The physical aspects of sustainability are partly understood. Practices that can cause long-term damage to soil include excessive tillage (leading to erosion) and irrigation without adequate drainage (leading to salinization). Long-term experiments have provided some of the best data on how various practices affect soil properties essential to sustainability. The most important factors for an individual site are sun, air, soil and water. Of the four, water and soil quality and quantity are most amenable to human intervention through time and labour.
Sistem Produksi Primer Plants produce plant matter from soil nutrients, water and carbon dioxide, using the energy of light. It is called primary production. The diagram shows the carbon flows (is equal to energy flows). At left one sees a plant receiving light and CO2 from the air and returning oxygen. At night, when there is no sunlight, plants respire like animals do, taking up oxygen and returning CO2. Surprisingly, a large proportion of a plant's primary production (50%) disappears underground, where it grows the root system and feeds soil organisms. Only 50% is used for above-ground growth. Of this, between 10 and 40% is used for growing, depending on plant type, age and kind of harvesting. If the plant is grazed regularly, the grown biomass will be grazed, amounting to no more than 40%. The remaining 10% is lost by leaf drop. This leaf litter is decomposed by fungi and bacteria, contributing energy to the soil biota, while returning nutrients to the plant
Udara dan sinar matahari tersedia di mana-mana di muka Bumi ini, namun tanaman juga tergantung pada ketersediaan hara dalam tanah dan ketersediaan air dalam tanah. Ketika petani menanam dan panen tanaman , mereka mengambil sejumlah hara dari dalam tanah . Tanpa pengembalian sejumlah hara ke tanah, maka tanah akan mengalami penurunan tingkat kesuburannya dan dapat berdampak pada penurunan pertumbuhan dan hasil panen. Pertanian berkelanjutan tergantung pada pengelolaan tanah dan meminimalkan penggunaan sumber daya yang tidak terbarukan, seperti gas alam (digunakan dalam mengkonversi nitrogen atmosfer menjadi pupuk sintetis) , atau bijih mineral ( misalnya fosfat ) . Sumber nitrogen yang dapat tersedia secara berkelanjutan , meliputi:
Daur ulang limbah tanaman dan ternak atau kotoran manusia yang telah diolah
Menanam tanaman legume dan hijauan seperti kacang-kacangan atau alfalfa yang membentuk mampu bersimbiosis dengan bakteri fiksasi nitrogen yang disebut rhizobia
Produksi industri pupuk nitrogen dengan proses Haber menggunakan hidrogen , yang saat ini berasal dari gas alam , tetapi hidrogen ini sebenarnya dapat dibuat dengan elektrolisis air menggunakan listrik ( mungkin dari sel surya atau kincir angin )
Rekayasa genetik tanaman non - legume untuk membentuk simbiosis penambat nitrogen atau fiksasi nitrogen tanpa simbion mikroba .
The last option was proposed in the 1970s, but is only recently becoming feasible. Sustainable options for replacing other nutrient inputs (phosphorus, potassium, etc.) are more limited. More realistic, and often overlooked, options include long-term crop rotations, returning to natural cycles that annually flood cultivated lands (returning lost nutrients indefinitely) such as the Flooding of the Nile, the long-term use of biochar, and use of crop and livestock landraces that are adapted to less than ideal conditions such as pests, drought, or lack of nutrients. Crops that require high levels of soil nutrients can be cultivated in a more sustainable manner if certain fertilizer management practices are adhered to.
Air - Pertanian
In some areas, sufficient rainfall is available for crop growth, but many other areas require irrigation. For irrigation systems to be sustainable they require proper management (to avoid salinization) and must not use more water from their source than is naturally replenished, otherwise the water source becomes, in effect, a non-renewable resource. Improvements in water well drilling technology and submersible pumps combined with the development of drip irrigation and low pressure pivots have made it possible to regularly achieve high crop yields where reliance on rainfall alone previously made this level of success unpredictable. However, this progress has come at a price, in that in many areas where this has occurred, such as the Ogallala Aquifer, the water is being used at a greater rate than its rate of recharge.
Suplai dan Penggunaan Air
An extensive water storage and transfer system has been established which has allowed crop production to expand to very arid regions. In drought years, limited surface water supplies have prompted overdraft of groundwater and consequent intrusion of salt water, or permanent collapse of aquifers. Periodic droughts, some lasting up to 50 years, have occurred in any areas. Several steps should be taken to develop drought-resistant farming systems even in "normal" years, including both policy and management actions: 1) improving water conservation and storage measures, 2) providing incentives for selection of drought-tolerant crop species, 3) using reduced-volume irrigation systems, 4) managing crops to reduce water loss, or 5) not planting at all.
The most important issues related to water quality involve salinization and contamination of ground and surface waters by pesticides, nitrates and selenium. Salinity has become a problem wherever water of even relatively low salt content is used on shallow soils in arid regions and/or where the water table is near the root zone of crops. Tile drainage can remove the water and salts, but the disposal of the salts and other contaminants may negatively affect the environment depending upon where they are deposited. Temporary solutions include the use of salt-tolerant crops, low-volume irrigation, and various management techniques to minimize the effects of salts on crops. In the long-term, some farmland may need to be removed from production or converted to other uses. Other uses include conversion of row crop land to production of drought-tolerant forages, the restoration of wildlife habitat or the use of agroforestry to minimize the impacts of salinity and high water tables
Indicators for sustainable water resource development are: ¤ Internal renewable water resources. This is the average annual flow of rivers and groundwater generated from endogenous precipitation, after ensuring that there is no double counting. It represents the maximum amount of water resource produced within the boundaries of a country. This value, which is expressed as an average on a yearly basis, is invariant in time (except in the case of proved climate change). The indicator can be expressed in three different units: in absolute terms (km3/yr), in mm/yr (it is a measure of the humidity of the country), and as a function of population (m3/person per yr).
¤ Global renewable water resources. This is the sum of internal renewable water resources and incoming flow originating outside the country. Unlike internal resources, this value can vary with time if upstream development reduces water availability at the border. Treaties ensuring a specific flow to be reserved from upstream to downstream countries may be taken into account in the computation of global water resources in both countries.
¤ Dependency ratio. This is the proportion of the global renewable water resources originating outside the country, expressed in percentage. It is an expression of the level to which the water resources of a country depend on neighbouring countries.
¤ Water withdrawal. In view of the limitations described above, only gross water withdrawal can be computed systematically on a country basis as a measure of water use. Absolute or per-person value of yearly water withdrawal gives a measure of the importance of water in the country's economy. When expressed in percentage of water resources, it shows the degree of pressure on water resources. A rough estimate shows that if water withdrawal exceeds a quarter of global renewable water resources of a country, water can be considered a limiting factor to development and, reciprocally, the pressure on water resources can have a direct impact on all sectors, from agriculture to environment and fisheries.
Soil erosion is fast becoming one of the worlds greatest problems. It is estimated that "more than a thousand million tonnes of southern Africa's soil are eroded every year. Experts predict that crop yields will be halved within thirty to fifty years if erosion continues at present rates." Soil erosion is not unique to Africa but is occurring worldwide. The phenomenon is being called Peak Soil as present large scale factory farming techniques are jeopardizing humanity's ability to grow food in the present and in the future. Without efforts to improve soil management practices, the availability of arable soil will become increasingly problematic.