Book Review-Soil fertility decline in the Tropics, with case studies on plantations

Authors

Lynch, J.P.; Jaramillo, R.E.

Source

Field Crops Research, Volume 96, p.481-483 (2006)

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The global population is expected to reach almost 9 billion by the middle of this century, and the vast majority of this growth will occur in the tropics. A primary constraint to agricultural production in much of the tropics is low soil fertility, exacerbated by decades of soil degradation and limited availability and use of fertilizers. The status and trends of soil fertility in the tropics are therefore of considerable importance in the food security and consequently, in the economic development and political stability of the developing world.

Despite the truth of these statements, this issue receives very little attention by agricultural researchers and development agencies. A noteworthy exception is the tradition of Dutch tropical agronomy. With a cultural orientation to horticulture and trade, a gloomy climate in their native land, and a long colonial history exploiting agricultural plantations in the tropics, the Dutch continue to have a profile in tropical agriculture that is far in excess of the proportion of agricultural scientists fortunate enough to actually have been born in The Netherlands.

Alfred Hartemink continues this tradition in his book 'Soil Fertility Decline in the Tropics, with Case Studies on Plantations'. The goal of the book is to provide some "hard data" documenting soil fertility trends in the tropics, with specific focus on plantation agriculture. While both plantation agriculture and soil fertility in the tropics have received attention in separate books, this is the first book to specifically address soil fertility in plantation systems. Plantation systems occupy less than 10% of the arable land in the tropics, but often have disproportionately large contributions to trade and GNP compared with smallholder production, because of greater yields and market integration. Hartemink focuses closely on assessing soil fertility in plantation systems from chemical measures of fertility such as organic C, pH, CEC, and plant nutrient availability. He does not consider micronutrients, biological and physical measures of soil quality, animal husbandry, nor does he examine socio-economic dimensions of soil fertility decline. The 360 pages of this book therefore provide a comprehensive treatment of this tightly focused subject.

The book is organized into three sections: the first (Chapters 1-4) gives an overview of human population and soil degradation, plantation agriculture, and soil fertility decline; the second (Chapters 5-10) discusses soil fertility trends in annual crops and various plantation systems, with detailed case studies of sugarcane in Papua New Guinea and sisal (Agave sisalana) in Tanzania; the final section (Chapters 11 and 12) provides synthesis and summary.

The overview chapters are noteworthy in providing a useful historical perspective about the development of our notions of tropical soil fertility, the origins of plantation agriculture, and the nontrivial methodological issues in seeking to characterize long-term trends over vast areas with diverse soils and cropping systems. Hartemink considers how sociocultural and historical conditions influenced scientific perceptions of these issues, and includes interesting citations from early literature. This section provides an insightful and highly readable introduction to these themes. The upshot of the first section is that soil fertility is increasingly important for a growing population, plantation agriculture is an important agricultural sector in tropical economies, and that given available data and methods, quantification of net changes in soil chemical properties is more reliable than competing approaches to assess fertility changes.

Hartemink makes an important and possibly controversial point towards the end of this section, which is that the quality of baseline soils data commonly used for global analysis and simulation is sorely lacking. Much of the historical data is too local and too old to be completely trusted. This in itself is an important justification for more efforts to consolidate information, like this book.

Before the different land uses are discussed in detail, Hartemink provides an in-depth description of the methods used in the rest of the book. Two main avenues of analysis are used: the comparison of data across time in a single location or plot and the comparison of multiple locations within some boundaries. The first approach is most commonly associated with controlled, long-term experiments while the second is used in regional and supra-regional studies, and both should reflect the effects on soil fertility through time.

Hartemink also emphasizes the importance of bulk density and the resilience of soils in the analysis of soil fertility trends. An increase in bulk density through time could lead to an unexpected increase in nutrient content per volume while concurrently reducing nutrient availability by restricting rooting, an interaction of considerable importance for the estimation and interpretation of soil nutrient content. Soil resilience is related with sampling intervals: some soil properties could be rapidly changing while others are more stable. The moral of this section is that a thoughtful selection of methods and sampling design is not only advisable, but critical in interpreting soil fertility trends.

The chapter on annual crops, perhaps the most studied cropping system, does not show a clear tendency of soil fertility decline after decades of research. In a regrettably common pattern, the variability and unreliability of the data preclude general, universal conclusions. Nonetheless, Hartemink points that often a large, rapid decline in soil fertility is followed by equilibrium at a new, lower level.

