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 | 25 |
The size and machinery complement of each synthetic farm was determined from a survey and analysis of Nebraska farms (Bernhardt, 1994), and a schedule of operations was developed for each farm based on best management practices for east‐central Nebraska. The economic performance of the two systems was then quantified with a model developed by Olson (1998), and erosion and nutrient losses were evaluated with PLANETOR, a farm‐scale environmental and economic model (Center for Farm Financial Management, University of Minnesota). Energy and nutrient budgets for each farm were compiled from published values of the embodied energy of farm inputs (Pimentel, 1980) and crop nutrient and energy contents (Church, 1984; Holland, Welch, et al., 1991). The values of each indicator for the two farms are given in Table 3–4.
A rapid appraisal of Table 3–4 suggests that the agroforestry farm is more sustainable than the conventional corn–soybean farm. Although the systems perform similarly as measured by production indicators (e.g., harvest, energy capture efficiency, water use efficiency), the agroforestry farm does better economically (net income, capital borrowing) and in some measures of resource conservation (e.g., erosion, N loss). Neither system has a sustainable nutrient balance in that each exports considerably more N and P than it imports.
Of course, there is no way to tell from system‐level indicators how much of the improvement in the performance of the agroforestry farm is due to its woody perennial components. The underlying performance data (not shown) indicate that the tree components had a major impact on economic returns. Christmas trees and hazelnuts (Corylus L.) were very profitable, and windbreaks increased crop yields more than enough to compensate for the land taken out of production. Tree crops (with grassed alleys) eliminated
