EFFECTS OF SOIL COMPACTION AND ORGANIC AMENDMENTS ON SOIL PROPERTIES, GROWTH AND YIELD OF COWPEA ( Vigna unguiculata) IN SOUTH WESTERN NIGERIA.

Abstract

Field and pot (containerized soil) experiments were conducted in 2011 and 2012 respectively at the Teaching and Research Farm, Federal University of Technology, Akure, a humid rainforest zone of Nigeria. The aims were to examine the effects of compaction and organic amendment on soil physico-chemical properties and performance of cowpea (Vigna unguiculata). For the field experiment, compaction was imposed on the seed bed via tractorised multiple wheel passes using Messy Ferguson tractor (72 hp, 2 wheel drive 2590 kg weight) while Proctor Hammer weighing 5.5kg was employed to impose the varying degree of compaction in the containerized (potted) soil. At the commencement of the field experiment (Late august), penetration resistances ranged from 270 to 350 and 340 to 520 at 10 and 30 cm and at crop maturity, soil strength ranged from 280 to 400 and 330 to 500 respective at 10 and 30 cm. On the field experiment, treatments were 3 x 4 factorial combinations of 4 number of tractor wheel passes by 3 levels of organic manure application. Tractor passes were 4, 8 and 12 and the ploughed and harrowed only seed bed. Treatments were replicated 3 times on field plots of 60x 12m. In the screen house experiment, compaction was imposed using multiple blows (0, 7, 14 and 21 blows) using 2.5kg (5.5 1b) Proctor Hammer. The number of blows measures the extent of compaction and a penetrometer resistance was used to determine soil strength. Treatments which consisted of 4 by 3 factorial combinations of proctor hammer blows and organic manure application arranged in a complete randomized design. Treatments were replicated three times. Organic amendment consisted of soil application of 10 t/ha organic fertilizers (Ondo State Sunshine organic fertilizer) and poultry manure and an un-manured control. Data were collected on cowpea growth and yield parameters such as root and shoot biomass, pod and seed yields, and 100 seed weight. In both the field and pot experiments, soil compaction treatments affected growth and seed yield of cowpea. Shoot biomass, pod and seed weight were similar for control and 7 blows but were significantly higher over 14 and 21 blows. This trend is consistent for ploughed and harrowed seedbed and plots treated to 4 wheel passes in comparison with 8 and 12 tractor wheel passes. Organic amendment of the variously compacted soil enhanced root and shoot biomass, pod and seed yield of cowpea. Application of organic manures ameliorated compaction effects on soil properties and enhanced growth and yield of cowpea. Organic amendments of the variously compacted soil alleviate the consequence of compaction especially within the plow layer and brought about improvements in bulk density, moisture contents and cowpea performance. Application of manure produced decreases in soil strength 260 to 360 and 230 to 360 at 10 and 30 cm depths at the respective onset and termination of experiment. Soil moisture contents ranged from 15 to 14% and 14 to 12% while manuring enhanced soil moisture contents (15 to 14% at commencement to termination of experiment). Soil moisture were lower for tractor wheel passes alone (11.5, 12.8 and 14.7 % for 12, 8, and 4 wheel passes) and increased under organically amended wheel passes treatments. In general, depth wise trends in mechanical impedance (penetration resistance) and soil moisture were 230, 390 and 300 Psi and 11.5, 12.8 and 14.7 % at 10, 30 and 50 cm depths respectively. The time course of changes in mechanical impedance (penetration resistance) and moisture contents showed that penetration resistances decreased as soil moisture contents increased The decreases in rainfall amount and consequently decreases in soil moisture contents from commencement of experiment (Late August) associates with increases in soil resistance (mechanical impedance). The results show high soil strength within the plough layer and the active portion of crop root zone (10-30 cm) while declining trends in values of soil strength from planting to crop maturity followed trends of the increases in soil moisture contents. The declining trends in rainfall amount and consequently reductions in soil moisture contents from commencement of experiment (Late August) to crop maturity appeared to have provoked the increases in soil resistance (mechanical impedance) at crop maturity. The results show high soil strength was obtained within the plough layer and the active portion of crop root zone (10-30 cm) while there was increasing trends in values of soil strength from planting to crop maturity especially at depth (30 – 50 cm soil depth). In the pot experiment, following the imposition of the treatments (May), June and July and at planting, and 30, 60 and 90 days after planting (DAP) cowpea, the average values of penetration resistances ranged from 354; 320; 270 and 210 Psi (51.3, 46.4, 39.2, 30.5 kPa) at 10 cm depth and 520; 450,380 270 Psi (75.4 65.3, 55.1 and 39.2 kPa) at 30 cm depth for the respective levels of compaction (blows from Proctor hammer) imposed. Soil moisture contents ranged from 15 to 14% at 10 cm depth and around 15% at 30 cm and manuring enhanced soil moisture contents ( 16 to 15%) at commencement to termination of experiment.Soil moisture were lower for Proctor Hammer blows alone (14.5%) and increased under organically amended wheel passes treatments (16%) Organic amendment produced decreases in soil bulk density, mechanical impedance (penetration resistance) and enhanced moisture content and cowpea performance. Close relationships among soil moisture status (content), mechanical impedance and cowpea performance. Root and shoot biomass and seed yields were affected by soil moisture contents and mechanical impedance (penetration resistance). Organic amendment alleviated the consequences of compaction especially within the plow layer and increased moisture contents within crop root zone and consequently enhanced root development, growth and yield characters of cowpea. The results showed that mechanical impedance is a major limitation to root elongation and crop performance in the soils of the study area. Key words; Soil amendments, Soil compaction, Soil bulk density, Porosity, Yield and Cowpea