Impact of planted fallows and a crop rotation on nitrogen mineralization and phosphorus and organic matter fractions on a Colombian volcanic-ash soil

Soil fertility replenishment is a critical factor that many farmers in the tropical American hillsides have to cope with to increase food crop production. 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). 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. Tithonia diversifolia had a significantly higher contribution to exchangeable Ca, K and Mg as well as B and Zn; the order of soil nutrient contribution was TTH > CAL > IND > NAT > ROT. On the other hand, lND had significantly higher soil NO 3 ?–N at both experimental farms as compared to all the other fallow system treatments. For the readily available P fraction, CAL and ROT had significantly higher H2O–Po and resin-Pi, respectively, in the 0–5 cm soil layer; whereas TTH showed significantly higher values for both H2O–Po and resin-Pi in the 5–10 cm soil layer. Significant effects were observed on the weights of the soil organic matter 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 soil organic matter fractions than all the other fallow treatments, with NAT having the lowest values. It is concluded that planted fallows can restore soil fertility more rapidly than natural fallows

Critical soil organic carbon range for optimal crop response to mineral fertiliser nitrogen on a ferralsol.

Published online: 18 January 2016Soil Organic Carbon (SOC) is a major indicator of soil fertility in the tropics and underlies variability in crop response to mineral fertilizers. Critical SOC concentrations that interact positively with N fertilizer for optimal crop yield are less understood. A study was conducted on a Ferralsol in sub-humid Uganda to explore the critical range of SOC concentrations and associated fractions for optimal maize (Zea mays L.) yield response to applied mineral N fertiliser. Maize grain yield response to N rates applied at 0, 25, 50 and 100 kg N ha−1 in 30 fields of low fertility (SOC 1.7%) was assessed. Soil was physically fractionated into sand-sized (63–2000 µm), silt-sized (2–63 µm) and clay-sized (1.2% SOC registered the highest agronomic efficiency (AE) and grain yield. Non-linear regression models predicted critical SOC for optimal yields to be 2.204% at the 50 kg N ha−1 rate. Overall, models predicted 1.9–2.2% SOC as the critical concentration range for high yields. The critical range of SOC concentrations corresponded to 3.5–5.0 g kg−1 sand-sized C and 9–11 g kg−1 for clay-sized C

Tillage effects on maize yield in a Colombian savanna oxisol : Soil organic matter and P fractions

Soil organic matter (SOM) and phosphorus (P) fractions play a key role in sustaining the productivity of acid-savanna oxisols and are greatly influenced by tillage practices. In 1993, a long-term experiment 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 cereal crops. Five agropastoral treatments (maize monoculture MMO, maize soybean rotation MRT, maize soybean green manure rotation MGM, native savanna control NSC and maize-agropastoral rotation MAP) under two tillage systems (no till-NT and minimum tillage-MT) were investigated. The effects of NT and MT on SOM and P fractions as well as maize grain yield under the five agropastoral treatments were evaluated. Results showed that soil total C, N and P were generally better under no-till as compared to minimum-tilled soils. While P fractions were also generally higher under no-till treatments, SOM fractions did not show any specific trend. Seven years after establishment of the long-term ley farming experiment (5 years of conventional tillage followed by 2 years alternative tillage systems), MT resulted into moderately higher maize grain yields as compared to NT. The MGM rotation treatment had significantly higher values of maize yield under both tillage systems (4.2 Mg) compared to the NSC (2.3 Mg ha?1). Results from this study indicate that the rotational systems (maize soybean green manure and maize-pastures) improved the soil conditions to implement the no-till or minimum tillage systems on Colombian savanna oxisol