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 (inputs/outputs = 1). Values greater than or less than 1 would indicate potential environmental problems or depletion of fertility.
j System outputs (harvest and losses) within ±20% of inputs (imported P) is considered close to balance (inputs/outputs = 1). Values greater than or equal to 1 would indicate potential environmental problems or a depletion of fertility.
k Bender (1994) grows 12 crops on his eastern Nebraska organic farm. Diversity of this magnitude is required to implement flexible rotations for weed control and fertility and to provide sod and pasture crops for grazing and erosion control.
l Irrigated corn in Nebraska requires 5 h labor ha−1 (2 h labor acre−1) (Selley, 1996).
m A 172‐ha (425‐acre) farm would have to generate $89 ha−1 ($36 acre−1`) in net income to keep a four‐person family above the official poverty line ($15,141; U.S. Census Bureau, 1997, Table 732). An average size Nebraska cash grain farm (255 ha [630 acre]) generating $235 ha−1 ($95 acre−1) would be in the 90th percentile of net farm income for that type of farm (Johnson, 1995).
n A value of 1 indicates that the income remaining after fixed costs are covered is just sufficient to repay operating loans plus interest.
o This is very difficult to quantify, but it is assumed to be positively correlated with the number of crops and enterprises on the farm.
Once indicators of sustainability have been defined, they can then be used to evaluate the effect of agroforestry practices on the sustainability of a farming system. Thevathasan et al. (2014) have suggested utilizing a common method for visualizing sustainability indices through the use of “amoeba diagrams,” originally developed by Bell and Morse (2000). Amoeba diagrams are two‐dimensional, multi‐axis diagrams where the axis scale can be ordinal or relational (Figure 3–4). Using relational axes makes visual interpretation easier. In the absence of distinct values (or ranges of values) that are deemed thresholds of sustainability, data can be normalized against a reference state. The reference state may be determined by collecting information from a local site that reflects an ideal state of the ecosystem. This could be a site that has minimal disturbance and native vegetative cover, or it could be farmland that is currently managed under the best management practices.
Fig. 3–4. Example of an amoeba diagram (NPV, net present value; BOD, biological oxygen demand; GHG, greenhouse gas)
(adapted from Bell & Morse, 2000).
Amoeba diagrams do not provide a composite value for sustainability. They are a visual representation that effectively gives equal weight to each index that will allow comparison and interpretation. Collecting the same set of data on the sustainable indicators with time, the user can see which areas are improving and which are declining while still getting a sense of the overall sustainability of the system.
Sustainability indices can also be assessed in more quantitative terms. We have undertaken a quantitative comparison of two synthetic farms modeled from regional data (Table 3–5). One of the synthetic farms is the conventional corn–soybean farm described in Appendix 3‐1, while the other is a more diversified farm that incorporates windbreaks, an herbaceous perennial crop, and two woody perennial crops in block plantings.
Table 3–5. Characteristics of two model farms in eastern Nebraska representing a conventional cash grain operation and an agroforestry alternative, both on a Sharpsburg silty clay loam with 4–6% slope.
| Characteristic | Conventional farm | Agroforestry farm |
|---|---|---|
| Size, ha (acres) | 264 (650) | 172 (425) |
| Rented land, % | 55 | 0 |
| Crops, ha (acres) | ||
| Corn | 132 (325) | 34 (83) |
| Soybean | 132 (325) | 61 (151) |
| Grain sorghum | 34 (83) | |
| Alfalfa | 24 (60) | |
| Christmas trees | 4 (9) | |
| Hazel nut production | 6 (16) | |
| Windbreaks | 9 (23) | |
| Area in perennials, % | 0 |