Abstract - Twaha Ali Basamba Ateenyi

Institutt for plante- og miljøvitenskap

In many tropical areas, soils are rapidly degrading because of over-grazing and only short periods of fallow in agriculture. Methods to improve and/or maintain soil fertility, for example intercropping with trees, improved grass and tree fallows, crop rotation or better tillage systems, are therefore of great research interest. In the South American region, soil organic matter (SOM) studies are critical because of the SOM’s capacity to regulate nutrient cycling and improve the soil’s physical condition, more so in the low phosphorus fertility volcanic-ash soils and the highly weathered Oxisols.

Studies were conducted on severely degraded Colombian volcanic-ash soils, 28 months after fallow establishment, at two on-farm experimental sites (BM1 and BM2) in the Cauca Department. Experimental site BM1 was at San Isidro farm while BM2 was at Benizio Velazco farm, all in Pescador village. The effect of three planted fallow systems (Calliandra houstoniana-CAL, Indigofera zollingeriana-IND, Tithonia diversifolia-TTH) and a crop rotation (maize/beans-ROT) on soil nitrogen mineralization, organic matter and phosphorus fractions was compared to the usual practice of allowing natural regeneration of native vegetation or natural fallow management (NAT). Significant contribution of TTH to exchangeable Ca, K and Mg as well as B and Zn concentration was observed. The order of soil nutrient contribution by different species and management practices was TTH > CAL > IND > NAT > ROT. Indigofera zollingeriana showed significant impacts on soil NO₃--N at both experimental farms as compared to all the other fallow system treatments. Calliandra houstoniana and ROT had significantly higher H₂O-Po and resin-Pi, respectively, in the 0-5 cm soil layer; whereas TTH showed significant results for both H₂O-Po and resin-Pi in the 5-10 cm soil layer. Significant results were obtained on the weights of the SOM fractions, which decreased in the order LL (Ludox light) > LM (Ludox intermediate) > LH (Ludox heavy). Indigofera zollingeriana showed greater C, N and P in the SOM fractions than all the other fallow treatments, with NAT having the lowest values.

The influence of planted fallows with chicken manure application on soil nitrogen availability, and phosphorus and organic matter fractions, and on grain yield of maize was investigated in the same experiment as described above. The sites had the same soil type and recent cropping history. In BM1, TTH with chicken manure application had a positive impact on nitrogen and phosphorus availability as well as maize yield as compared to the other fallow treatments. In BM2, IND, with the same manure treatment, performed better.

A long-term field experiment (> 5 years) on sustainable crop rotation and ley farming systems was initiated on a Colombian acid-savanna Oxisol to test the effects of grain legumes, green manures, intercrops and leys as possible components that could increase the stability of systems involving annual crops. Five agropastoral treatments (Maize monoculture-T6, Maize-soybean rotation-T7, Maize-soybean green manure rotation-T8, Native savanna (control)-T9 and Maize-agropastoral rotation-T10) under two tillage systems (no till-NT and minimum tillage-MT) were investigated. One year after implementation of the no-till (NT) and minimum tillage (MT) systems, the effects of NT and MT on soil physical properties, root distribution and maize grain yield under the mentioned treatments were evaluated. Lower bulk density and higher total porosity for all treatments and soil layers was found in NT as compared to the MT system. Minimum tillage seemed to improve root length, root biomass and specific root length distribution. Significantly higher maize grain yield (P < 0.1) was obtained under NT agropastoral treatments as compared to the same treatments under MT. Maize yields on native savanna soils were markedly lower than in the rest of the treatments.

Soil organic matter and phosphorus (P) fractions play a key role in sustaining the productivity of acid-savanna Oxisols and are greatly influenced by tillage practices. The effects of NT and MT on SOM and P fractions as well as maize grain yield under the five treatments mentioned above were evaluated, two years after implementation of the NT and MT systems. Results showed that, on average, soil chemical characteristics, P fractions and nutrient contents of the SOM fractions are better or higher under NT as compared to MT system. The weights and nutrient contents of the SOM fractions decreased in the order LL > LM > LH. Treatment T8 had significantly higher values for the P fractions under both tillage systems. However, MT resulted into slightly higher maize grain yields as compared to NT.

In conclusion, this study showed that planted fallows and tillage practices affect availability of phosphorus and nitrogen as well as organic matter fractions and crop yields in volcanic-ash soils and Oxisols. The improved soil conditions would enhance and ensure sustained productivity of these tropical soils.

Key words: Colombia, Maize yield, Oxisols, Planted fallows, Root distribution, Savanna, Soil physical properties, Soil organic matter fractions, Soil phosphorus fractions, Tillage, Volcanic-ash soils


Oppdatert: 15.01.05
Utskriftsvennlig versjon : Arkiv : Tips en venn