Effect of different multipurpose tree prunings and placement method on the growth and development of rape (Brassica napus L.)

ABSTRACT

Production of vegetables in the smallholder sector of sub-Saharan Africa (SSA) is affected by

poor soil fertility with nitrogen (N) being the most limiting nutrient. Continued nutrient

mining without adequate replenishment has led to a corresponding decline in soil fertility and

crop yields due to failure by farmers in SSA to purchase inorganic fertilisers. Leguminous

multipurpose tree (MPT) prunings have widely been promoted as alternative sources of N but

nutrient release is affected by type of MPT and quality attributes such as total N, total

carbon(C), lignin, polyphenols and C:N ratio. The aim of this study was to evaluate various

MPTs which can provide adequate N to Brassica napus L. (rape) which is a short season but

high N demanding crop harvested over multiple times. An experiment was therefore,

conducted at the Midlands State University to determine the yield response of rape to legume

prunings of different quality. The experiment was arranged as a 2 x 7 factorial treatment

structure in a randomised complete block design (RCBD) with placement method (mulching

or soil incorporated) being the first factor and pruning type (Tithonia diversifolia, Gliricidia

sepium, Calliandra calothyrsus, Acacia karoo, Acacia angustissima, Leucaena pallida and

Leucaena trichandria) being the second factor. The tree prunings were applied at a rate to

achieve 150 kg N ha-1. The crop was transplanted at 4 weeks after sowing. Data on LAI, fresh

and dry weight readings as well as leaf chlorophyll readings were taken fortnightly and

subjected to an analysis of variance (ANOVA). Incorporation of prunings achieved the

highest total dry mass of 1,046 t ha-1 compared to mulching which had 0.580 t ha-1. The two

Acacia species achieved the lowest dry mass (DM) yields while G.sepium and C.calothyrsus

achieved the highest DM yields. A. angustissima and A. karoo achieved the lowest total fresh

leaf yield of 1.681 and 1.910 t ha-1 respectively while L. trichandria was the highest yielding

at 1.996 t ha-1 for mulched treatments. Under incorporated treatments L. trichandria and A.

angustissima gave the lowest total yields of 2.206 and 2.033 t ha-1 respectively while T.

diversifolia was the highest yielding at 2.377 t ha-1 fresh weight. These results showed that

soil incorporation gives the highest yields for fresh and dry weight. From the results it can

therefore be recommended that farmers should use T. diversifolia and L. trichandria prunings

over the Acacia species prunings for high fresh weight yields.