Fertilization practices for phosphorus (P) and potassium (K) during the last three decades have increased soil-test P (STP) values and soil-test K (STK) values of most agricultural soils in Iowa. This tendency for soil-test values to increase with time generally has been considered a desirable consequence of fertilization. This was especially true about 30 or 40 years ago when many Iowa soils tested very low or low in STP and STK. Recent soil-test summaries (Killom, Voss, and Eik, 1990) and surveys (Blackmer and Mallarino, unpublished), however, indicate that more than 70% of Iowa soils test high or very high in STP and STK. This situation suggests the possibility that many crop producers may be using more P and K fertilizers than needed. This possibility deserves attention because large areas of Iowa are planted to com and soybean and because P and K fertilization represent a significant cost for producers. Here I present an overview of various long-term and short-term P and K trials with corn and soybean conducted by various researchers in the Department of Agronomy. Some of the data presented have been published in more detail (Mallarino, Webb, and Blackmer, 1991a and 1991b; Mallarino and Blackmer, 1992; Webb, Mallarino, and Blackmer, 1992). The results shown here include changes of soil-test values over time, yield responses, critical soil-test values, maintenance rates, and economic returns from fertilization.