Patterns of fertility changes in perennials and forests are generally clearer than in annuals. Fertilizer components such as P or K may accumulate over time in perennial systems, but cultivation still results in a net loss of organic matter compared with forests. Perennial crops differ in their effects on soil fertility. For example, oil palm plantations are more exposed to erosion losses than coffee and cocoa plantations, although all of them are exposed to substantial erosion losses at the establishment of the plantation. Intercropping with legumes such as Erythrina with cocoa could substantially increase the availability of nitrogen and accumulate organic matter in the long term, although this could also cause soil acidification and reduced nutrient availability.

Of all the land uses discussed, it appears that forest plantations are the most sustainable, as a net increment in organic matter can occur, an effect more evident on impoverished soils. Some soil properties could be negatively affected however: Pinus and Euclayptus, for instance, can ultimately reduce the coverage of understory vegetation, increasing the risks of losses by erosion; Pinus is also a frequent acidifier of soils. Both plantation and forest systems could be very valuable in carbon sequestration: oil palm for instance can accumulate comparable amounts of carbon as native rainforest. Nevertheless, all forest species, but specially Pinus, can be less effective for C sequestration than expected due to low soil fertility. These conclusions come from relatively few studies and highlight the unbalanced research efforts on annual vs. perennial and forest crops.

The last three chapters in this section provide detailed case studies on plantations. Sugar cane and sisal reflect contrasting types of management. Sugarcane is typically an intensive, fertilizer-dependant crop, whereas sisal plantations receive very low inputs in a negative cycle of low yields and low profitability. With sugarcane the management of residues is a critical consideration: burning is more prone to soil carbon depletion than the 'trash-harvesting' that accumulates residues in situ and that also reduces N losses via nitrification. Case studies from New Guinea and Australia report significant soil acidification, probably caused by mineralization of organic matter and the use of N fertilizers.

Sisal, due to the paucity of inputs, clearly shows the importance of soil resilience in sustained production under long-term exploitation. Highly weathered soils such as Ultisols and Oxisols have experienced larger changes in their chemical properties than younger Inceptisols or the more fertile Alfisols. Declining sisal productivity (a 30% drop over the last 40 years) driven by declining soil fertility and reduced availability of fertile soils for new plantations is transforming sisal into a marginal crop, with declining profitability and sustainability.

While the exclusion of socio-economic factors gave this volume a clear focus and allowed the author to base his conclusions on "hard data" rather than more elusive notions from the social sciences, it also prevented him from discussing some of the factors that are important drivers of nutrient dynamics in these agroecosystems as well as being compelling humanitarian concerns. Plantation agriculture in the tropics typically represents an industrial model of agriculture, in which local natural resources are exploited for export of a commodity, with economic returns accruing largely to a small group of (possibly foreign) managers and investors rather than local communities. As Hartemink points out, the origins of plantation agriculture are found in European cultivation of sugar in South America using African slaves. Land husbandry and soil conservation have very different connotations in this context than they do in traditional smallholder or communal production systems. A striking example of this can be seen in the banana plantations of Central America, which despite having received intense production inputs and having generated tremendous wealth for multinational corporations over more than a century, have brought very little benefit, and many political and ecological liabilities, to their local communities. In the opening chapters of this book Hartemink raises the issues of sustainability and food security and the carrying capacity of the soil, but it is difficult to resolve these themes without linking soil fertility in plantations to the social, economic, and political forces that shape them. Who benefits or suffers from declining plantation fertility? What are the incentives and disincentives for maintaining the fertility of these systems? What implications do the soil fertility trends have for policy?

Hartemink's review is impressive; the collection of information and the analysis of elusive trends within land uses and soil types make the book one of the most comprehensive efforts of this type. There are gaps, unavoidable perhaps, many of which come from the lack of universal analytical standards, incompatible soil classifications and incomplete site characterizations. All of this unfortunately leaves the book without a definite conclusion on the central questions: is there a real fertility decline, have we caught the trend, and can we ring the alarm on food security? For many, the answer is probably yes -actually it has been proved at the local level, but a universal verification of fertility decline for the tropics seems to be elusive. If we truly want to estimate future trends of soil fertility in a changing climate and take advantage of rapidly increasing modeling and spatial analytical capabilities, more attention needs to be focused on the tropics. This book is a useful step in that direction