Sustainability

• Introduction
• The problem of soil degradation
• Erosion
• Organic Matter
• Environmental aspects

Summary

The key problem of tropical agriculture is the steady decline in soil fertility, which is closely correlated to duration of soil use. This is due primarily to soil erosion and the loss of organic matter associated with conventional tillage practices, that leave the soil bare and unprotected in times of heavy rainfall and heat. The implications of soil preparation on soil erosion and the sustainability of agricultural production was studied with special reference to experience and projects carried out in Paraguay, Brazil and Argentina under tropical and subtropical conditions.

Scientific data show that under tropical and subtropical conditions, tillage generally has a detrimental effect on chemical, physical and biological soil properties. Investigations also show that erosion damage is enhanced when the soil is bare. Water infiltration rates are increased and consequently erosion is reduced when mulch covers the soil in a no-tillage system. Tillage also releases considerable amounts of CO₂ into the atmosphere contributing to global warming. In order to achieve sustainable agriculture in the tropics and avoid global warming, soil tillage has to be reduced to a minimum or avoided completely and the soil has to stay as long as possible covered with mulches, sod and growing crops. No- tillage in mulches of previous crops or green manures in combination with adequate crop rotations is the production system of the future if sustainable agriculture is to be achieved.

Introduction

One of the main factors to be considered in relation to agro- ecological sustainability is the soil, as it is the basis for food production for humanity. Therefore, an effort has to be made to minimise soil erosion so that soil is not transported by runoff to rivers, lakes or to the sea, and to ensure sustainability of food production.

In this paper, sustainable agriculture is defined as establishing high, lasting and economic soil productivity, without damaging the soil and the environment, improving quality of life. Definitions of sustainability that consider only one dimension (i.e. soil fertility) are insufficient. Ecological, social and economic dimensions must always be considered (Hailu and Runge- Metzger, 1993).

The results of exploiting agricultural systems are evident in those regions where the soil is cultivated intensively and continuously, without considering soil degradation caused by soil preparation under hot/ humid conditions. In Central Paraguay, the regions which used to be the granaries of the country and where food used to be produced and exported to Argentina, many soils are so degraded and depleted that it is not possible to obtain economic production of basic products such as maize, cassava and cotton, and are gradually being abandoned. In southern Chile a hilly region close to the city of Concepción named "Cordillera de la Costa", also a granary of the country some 40 or 50 years ago, has suffered such catastrophic erosion that some areas cannot be used even for forestry. In the Andean region of Bolivia and Peru deep erosion gullies are destroying entire landscapes.

Such examples can be found not only in Latin America but world- wide. Rapid depletion of soil fertility and non- sustainable land use particularly in developing countries is both the cause and the consequence of widespread poverty. It is therefore necessary to change actual soil- degrading agricultural systems based on intensive soil preparation which leave the soil bare and unprotected, to sustainable production systems based on permanent soil cover with plant residues and mulches.

Soil is a non- renewable resource and it is available only in limited quantities. Conventional soil tillage that leaves the surface of the soil bare, is one of the major causes of the occurrence of erosion on agricultural land. Highest sediment amounts as well as phosphorus and nitrogen content in the water of the Itaipú dam (shared by Paraguay and Brazil), was measured in times of soil preparation for winter and summer crops (Derpsch et al., 1991).

The problem of soil degradation

The key problem of conventional agriculture in the tropics is the steady decline in soil fertility, which is closely correlated to duration of soil use (Fig. 1). This is due primarily to soil erosion and the loss of organic matter associated with conventional tillage practices, that leave the soil bare and unprotected in times of heavy rainfall and heat.

Despite progress in genetics and breeding, fertilisation, plant protection and management, there is a clear tendency of diminishing yield over time. FAO predicts, that if soil losses continue unchecked the potential rainfed crop production will decline by about 15% in two decades in Africa, about 19% in Southeast Asia, and by more than 41% in Southwest Asia (Kelly, 1983).

The result of soil degradation is not only that farm land has to go out of production, but also that there is an increasing need for more inputs and investments to maintain high levels of productivity. In the United States, 50% of fertiliser needs is applied only to compensate for the losses in soil fertility due to soil degradation. In Zimbabwe, soil nutrient losses by erosion are three times higher than the total quantity of fertilisers applied ( Stocking, 1986).

Erosion

Occurrence of erosion can be considered the most important factor causing soil degradation. Under the concept of sustainability, the first negative factor in relation to productivity and profitability, and the major aggressor of the environment is soil erosion. Consequently, sustainability can only be achieved if soil erosion is stopped completely.

When agriculture is practised on slopes in undulating topography, and rains of a certain intensity occur, soil preparation especially with disc implements results in bare soil, and this results in water erosion, or in regions of heavy winds in wind erosion.

