Organic Methods Of Maintaining Soil Tilth And Fertility
Soil fertility is a function of two major characteristics: the tilth of the soil, and the ability of the soil to supply essential nutrients to crop plants.
Tilth refers to the physical structure of soil, and is strongly influenced by the concentration of humified organic matter. In soils with good tilth the ability to hold water is great, so that excessively rapid drainage is avoided and rainwater can be used more effectively by growing plants. The organic matter also helps to bind nutrients, thereby preventing them from being lost by leaching, and releasing them slowly for more efficient uptake by growing plants. In addition, soils with good tilth have their sand-sized and smaller inorganic particles loosely aggregated into lumpy structures, which improves soil aeration and eases the growth and penetration of plant roots.
Typically, soil tilth becomes badly degraded in conventional, intensively managed agricultural systems. This happens because soil organic matter is progressively lost through plowing and decomposition, while inputs with plant debris are relatively small. Compaction by heavy vehicles also helps to degrade soil tilth. In contrast, a major goal of organic agriculture is to maintain or increase the concentration of organic matter in the soil (using methods that are described below, in regard to nutrients).
Plants require more than 20 nutrients for proper growth. Some of these nutrients are obtained primarily from the soil, especially compounds of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. These nutrients are primarily taken up by plants as inorganic compounds. For example, nitrogen is mostly assimilated from soil as nitrate or ammonium, while phosphorus is taken up as phosphate. In natural ecosystems, these inorganic compounds are steadily recycled by microorganisms from dead organic matter such as plant litter. The microorganisms have the ability to metabolize complex organic forms of nutrients and convert them to simple, inorganic forms, such as the ones just listed. As they perform this function, the microorganisms gain access to the fixed energy and nutrients of dead biomass that they require for their own growth and reproduction. Therefore, soil fertility in natural ecosystems is largely associated with organic matter, from which inorganic nutrients are slowly released from complex, organic forms. These are then efficiently taken up by plants, so that little of these precious nutrients is lost to ground or surface waters or to the atmosphere.
However, in intensively managed agricultural systems, inorganic nutrients are usually added directly, in the form of manufactured fertilizers of various sorts. Synthetic inorganic fertilizers are manufactured industrially from raw materials. For example, rock phosphate mined in Florida or elsewhere is manufactured into super- and triple-superphosphate fertilizers. Inorganic nitrogen fertilizers such as urea and ammonium nitrate are manufactured by combining atmospheric dinitrogen (or nitrogen gas) with hydrogen obtained from methane (or natural gas). Inorganic potassium is obtained from potash, a mined material rich in that chemical, while calcium and magnesium are obtained from limestone (calcium carbonate) or dolomite (calcium, magnesium carbonate). Sulfur fertilizers are manufactured from elemental sulfur or sulfuric acid obtained from sour natural gas or from air-pollution control at metal smelters. The manufacturing of all of these fertilizers has large costs in terms of energy and the depletion of non-renewable material resources.
Often, the rates of fertilization in intensively managed agriculture are intended to satiate the needs of crop plants for these chemicals, so their productivity will not be limited by nutrient availability. However, excessive rates of fertilization have important environmental costs. These include: the contamination of ground water with nitrate; eutrophication of surface waters caused by nutrient inputs (especially phosphate); acidification of soil because of the nitrification of ammonium to nitrate; large emissions of nitrous oxide and other nitrogen gases to the atmosphere, with implications for acid rain and Earth's greenhouse effect; and the need to use herbicides to control the weeds that flourish under artificially nutrient-rich conditions.
In contrast, organic methods of maintaining site fertility focus on soil organic matter. Much action is expended on maintaining or increasing the amount of organic matter in the soil, because this is the reservoir from which inorganic nutrients are slowly made available to growing crop plants. Organic matter is also critical to soil tilth, as was previously described. Organic farmers add nutrient-containing organic matter to their soils in three major ways.
First, as dung and urine of animals, which contains both organic matter and large concentrations of nutrients. However, care must be taken to avoid the contamination of surface and ground waters with pathogenic bacteria. This method of organic fertilization also causes local air pollution with ammonia and distasteful smells.
Second, as green manure, which is growing or recently harvested plant material that is directly incorporated into the soil, usually by plowing. The most fertile green manures are the biomass of plants in the legume family, such as alfalfa or clovers. This is because legumes have a symbiosis with a bacterium that can fix atmospheric dinitrogen (N2) into biologically useful nitrogen. Consequently, legume-derived green manure is a commonly used organic means of fertilization with nitrogen.
Finally, as compost, or partially decomposed and humified organic material. Composting is an aerobic process by which microorganisms aided by soil animals break down and metabolize organic material, eventually forming complex, large molecular-weight materials known as humic substances. These are resistant to further decay, and are very useful as a soil conditioner and to a lesser degree as an organic fertilizer.
It is important to understand that growing plants take up the same, simple, inorganic forms of nutrients from soil (for example, nitrate, ammonium, or phosphate), regardless of whether these are supplied by organic matter or manufactured fertilizer. The important difference between fertilization using organic or synthetic materials is in the role of ecological processes versus manufacturing ones. Organic methods rely more heavily on renewable sources of energy and materials, rather than on non-renewable materials and fossil fuels. Overall, the longer-term environmental implications of maintaining soil tilth and fertility using organic methods are much softer than those associated with conventional, intensive agriculture.
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