Consequences of not fertilizing onions on organic soils with high soil test values

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Abstract
Two experiments of 4 year duration were conducted on organic soil sites which had accumulated P and K from several years of previous fertilization. There were no adverse consequences to an onion crop (Allium cepaL.) when P and K were omitted from the fertilizer program until the third year in one experiment. In the other there was no response to P and K even after 4 years of omission. Soil test levels declined substantially when P and K were not applied and also, although to a lesser degree, when they were applied at 112 and 168 kg/ha as P2O5and K2O respectively In contrast there were yield responses to N in most years. Nitrogen stimulated early development and always accelerated maturity as measured by the percentage of onion tops down in middle or later August. Concentrations of N, P and K in leaves or immature bulbs at midseason were always higher when these nutrients were applied indicating increased uptake even though yield responses did not always occur. When soil tests are relatively high, there appears to be little risk in reducing P and K providing the resulting decline in soil tests are monitored yearly. Eliminating N, however, could be very risky for early planted onions, even though substantial quantities can be released from organic soils later in the season.

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