ABSTRACT
Summary In recent field trials on all-grass swards on clay soils, fertilizer rates of 480 kg/ha and higher showed distinct residual effects in the following year. This paper analyzes the considerable residual effects of one treatment on heavy clay soil over 2 experimental years, after very high rates of N had been applied during 3 pretreatment years (totaling 800 kg N or more/ha/yr). The residual effect was measured through the increase in yield over a reference treatment, at one rate of N application (40 kg N/ha/cut, totaling 200 and 240 kg N/ha in the first and second experimental years, respectively). Over the 3 pretreatment years the amount of N applied exceeded the amount of N harvested by 941 kg N/ha. The excess N was reflected in accumulation of mineral N in the soil. At the beginning of the first experimental year the soil contained 314 kg mineral N/ha against the reference treatment 21 kg N. The accumulated mineral N was nearly depleted by the end of the first season. A residual effect was evident at every cut, and over the whole growing season the increase in yield was 6 metric tons (t) dry matter (DM) and 199 kg N/ha. In the second experimental year, there was no difference in soil mineral N status of the treatments. Still, a residual effect was observed through an increase in yield of 1.1 t DM and 21 kg N/ha. The study showed that on heavy clay soils—despite N losses from the soil, especially through leaching in winter—part of the applied N may remain in the soil-plant ecosystem and will be available for grass growth in the following season(s).
