Barley yield response to soil organic matter and texture in the Pampas of Argentina
Introduction
The role of organic matter in sustaining soil fertility is widely recognized (Tiessen, 1994, Reeves, 1997). A decline in soil organic matter (SOM) caused by cultivation of soils has been studied in many long-term experiments (Grace et al., 1995, Golchin et al., 1995, Bowman et al., 1999). However, these studies have been conducted in designs where the effect of very few other soil properties on SOM variation could be evaluated. In Mollisols of the semiarid Pampa region, Buschiazzo et al. (1991) found a strong relationship between SOM level of both virgin and cultivated soils and soil particle size distribution. Soils with higher clay content had higher level of SOM, which was retained over extended periods of cultivation, making it difficult to compare the effect of SOM on soil fertility in this region across a wide range of textural classes. Any particular level of SOM can therefore be considered high, medium or low, depending on soil texture.
The loss of SOM under cultivation can mainly be attributed to the loss of the labile or young C fraction (Wadman and De Haan, 1997), which is very important for nutrient supply to plants (Körschems et al., 1998). Kapkiyai et al. (1999) also indicated that the young C fraction is crucial for the interpretation of soil fertility changes and that it could be potentially used as a soil quality indicator. Biederbeck et al. (1998) found labile organic matter fractions to be more sensitive indicators than total C and N. Quiroga et al. (1998) compared virgin, pasture and cultivated soils of the semiarid Pampa region and found soil variables that discriminated these land uses related to SOM (e.g. susceptibility to compaction, aggregate stability and particulate organic matter). Within a management type, SOM is strongly related to clay + silt content of soil. For any given soil texture, the SOM/clay + silt ratio will be high in soils with good conditions for plant growth, whereas in deteriorated soils values will be low. Pieri (1995) reported that critical SOM levels in Africa were dependent upon clay + silt content.
We assumed that SOM loss with cultivation and the resulting change in the relation between SOM and texture (expressed as clay plus silt content) was mainly through the decrease in young SOM, which affects soil physical properties and plant nutrition. Therefore, the simple soil quality indicator, SOM/clay + silt, should reflect the response of plant growth to physical soil properties and nutrient availability. We tested this hypothesis using barley (Hordeum vulgare) grain yield response to N fertilization.
Section snippets
Materials and methods
The study area was within a 20,000 km2 region of the semiarid and subhumid Pampa of central Argentina (35°50′S to 37°30′S latitude, 63°40′W to 64°20′W longitude). This area contained calcareous plain and sandy plain subregions. Soils were Haplustolls with texture ranging from sand to loam. Thirty-eight experimental sites were selected on the basis of their SOM content and SOM/clay + silt ratio. Location of the sites is shown in Fig. 1, and climatic conditions for the growing seasons of 1998, 1999
Results and discussion
Soil properties varied widely among sites, with extremes of 240 and 670 g kg−1 for the clay + silt fraction, 5.7 and 28.6 g kg−1 for SOM and 1.4 and 41 mg kg−1 for available P, respectively. During the three growing seasons, crops developed without water stress and water stored in the soil at seeding contributed to a considerable portion of the plants’ water requirements (Table 1). Soil nitrate content in the surface 0.4 m of soil at seeding varied from 24 to 111 kg ha−1.
Grain yield in the control plots
Conclusions
Malt barley yield and water use efficiency were strongly affected by SOM/clay + silt ratio. The strong relationship between SOM/clay + silt ratio and nitrate availability reflects the capacity of this indicator to integrate soil physical conditions related to crop water use. Water use efficiency was influenced by SOM/clay + silt ratio and initial nitrate content. The SOM/clay + silt ratio was a better predictor of malting barley grain production in the semiarid Pampas region of Argentina than nutrient
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