Fertigation frequency for subsurface drip-irrigated broccoli

Subsurface-drip irrigation and fertigation with fluid N fertilizer sources offers substantial flexibility for N fertilizer management. Fertigation events can be scheduled as often as irrigation, up to several times per day. However, because of system or management constraints very frequent fertigation may not be possible or desirable for some growers. Optimum fertigation interval for subsurface drip-irrigated crops has not been well researched. A 3-yr field experiment was conducted on a sandy loam soil in southern Arizona with subsurface drip-irrigated broccoli (Brassica olearacea L. Italica) to i) determine the effects of N rate and fertigation frequency on crop yield, quality, and crop N status, and ii) estimate a N balance. Broccoli was planted in two rows per raised bed 1.02 m apart, with one drip line buried 0.15 to 0.20 m deep within each bed. The experiment included factorial combinations of two N rates and four fertigation frequencies (intervals of 1, 7, 14, and 28 d). Broccoli marketable yield and quality were responsive to N rate, but not to increased fertigation frequency. During one of three seasons, fertigation frequency significantly (P ≥ 0.05) affected crop N uptake, but there was no trend of increasing N uptake with increasing fertigation frequency. Unaccounted fertilizer N and apparent N use efficiency (ANUE) were calculated for two seasons. Unaccounted fertilizer N averaged 20 and 75 kg ha^-1 and ANUE 90 and 81% with 250 and 350 kg N ha^-1 applied, respectively. Neither was significantly affected by fertigation frequency. We conclude, therefore, that for broccoli production with subsurface-drip irrigation on sandy loam or finer soils, fertigation can be applied as infrequently as monthly, without compromising crop yield or quality, or causing excessive N losses.