Willem de Clercq

Nutrient leaching under zero tension in a subtropical clonal eucalypt plantation on a sandy soil in South Africa

Little is known about the effects of residue burning or retention on nutrient leaching during the inter-rotation of clonal Eucalyptus grown on the sandy soils of subtropical Zululand, South Africa. A study compared zero-tension nutrient leaching through the top metre of soil at depths of 0.15, 0.5 and 1.0 m in an undisturbed crop with adjacent clearfelled areas subjected to residue burning and residue retention. Leaching at 1.0 m in the undisturbed crop was 80% less than at 0.15 m leaching due to high water use of the mature trees. Loss of nutrients past 1.0 m in the undisturbed crop amounted to 7.0, 13.1, 6.6, 15.1 and 60.7 kg ha−1 of nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na) over the period between felling and new crop canopy closure (22 months). Annualised 1.0 m leaching amounted to 4.2, 7.8, 4.0, 9.0 and 36.3 kg ha−1 of N, K, Ca, Mg and Na, respectively. Clearfelling induced an increase in N and cation leaching that was apparent five months after clearfelling and persisted for nine months. Leaching loss declined rapidly in the new crop after planting to levels similar to the undisturbed crop by six months of age. Leaching past 1.0 m soil depth under residue retention amounted to 30.6, 132.0, 82.5, 108.7 and 299.1 kg ha−1 of N, K, Ca, Mg and Na, respectively, between felling and canopy closure. Although some weakly significantly differences were found between residue burning and retention, residue burning did not substantially alter leaching past 1 m soil depth. Burning rather induced a large loss of N (121 kg ha−1) through oxidisation, around half the residue N content. Residue retention or burning followed by rapid re-establishment can therefore be practiced to retain most nutrients on this site. Burning of residues should be practiced conservatively on low N soils or be followed by N fertilisation.

Effect of soil amendments on yield of wild okra (Corchorus olitorius) in northern KwaZulu-Natal, South Africa

The consumption of semi-domesticated indigenous vegetables such as Corchorus olitorius is being promoted in South Africa. Presently, cultivation of indigenous vegetables is hampered by the absence of cultivation guidelines due to lack of agronomic research on the various production aspects. The current study evaluated the yield of C. olitorius in response to cattle manure (5 000 kg ha−1) and NPK inorganic fertiliser (500 kg ha−1). Plant height, number of branches, marketable fresh yield and shoot dry mass responded significantly (p<0.05) to the applied fertilisers. Growing Corchorus without basal fertiliser gave significantly the lowest yield even when top-dressing nitrogen fertiliser was applied. There were significant interactions between the different basal and nitrogen top-dressing fertiliser. It was concluded that the application of basal soil amendments and the interaction of basal and top-dressing resulted in increased marketable yield for C. olitorius. The highest marketable yield for both cattle manure and NPK fertiliser were obtained when these were combined with 200 kg ha−1 lime ammonium nitrate.