NUTRIENT DYNAMICS, ALLELOPATHY LEVELS AND CROP PERFORMANCE UNDER EUCALYPTUS GRANDIS TREES IN KENYA

Abstract

Successful Eucalyptus-crop intercropping is limited by; soil nutrients and water dynamics, allellochemical accumulation in the soil and competition for light between trees and crops among other factors. A study with the objective to evaluate the effects of nutrient dynamics and allelopathy on performance of selected crops i.e. common beans (Phaseolus vulguris), Irish potatoes (Solanum tuberosum L.) and black Nightshade (Solanum villosum) under Eucalyptus trees was carried out. The study characterized and monitored changes in soil nutrients under Eucalyptus grandis tree stands at different ages (1.5, 3, 6, 12, 20 and 40 years) before and during intercropping. For allelopathy studies, the quantity of polyphenols in litter, fresh leaves and soils under trees were determined and tested for their effects on crop germination and soil water repellency. Radial cluster sampling in RCBD design was used for sampling soils and plant materials. Furthermore, the performance of crops under Eucalyptus (3 and 6 years) was evaluated. The crops were planted along rows of Eucalyptus in plot sizes of 4 m by 2 m adopting a factorial arrangement in RCBD where germination, photosynthetically active radiation (PAR), leaf area index (LAI) and yields were measured. Data analyses involved ANOVA, correlations and regressions. From the results; organic carbon, nitrogen and calcium in the soil significantly reduced with increasing soil depth under Eucalyptus trees. Soil available phosphorus, pH, iron, calcium, potassium and magnesium were reduced significantly as age of the stand increased. Crop cultivation under Eucalyptus trees reduced nitrogen and potassium in the soil while available phosphorus, pH, magnesium and manganese increased significantly. Soil organic carbon, exchangeable calcium and extractable iron were unchanged. Potassium, magnesium, manganese and organic carbon were above normal levels in the soil, Eucalyptus litter and its leaves. The soil polyphenol content was 50 to 100 times less than those present in the litter and leaves of Eucalyptus and increased with tree age and reduced down the soil profile. The polyphenol extract from litter and fresh leaves completely inhibited the seeds germination of common bean but not the soil extract (80% germination). Soil water repellency increased with Eucalyptus tree age, was severe during dry spells (less moisture) and reduced down the soil profile. Germination of crops under trees was high (beans 90%, potatoes 80%, and nightshade 100%) but did not differ when planted in the open field. The leaf area index (LAI) and yield of crops reduced under Eucalyptus trees (p ≤ .001) except Nightshade. The age differences in Eucalyptus trees had no significant effect on the amount of PAR reaching the understory crops. In conclusion, phosphorus deficiency and manganese toxicity were the major limitations to optimum crop production under Eucalyptus trees. Soil soluble polyphenol and moisture contents influenced soil water repellency under Eucalyptus trees. It is recommended to continuously cultivate and mix litter with soils under Eucalyptus trees during intercropping to reduce soil water repellency. Nightshade vegetable performed well under Eucalyptus trees and should be adopted for intercropping.