Yield development in cassava under different soil physical conditions

Abstract

We compared storage root yield of cassava ( Manihot esculenta Crantz) cultivar T 02/72 in three soils, tilled and untilled, in a field experiment in Central Trinidad. In the wet season, fresh yield at 4.5 months after planting varied as minus the logarithm of the time-averaged soil penetrometer resistance measured at principal rooting depth ( r 2 = 0.80; P<0.001), provided that soil air was adequate. Yield was sharply reduced by reduction in season-mean soil air content below 12 ml air per 100 ml soil in the 0–150-mm depth. In continued growth into the dry season, fresh yield increase was linearly related to the amount of plant-available water in either the 0–150 or 0–300-mm soil layer ( r 2=0.70; P<0.05), and was unrelated to penetrometer resistance. These yield differences were explained by variation in total crop dry matter ( tdm) production ( r 2=0.95; P<0.001; dry yield basis), while marginal dry matter partition to storage roots was not noticeably affected by the soil conditions. The storage root number and size (individual mass) components of yield were mutually proportional at 4.5 months. Both components continued to increase after this time if soil physical conditions were favourable. Storage root number was related primarily to tdm ( r 2=0.86; P<0.001, at 6 months), any deviation from this relationship was complemented by compensatory variation in size.