It is estimated that soil losses in cropland in Latin America reach 10 to 60 t/ ha/ year (Steiner, 1996; Derpsch et al., 1991). Average soil losses in the State of Paraná, Brazil, where good soil conservation is practised, are as high as 16 t/ ha/ year. In Paraguay, on 4000 m² plots with 6% and 8% slope on high clay content Oxisols, average soil losses of 21.4 t/ ha were measured in conventional soil preparation, while only 633 kg/ ha of soil loss were measured in No- tillage (Venialgo, 1996). For the same experiment after extreme precipitations of 186 mm on June 9 and 18, 1995, soil losses of 46.5 t/ ha were measured under conventional tillage, as compared to soil losses of only 99 kg/ ha under No- tillage (both plots on 8% slopes). This resulted in 470 times higher soil losses when soil was prepared. (Venialgo 1996)

The high losses from agricultural soils have to be compared against the annual rates of soil regeneration that are estimated to be not more than 250 to 500 kg/ ha/ year. When soil losses are higher than natural soil regeneration rates, sustainable agriculture is not possible.

Recent studies show that soil erosion is a selective process, with the most fertile soil particles taken away. Eroded soil sediments usually contain several times more nutrients than the soils they originated from (Stocking, 1988).

Applied fertilisers are also transported by erosion to streams, rivers, lakes and to the sea, and therefore lost forever. Considering that world phosphate reserves are going to be exhausted in 40 to 50 years (Hoffman et al., 1983), present generations are acting irresponsibly when allowing soil management practices that produce high erosion rates. Even under the assumption that phosphate reserves are going to last much longer, it has to be kept in mind that reserves are finite.

Research has shown, that soil cover is the most important factor that influences water infiltration into the soil, thus reducing runoff and erosion (Mannering and Meyer, 1963).

Organic Matter

In the tropics and subtropics organic matter content of the soil has an overriding importance in relation to soil fertility. According to Cannel and Hawes (1994), organic matter content of the soil is probably one of the most important characteristics in relation to soil quality, due to its influence on soil physical, chemical and biological properties.

Due to the fact that cation exchange capacity of most tropical soils is very low (Sánchez, 1976), organic matter has a much higher importance to store nutrients in the tropics than in temperate regions. Therefore the efficiency of mineral fertilisers is greatly reduced if at the same time organic matter is not added. On the other hand it is necessary to consider that organic matter is mineralised about five times more rapidly in the tropics than in temperate regions.

Therefore we can state that any agricultural production system that does not add sufficient organic matter and/ or gradually reduces organic matter content of the soil below an adequate level, is not site appropriate, will result in soil degradation and is not sustainable.

Influence of soil preparation on soil organic matter content and yield:

Soil tillage results in rapid mineralisation of organic matter stored in the soil, liberating nitrogen that will be available for plants. This can lead during a few years to an increase in yield. However, when soil tillage is performed under favourable conditions for mineralisation of organic matter (heat, humidity, good aeration) leaving the soil under fallow (bare), valuable nitrate reserves are lost by lixiviation (washed into deeper soil layers), without crops being able to utilise them.

Once organic matter has been consumed, more nitrogen cannot be liberated and yields of crops remain low. The result is a depleted soil, where the indispensable organic matter is missing.

Many depleted soils of Paraguay and other countries of Latin America are an example of bad land management, with excessive soil tillage resulting in organic matter exhaustion. The long term influence (100 years) of soil preparation on the organic matter content in northeastern United States (temperate climate) is described by Rasmussen and Smiley (1989). In that period a reduction in the organic matter content of the soil from 2.7 to 1.5% could be observed when plant residues were not burned. When 22 t/ ha/ year of manure was applied from 1930 to 1981 only a small increase in the organic matter content of 1.9% to 2.1% was measured. This shows how difficult it is to raise organic matter content of the soil once it has fallen.

Here it is necessary to remember that in tropical climates organic matter reduction is processed much more quickly, and reductions below 1%, sometimes as low as 0.2% can be reached in only one or two decades of intensive soil preparation.

The influence of 20 years of different soil preparation on the organic matter content of the soil in Kentucky, USA, is reported by Thomas (1990) (Table 1).

These organic matter contents were also reflected on maize yields after 20 years in the same experiment (Thomas, 1990).

Yields of maize without nitrogen were initially much lower in no- tillage than in conventional tillage. The situation changed after 13 years due to organic matter depletion in conventional tillage, and since then yields under no- tillage without nitrogen have been always higher (G. Thomas, 1996, pers. commun.).

