Does certified organic agriculture increase agroecosystem health? Evidence from four farming systems in Uganda

Over the past decades, the number of certified organic farms have increased significantly in Uganda. One assumption is that certified organic agriculture contributes to economic, social and ecological health of agroecosystems. In the literature, however, there is thin empirical evidence to support such claims. We therefore developed health indicators and contrasted data from four Ugandan farming systems with principles and objectives of organic agriculture. We identified four health patterns (ecology-driven, economically struggling, socially-driven, and hanging in) demonstrating the impact of farm management on agroecosystem health and trade-offs between health domains. Ecological farm health is strengthened only if the conversion goes beyond ‘organic by default’. Market-oriented specialization can create lock-in situations if production strategies cannot be changed easily. Food shortages occur when additional income from certified production does not compensate for the reduced area and effort devoted to food crops. We conclude that the positive effects of organic certification on agroecosystem health cannot be taken for granted. Interventions promoting organic agriculture should acknowledge risks smallholder farmers take by converting to cash crop-oriented certified organic farming. A challenging question will be how aspects of wellbeing and social health can be translated into certification standards and thus product attributes

Effect of lime, urea and triple super phosphate on nitrogen and phosphorus mineralisation in an acid soil during incubation

A study to investigate N and P mineralisation in an acid mollic Andosol was conducted through laboratory incubation for 120 days. Soil samples were taken from 0-15, 15-30 and 30-60 cm depths. Treatments applied were lime, urea and triple superphosphate (TSP) at rates equivalent to 2.5 t ha-1, 50 and 75 kg ha-1, respectively. Soil moisture of the treated soil samples was adjusted to field capacity and transferred into polythene bags, sealed and incubated in the laboratory at room temperature (19-24 OC). Changes in soil available N and P were measured at specific time intervals and their concentrations recorded. Nitrogen and P mineralisation potential in the soil profile (0-60 cm depth) were 4.7 µg N g-1 and 0.37 µg P g-1 dry soil day-1, with the 0-15 cm depth contributing 42.7 and 33.3 % of the mineral N and P, respectively. Total net N mineralised in lime, control TSP and urea treatments was 666.1, 563.3, 380.2 and 282.2 µg N g-1 dry soil, respectively with significant differences observed among treatments. Net P mineralised (µg P g-1 dry soil) was significantly higher in TSP (74.1) than control (43.9), lime (47.4), and urea (34.2) treatments. Liming and P application enhanced soil N and P mineralisation, respectively but urea retarded both.Key Words: Acidity, Andosol, incubation, Kenya, lime, TSP, urea RÉSUMÉUne étude a été conduite pour examiner la mineralisation du P et de l'N dans un Andosol mollique acide par l'incubation pendant 120 jours au laboratoire. Les échantillons de sol ont été pris à une profondeur de 0-15, 15-30 et 30-60 cm. Les traitements utulisés étaient la chaux, l'urée et le superphosphate triple (TSP) à des taux de 2.5 t ha-1, 50 et 75 kg ha-1 respectivement. L'humidité du sol des échantillons traités a été ajustés à la capacité au champs et transferrée dans des sacs de polythéne, puis fermée et incubée au laboratoire à une temperature de la salle (19-24 0C). Les variations de l'N et du P disponble ont été mesuré à un untervalle de temps spécifique et leurs concentrations enregistrées. Le potentiel de mineralisation de l'N et du P des profiles de sol (0-60cm) était 4.7µgNg-1 et 0.37µgPg-1 de sol sec par jour, et le profile de 0-15cm de profondeur contribuait 42.7 et 33.3% de l'N et du P respectivement. L'azote total net minéralisé des traitements chaux, temoin et 'ur ée était 666.1, 563.3, 380.2 et 282.2 µgNg-1 de sol sec respectivement avec des differences significatives entre les traitements. Le P net minéralisé (µgPg-1) était significativement élevé dans SPT (74.1) plus que le temoin (43.9), la chaux (47.4) et l'urée (34.2). Le chaulage et l'application de P accélère la minéralisation de l'N et du P respectivemet, mais l'urée retarde les deux.Mots Clés: Acidité, Andosols, incubation , Kenya, chaux, SPT, urée (African Crop Science Journal 8(3) 2000: 327-336