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Without irrigation, crop yields are usually water‐limited on the acid soils of the southeastern USA. Economically feasible irrigation systems that require less water are needed. As part of a feasibility study of subsurface microirrigation for corn (Zea mays L.) production, the effects of irrigation line spacing and irrigation water amount were investigated in a 5‐yr study at Suffolk, VA. The soil was a Uchee loamy sand (loamy, siliceous, thermic Arenic Hapludult) with inclusions of Emporia loamy sand (fine‐loamy, siliceous, thermic Typic Hapludult). Treatments were three line spacings (0.91, 1.82, and 2.74 in apart, buried 0.38 m below and parallel to the crop row), low and high water amounts (about 41 and 62% of average pan evaporation, respectively), and a nonirrigated control. Nonirrigated grain yields ranged from 3.12 Mg ha−1 (1987) to 9.73 Mg ha−1 (1989). Irrigated grain yields ranged from 8.27 to 12.02 Mg ha−1. Except for 1987 (the driest year), there was no significant difference between the low and high water application amounts. In 1987, corn irrigated with the higher rate yielded 34% more. Grain yields were highest with irrigation lines placed under each row (narrow spacing) and ranged from a low of 8.55 Mg ha−1 (wide spacing) in 1989 to a high of 12.25 Mg ha−1 (narrow spacing) in 1990. Increased amounts of irrigation water were needed with the wider line spacing to obtain corn grain yields comparable with the other two spacings. It is unlikely that the lowered yield or increased irrigation water required with wide line spacing could offset additional installation costs for the narrow spacing. Irrigation lines spaced in alternate corn row middles (1.83 m) or under the third row (2.74 m) are recommended.

Simulation is used to compare water use, yields, and the relative profitably of fixed and towable center‐pivot sprinkler irrigation and subsurface microirrigation for a corn (Zea mays L.) and soybean (Glycine max L.) rotation. Mean water use is somewhat lower on corn for microirrigation compared to sprinkler irrigation while water use differs little on soybean. Yield responses to irrigation differ little between the two systems. Relative profitability of systems depends on size. At 75 acres, subsurface microirrigation is the most profitable system while the towable center‐pivot system is most profitable at 150 and 300 acres. The towable center pivot is more profitable than the fixed center pivot for all system sizes. Microirrigation is more profitable for smaller systems because of its lower per‐acre investment cost and lower pumping cost. Towable center‐pivot irrigation is more profitable for larger systems because its peracre investment cost declines with increasing size.

A 3-year field experiment was conducted in 1996-98 near Penicuik, Scotland, to investigate the fate of N released after cultivation of previously long-term grass and grass-clover swards. The effects of timing of cultivations (autumn and spring), tillage methods (no tillage, ploughing to 200 mm and ploughing to 300 mm) and fertilizer N for spring (0, 40, 80 and 120 kg N/ha) and winter barley (0, 60, 120, 180 kg N/ha) on yield, N uptake and nitrate leaching were measured.In 1996, after spring cultivation, on plots previously in grass, spring barley grain yield and N uptake did not respond to N fertilizer, but on plots previously in grass-clover there was a nearly linear response to N. In 1997, the spring barley responded to N fertilizer at all levels. Yields of 1997 winter barley after grass did not show a response above 60 kg N/ha, but increased with fertilizer N up to at least 120 kg N/ha after grass-clover. In 1998, there were strong effects of N fertilizer and cultivation method on grain yield and N uptake of both spring barley and winter barley. Winter barley grain yield was significantly higher in plots previously in grass than in plots previously in grass-clover in 1998, though not in 1997. Winter barley yields were higher than spring barley at the same fertilizer N level.Throughout the 3 years, the no-tillage plots had consistently lower yields than the ploughed plots, but there was no consistent difference between the ploughed and deep ploughed treatments. There were strong interactive effects between tillage and previous sward in 1997. No-tillage under spring barley generally yielded lower than ploughing due to difficulties in weed control and the frequent anaerobic conditions in the soil.Annual leaching losses were relatively small (6 . 4-19 . 6 kg N/ha). In 1996 -97, more N was leached from the plots left in stubble following spring barley than from those planted with winter barley after either spring barley or grass in 1996, but in 1997-98 more N was leached from plots in winter barley than from those in over-winter stubble. Nitrate leaching was least under no-tillage, though the effect was not significant.

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