Effects of planted fallows and tillage practices on organic matter fractions, nutrient availability and maize yield in Colombian soils

Abstract

In many tropical areas, soils are rapidly degrading because of overgrazing 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 interests. 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 soils physical condition, more so in the low phosphorous fertility volcanic-ash soils and the highly weathered Oxisols. Studies were conducted on several degraded Columbian 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 phosphorous 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 impact on soil NO,-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 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 phosphorous 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 phosphorous 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 Columbian acid-savanna Oxisol to test the effect 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 rotation 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 biomas 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 phosphorous (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 the implementation of 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 plant fallows and tillage practices affect availability of phosphorous 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