Influence of no-tillage (NT) on different soil properties

There is enough scientific evidence from warmer areas that shows, that no- tillage has positive effects on chemical, physical and biological soil properties compared to conventional soil preparation (Kochhann, 1996). First, because erosion is drastically reduced, and second, because organic matter levels in the soil are not only maintained, but are increased in this system, and third, because soil temperatures are kept low.

Influence of NT on chemical soil properties

Compared to conventional tillage, no- tillage has positive effects on the most important chemical properties of the soil. Under no- tillage, higher values of organic matter, nitrogen, phosphorus, potassium, calcium, magnesium and also a higher pH and cation exchange capacity, but lower Al values are measured (Lal, 1976; Lal, 1983; Sidiras and Pavan, 1985; Crovetto, 1996).

Influence of NT on physical soil properties

Under no- tillage higher infiltration rates have been measured compared to conventional tillage (Roth, 1985), and this results in a drastic reduction of erosion. In no- tillage a higher soil moisture content and lower soil temperatures as well as higher aggregate stability have been measured (Kemper and Derpsch, 1981; Sidiras and Pavan 1986; Derpsch et al., 1988). At the same time a higher soil density occurs under no- tillage (Lal, 1983; Derpsch, et al., 1991), which is considered negative by many scientists. Despite this fact, higher yields of crops are obtained in Paraguay, Brazil and Argentine with this system, as compared to conventional tillage.

Influence of NT on biological soil properties

Due to the fact that no mechanical implements are used that destroy the "nests" and channels built by micro- organisms, higher biological activity occurs under the no- tillage system. Also, micro- organisms do not die because of famine under this system (as is the case under bare soils in conventional tillage) because they will always find organic substances at the surface to supply them with food. Finally, the more favourable soil moisture and temperature conditions under no- tillage also have a positive effect on micro- organisms of the soil. For these reasons more earthworms, arthropods, (acarina, collembola, insects), more micro- organisms (rhyzobia, bacteria, actinomicetes), and also more fungi and micorrhyza are found under no- tillage as under conventional tillage (Kemper and Derpsch,1981; Kronen, 1984; Voss and Sidiras, 1985). Despite the fact that chemicals are used to kill weeds, higher biological activity occurs under no-tillage, an indicator of a healthier soil.

Water Quality

Water quality is improved in no- tillage. While drainage water from conventional tillage watersheds are brown in colour and carry a lot of sediments, watersheds in Brazil that have changed to no-tillage have been found to drain clear water even in times of heavy rainfalls.

Sanitary aspects

Some diseases of crops increase under no- tillage (Igarashi, 1981; Homechin, 1984; Reis, 1985; Reis et al., 1988). For this reason no- tillage should not be practised in monoculture. In general, a well balanced crop rotation with the use of green manure crops is sufficient to neutralise this negative aspect of no- tillage. In relation to pests, no- tillage can have positive or negative effects, and this depends on the specific pest and also on prevailing climatic conditions. In general, the diversity of insects, spiders, etc., increases under the mulch covered soil, where they find more favourable conditions for reproduction. As a result, many useful insects (predators) develop, and this leads to a better biological equilibrium, where pests may be controlled by predators, thus reducing the necessity for chemical pest control.

Environmental aspects

Intensive soil tillage accelerates organic matter mineralisation and converts plant residues in carbon dioxide, which is liberated into the atmosphere contributing to the green house effect and to global warming. Recent research performed in the USA by USDA/ ARS shows that soil carbon is lost very fast -as carbon dioxide- within minutes after the ground is intensively tilled, and the amount is directly related to the intensity of tillage. After 19 days, total losses of carbon from ploughed wheat fields were up to five times higher than for unploughed fields. In fact, the loss of carbon from the soil equalled the amount that had been added by the crop residue left on the field the previous season (Reicosky, 1997). While fossil fuels are the main producer of carbon dioxide, estimates are that the widespread adoption of conservation tillage could offset as much as 16% of world-wide fossil fuel emissions (CTIC, 1996).

Figure 2 (prepared by Reeves, 1995) illustrates the fate of soil carbon considering three hypothesis of adoption of conservation tillage in the USA until the year 2020. In the first hypothesis, in which conservation tillage adoption rates of 1993 (27%) are maintained, and where conventional tillage prevails, almost 200 million tons of carbon are lost to the atmosphere. In the second hypothesis, in which conservation tillage adoption would increase to 57%, some improvement can be observed in relation to the first. In the third hypothesis, when conservation tillage adoption rates would reach 75%, in conventional tillage almost half the carbon is lost in relation to hypothesis one, while no-tillage would contribute to increase carbon deposits into the soil by almost 400 million tons, where it contributes to increase soil fertility (Kern and Johnson, 1993a). Minimum tillage apparently is not able to retain additional carbon in the soil, but it does avoid a net loss.