Passage IV

To increase corn crop yield (amount produced), fertilizer nitrogen (fertilizer N) is added to soil to supplement soil test N (various naturally occurring N sources in soil). In a responsive soil, the yield is greater when fertilizer N is added than when fertilizer N is not added. In a nonresponsive soil, the yield does not change when fertilizer N is added.

A soil's amino sugar N (a naturally occurring N source different from soil test N) content can determine whether a soil will be responsive. Two studies were done over 2 consecutive years in 25 cornfields in the same 1,000-hectare area. The soils' amino sugar N and soil test N contents were examined, as well as how the yield was affected by adding fertilizer N.

For each field, just before the spring planting, samples of the top 15 cm of soil were collected in several locations, then thoroughly mixed and oven-dried for 24 hr. The dried mixed soil for each field was analyzed for ammo sugar N content and soil test N content, in milligrams per kilogram (mg/kg). The results are shown in Figure 1. No fertilizer N was added to any of the fields. At harvest, the yield for each field was recorded.

Study 2

The next spring, for each field, soil samples were collected, mixed, dried, and analyzed for amino sugar N as in Study 1. After collection and before spring planting, fertilizer N was added to each field at a rate of 120 kg N/hectare. No other fertilizer N was added over the growing season. At harvest, the yield for each field was recorded. Then, for each field, the percent increase in yield from the previous year was determined and plotted against the soil amino sugar N content (see Figure 2).