North American Agroforestry. Группа авторов

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14,091 (5,707) Energy output/input ^c ratio of energy in harvested crops to cultural energy inputs 5 <1 3.9 4.5 Energy capture efficiency ^d energy in harvested crops as proportion of growing season PAR,% 1.0 0 0.38 0.35 Water use efficiency ^e harvested biomass divided by AET (g m⁻¹ mm⁻¹) 1.15 0 0.61 0.61 Imported fertilizer ^f N + P, kg ha⁻¹ (lb acre⁻¹) 0 151 (135) 44 (39) 26 (23) N losses ^g losses through erosion and leaching), kg ha⁻¹ (lb acre⁻¹) 0 45 (40) 28 (25) 20 (18) Soil erosion ^h wind + water, Mg ha⁻¹ (tons acre⁻¹) 0 11 (8) 11 (5) 7.8 (2.5) N balance ⁱ N inputs/N outputs 1 <0.8 or >1.2 0.6 0.64 P balance ^j P inputs/P outputs 1 <0.8 or >1.2 0.7 0.46 Crop diversity ^k no. of crops per farm 12 1 2 7 Hired labor ^l h ha⁻¹ (h acre⁻¹) 0 5 (2) 1 (0.4) 5 (2) Net income ^m US$ ha⁻¹ (US$ acre⁻¹) 235 (95) 89 (36) 99 (40) 249 (101) Capital borrowing ⁿ debt/variable income 0 1 0.63 0.46 Farmer knowledge ^o total skills and knowledge held by farm family high low medium high

^a High value is dry weight of grain from Nebraska irrigated corn (9406 kg ha−1 (150 bu A−1)).

^b The value indicating low sustainability is the energy input per hectare to produce irrigated corn in Nebraska (Pimentel, 1980).

^c From Pimentel and Pimentel (1996), energy output/input ratio for U.S. soybean production is 4.15:1; Ohio alfalfa is 6.17:1; corn and wheat are around 2.5:1. So, 5:1 is a reasonable upper end to scale.

^d Loomis & Connor (1992) showed that the theoretical maximum daily energy capture efficiency of a crop is 12% of photosynthetically active radiation (PAR). However, Tivy (1990, p. 109) wrote that only in exceptional cases do crop efficiencies exceed 2% PAR for an entire growing season, and efficiency in terms of economic yields is only 0.3 to 0.4%. If 2% capture of PAR is a high efficiency, then 1% PAR in harvest (50% of total net primary productivity harvested) is a high upper bound for energy capture efficiency.

^e 1.15 is the water use efficiency for corn (grain only) on a central Iowa farm (Loomis & Connor, 1992).

^f Irrigated corn yielding 9406 kg ha⁻¹ (150 bu acre−1) would export 128 kg ha⁻¹ (114 lb acre−1) N and 22 kg ha⁻¹ (20 lb acre−1) P.

^g High value (45 kg ha⁻¹ [40 lb acre−1]) is 2× the estimated N losses for corn on a central Iowa farm (Loomis & Connor, 1992).

^h 11.2 Mg ha⁻¹ (5 tons acre−1) is the soil loss tolerance (T‐value) for a Sharpsburg silty clay loam with 4–6% slope.

ⁱ System outputs (harvest and losses) within ±20% of inputs (imported and N2 fixation) is considered close to balance

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