No abstract

With continuous cultivation replacing the traditional bush‐fallowing system in the West African savannah zone, farming is becoming more fertilizer‐dependent for high yields. It is imperative that cheaper means of augmenting soil fertility be explored to supplement the use of mineral fertilizers. A rational rotation scheme will enable the crops to utilize the limited amount of fertilizers in a complementary manner. The present study evaluated the effects of nine sequences of growing cotton (Gossypium hirsutum L.), sorghum [Sorghum bicolor (L.) Moench], and groundnuts (Arachis hypogaea L.) on soil productivity and crop nutrition at Samaru in northern Nigeria. The 8‐year study was conducted on a sandy loam soil (Typic Ustropept isohyperthermic family). The sequence treatments were: continuous sorghum, cotton, and groundnuts; sorghum followed by groundnuts or cotton; groundnuts followed by sorghum or cotton; and cotton followed by sorghum or groundnuts. All crops were supplied with adequate amounts of the appropriate basal fertilizers each year. Rotation effects on the yield of each crop were measured over 7 years and in the final year the whole field was ploughed, treated with uniform N and P fertilizers, and seeded to maize (Zea mays L.). Earleaf samples were taken at 50% silking stage for chemical analysis. Sorghum yielded consistently higher when preceded by groundnuts than when preceded by cotton. Continuous sorghum significantly depressed yield. On the average, groundnuts performed just as well when preceded by sorghum as when preceded by cotton but yielded significantly less when continuously cropped. Cotton following sorghum consistently and, on the average, significantly outyielded cotton following groundnuts or cotton following cotton. However, continuous cotton yielded just as well as when preceded by groundnuts. The sequences which included groundnuts gave the best maize yields with the highest, 4,480 kg/ha, from the continuous groundnuts, compared to 3,570 kg/ha from continuous cotton and 2,500 kg/ha from the continuous sorghum plots. Plant tissue analysis indicated greater N availability after groundnuts followed by cotton and sorghum in that order. Most of the rotational effects especially on sorghum were most attributable to residual soil N. A continuous monoculture of any of the three crop appears inadvisable. For maximum yields, the best sequence would be groundnuts followed by sorghum and cotton in that order.

Widespread Mg deficiencies have not been reported in western Nigeria. But with continuous intensive cultivation, heavier fertilization especially with K, and the introduction of high yielding, responsive crop varieties Mg deficiency is likely to become a serious problem in the near future. Knowledge of the critical available Mg levels can be useful in conjunction with routine soil analysis data to monitor the onset of these deficiencies. There is no previously published work on Nigerian soils that estimated the available Mg level below which response to Mg fertilization can be expected. The objective of this investigation therefore was to approximate this critical level under greenhouse conditions. Twenty‐three Nigerian soils collected from various ecological zones and soil types were cropped to maize (Zea mays L.) in the greenhouse with and without Mg application. The maize tops were cut after 35 days, dried, weighed, milled, and analyzed for Mg. Using the dry matter weights relative yields were worked out for all the soils and the new Cate‐Nelson (6) statistical procedure used to estimate the critical level. This procedure involved a step‐wise splitting of the data into two groups using successive tentative critical levels to determine the level which maximized the overall predictive ability (R2) with the means of the two groups serving as the predictor values. An approximate critical level of 0.28 to 0.35 meq/100 g was obtained by this technique. Tests on individual plot yields showed that 0.28 meg/100 g neutral N NH4OAc, extractable Mg was low enough for statistically signficant response to Mg application. All soils (except no. 10) that had less than 5% saturation of CEC responded to Mg application; those with more than 7% Mg saturation did not respond to Mg. Mg deficiency symptoms were associated with soil Mg level of 0.17 meq/100 g or lower prior to planting and with plant Mg content of less than 0.1%. Although the critical level of available Mg approximated in this study (0.28 to 0.35 meq/100 g) is much lower than the average available Mg content (1.2 to 1.5 meq/100 g) of western Nigerian soils. Caution is nonetheless needed in evaluating the findings of the present study as greenhouse results may not necessarily reflect the true situations in the field.