While most successful gardeners don't collect data and make calculations on the efficiency (the ratio of output per unit of input) of their gardens, it can be an important way for urban gardeners to become more knowledgeable and aware of garden dynamics and ways to increase efficiency. It can also contribute to the design and improvement of larger projects, and provide input for discussions of government policy.
Tom Orum, Nancy Ferguson, Daniela Soleri, and I were inspired to measure inputs (including water, manure and labor) and outputs (weights and market value of produce) in our Tucson, Arizona, household gardens in the early-1980s, because so many people we talked to in Tucson thought that gardening in the Sonoran Desert was a hobby for people who could afford to pay exorbitant prices for produce in terms of their water bills.
We found that this assumption was not at all accurate. We worked in our gardens only 2-3 hours per week, and did not try to increase the value of produce by growing crops with high market value, yet the value of the produce from these gardens was about eight times the cost of the irrigation water (although the net returns provided a return on labor of only about $1.00 an hour). In addition, the gardens supplied significant proportions of the Recommended Dietary Allowances (RDAs) for 10 nutrients, including over 50% of the RDAs for vitamins A and C for more than 6 months out of the year. Yields over 3 and 2.5 years in these small (77.4 and 58.3 m2) gardens were 1.2-6.5 kg/m2, compared with commercial vegetable production in the United States which averaged about 1.7 kg/m2 at the time of our study. These findings of the nutritional, economic, and agronomic efficiency of gardens are supported by other studies of gardens and small-scale intensive agriculture.
In dryland cities water is often the most expensive input in terms of money, time, and/or equipment to extract or deliver it. A key to improving overall efficiency is, therefore, increased water efficiency, in terms of the amount produced (measured as weight, market value or nutrients) per unit of water used (measured as volume, cost, or cost of labor to obtain and apply it).
Overall water costs can be reduced by adjusting cropping patterns to increase production in seasons when efficiency of output per unit of irrigation water is the highest. For example in one of our Tucson gardens rainfall supplied only 5% of the total water input in the hot, dry spring (April-June), 16% in the hot, wet summer (July-September), and 24% in the cooler, drier fall (October-December), but 53% in the cool, wet winter (January-March). Yet many people first coming to Tucson think only of planting in the spring. Storing food grown in periods of high water use efficiency for consumption during periods of low efficiency can also increase overall efficiency. The amount of water that needs to be imported into the household can be reduced by harvesting rainwater, and gray water can also be recycled.
Evapotranspiration (ET) can be reduced by reducing evaporation from the soil and leaf surfaces, and transpiration from plants in excess of that which occurs when crop requirements for water are fully met. Evaporation from the soil surface can be reduced by flooding diked beds quickly or by trickle irrigation, rather than watering the surface slowly or sprinkling. Applying water only to the root zone by subsurface irrigation, for example by filling buried pots with water, also reduces evaporation. Mulching, shading and windbreaks can reduce ET and materials from the garden, like sunflower stalks, carizo canes, tree branches and palm fronds can be used to construct them. Runoff can be reduced by increasing infiltration with diked beds or sunken beds that hold water in the garden, and by mulching to increase infiltration. In addition to not over-watering, deep percolation can be reduced by increasing soil water-holding capacity. The main way to do this is by adding organic matter, although in very sandy soils more clayey soil can be mixed in.
Composting is the most readily available source of organic matter, and composting is, therefore, a very important part of any urban garden. Making compost in dryland cities does not have to be labor and water-use intensive. Compost piles can be constructed whenever sufficient material has been accumulated, wet down and covered with soil, branches or palm fronds to reduce evaporation, and left until finished. While the outside layer of such a pile won't be broken down, the rest of it will.