|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|27. Plant production|
In the plant production sector, the following assessment criteria lend themselves to direct or indirect measurement:
- changes in the biotope (diversity of species of flora and
- impacts on finite natural resources (minerals, ores, water, atmosphere)
- impacts on global ecological relationships (net energy production: energy audit comparing energy fixed by a crop plant/harvested product and energy used in its production)
- contamination levels (chemical products, salts, dusts, gases)
Limits varying from one country to another have been laid down for many substances occurring in agriculture. Although many countries have maximum-quantity regulations covering immissions in water, air and soil, these are generally concerned with the effect of pollutants on human health.
As the properties and sensitivity of tropical soils vary greatly, a site survey must always be conducted before project planning commences. Such a survey involves mapping the soil types with regard to their heat, water, air and nutrient balances as well as their susceptibility to erosion. The soil type can be determined in the field or by means of granulometric analysis in a laboratory; once this has been done it is possible to assess the risk of compaction. Measurement of the infiltration rate permits more accurate appraisal of the erosion risk. Tolerance limits for humus decomposition can be formulated only on the basis of the soil conditions and the land use situation. The humus content can be roughly ascertained in the field; precise determination can be carried out in the laboratory by means of ignition loss, wet incineration or gas chromatography.
Spade analysis can be used for simple assessment of soil structure and biological activity; rooting characteristics are of particular importance. The findings can be substantiated in the laboratory by means of wet screening (aggregate stability), analysis of the C/N ratio (nitrogen availability) etc. The presence of effective root symbionts (nitrogen-fixing organisms, mycorrhiza) can be detected only by way of infection tests.
The extent of the leaching risk (particularly for nitrate and pesticides) can be ascertained by determining the field capacity of the soil profile down to the effective rooting depth. This can be estimated in the field with the aid of a drilling stock; it is advisable to conduct a pore analysis of typical soil horizons in order to calibrate the response. However, excavation of a profile is essential in some cases, above all if waterlogging or crusting is suspected.
Deficiency or toxicity symptoms in crops may prompt determination of nutrient status or contamination level. Measurement of the pH value as a function of soil depth can often reduce the necessary scope of analysis and provides information about the lime requirement. Measurement of effective cation exchange capacity and base saturation yields pointers regarding nutrient imbalances and the degree of salinisation. In the case of trace elements and heavy metals, plant analysis is to be preferred. The results allow appropriate recommendations to be made regarding fertilising or - where necessary - rehabilitation.
A body of water can be characterised with relative ease by means of quality classification, which is carried out by determining the pH value, temperature, oxygen content and important indicator organisms. If such organisms are not present or are unknown, the water's ammonium and phosphate content can also yield information about the trophic level. Analysis of biochemical and chemical oxygen demand (BOD, COD) allows conclusions to be drawn regarding the degree of pollution with degradable organic substances. The requirements to be fulfilled in terms of water quality will vary depending on the planned use.
It is above all in semi-arid regions that hydrogeological investigations are necessary for assessing the groundwater reserves. Such investigations can yield information on subsoil conditions and the location of the catchment areas. Current annual evaporation and groundwater recharge rate can then be estimated on the basis of the land use and soil distribution determined in the course of the site survey. If the rate at which the groundwater is tapped (drinking water, irrigation) permanently exceeds the recharge rate, lowering of the groundwater may cause severe damage to land which is in a near-natural state or has undergone reforestation. In such cases the groundwater must also fulfil more stringent quality requirements, since its use as drinking water must not be restricted.
Areas used for plant production often serve to neutralise or reduce emissions emanating from other areas. Correctly designed intensive agroecosystems can in fact sometimes perform such functions more effectively than the potential natural vegetation, because it becomes profitable, from a certain yield level upwards, to neutralise immission-induced damage through appropriate use of inputs (e.g. liming to offset the introduction of acid). The same applies to climatic effects, which can be positively influenced if suitable land and the correct forms of cropping are selected.
Summarising assessments of energy flows and natural cycling systems, which also yield information about loading capacities, will be highly unreliable in the absence of adequate familiarity with the species involved and their interrelationships.