Cassava (Manihot esculenta Crantz cv. MAus 7) was planted in Krasnozem soil on two dates; 6 October, 1981 (P1) and 26 November, 1981 (P2). On 14 January, 1982 (100 days after planting in P1, and 51 days after planting in P2) shade cloths were laid over the cassava stands to vary solar radiation inputs namely, 100% (control), 78% and 32% of the full solar input. The study was completed on 16 February, 1982.
Profiles of fibrous root length down to 120 cm were examined on four occasions. Soil cores were taken on these occasions, and the line intersection method was used to estimate root length density (RLD) in each core. Plant dry matter data at the beginning and at the end of the shading treatment period were made available to the author and were compared with the growth of fibrous roots.
distinctive stages of fibrous root development were observed within 133 days of planting: (1) root grew more vertically at the early stages resulting in small root length density at all depths, (2) continued root extension in the deeper soil layers and prolification of root length in the upper layers, (3) pronounced prolification of root length in the lower layers and retardation of growth in the upper layers.
Generally, fibrous root system of cassava was extensive. At 50 days after planting it had already reached the maximum observation depth and under non-shaded conditions the elongation rate of fibrous roots per unit ground area ranged from 0.51 to 0.62 km m-2 d-l. In the last sampling occasion, the older plants (133 days old) had an RLD of 4.83 cm cm-3 at the top layer, 3.85 to 2.66 cm cm-3 in 20-80 cm depth and 0.8 cm cm-3 at the 100-120 cm
depth. This had a computed total root length of 32 km m-2 (ground area).
The elongation rate of roots was adversely affected under reduced light i.e. in the order of 29% to 53% for the two shade treatments. The rate of increase in RLD was reduced in most of the strata. There were some indications that underground parts were more sensitive to reduced light than shoot growth. Moreover, when storage roots become a major sink for assimilates, the reduction of elongation rates of fibrous roots under low levels of light was more pronounced, indicating that competition between these two below ground organs for assimilates occurred under limited carbohydrate supply.
The root length density of several crops were compared with cassava and it was argued that cassava has more extensive root system which is more of advantage in areas with limited water supply because deep rooted plants with high RLD are more effective in
using stored water in lower soil depth.