
High rates of P fertilization for potatoes (Solarium tuberosumL.) over many years have resulted in significant increases in total and extractable soil‐P reserves. While fertilization practices for other crops grown in rotation with potatoes reflect both plant demand and soil‐test P, many producers continue to fertilize potatoes with little regard to crop requirements nor to existing soil‐P levels. If neither potatoes nor crops grown in rotation with them require such high rates of directly‐applied P, a considerable saving in fertilizer costs could be realized. This hypothesis was tested in two field experiments (on a Typic Umbraquult, fine‐loamy, mixed, thermic soil) designed to measure tuber yields and trace patterns of dry matter and P accumulation by ‘Pungo’ potatoes in response to row‐applied fertilizer P[FP (1969) = 0, 24, 48, and 72 kg/ha; FP (1970) = 0, 28, and 56] on soil with 35, 66, and 110 kg of extractable soil P/ha. The latter objectives were accomplished by weekly samplings and appropriate analyses of tops and tubers during tuber bulking. When soil P was low (35 kg) or medium (66 kg), row‐applied P increased yields by 38 and 22%, respectively; but no response to fertilizer P was obtained if initial soil P was high (110 kg). At tuber initiation, P concentration in whole plants ranged from 0.20% under severest deficiency to 0.45% under optimal nutrition; it dropped to less than 0.20% at harvest, regardless of treatment. On low‐P regimes (SP < 66, FP = 0) rates of dry matter and P accumulation were still positive at 84 days after emergence; but plants on other treatments had lost most of their leaves—and many haulms were dead—at 79 days. At tuber initiation, plants with and without row‐applied P had accumulated 20 and 10%, respectively, of their 72‐day total P. Dry matter at tuber initiation was about 10% of the 72‐day total for all treatments. About 75% of the total accumulated P and 70‐75% of total dry matter was in tubers at harvest. While massive P applications to these low P soils are needed to make them productive, my results suggest that even high‐demand crops like potatoes require relatively modest amounts of row applied P to maintain maximal productivity once the initial deficiency is eliminated. |
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