The yields of cotton from five three-course rotations are given; they show marked seasonal variations, the variations of the different rotations being in close accord with one another. The average yields of the rotations are related fairly closely to the average soil nitrates; in individual years this is not so, though considerable responses to nitrogenous manuring are always obtained on Gezira soil.The distribution of nitrate in Gezira soil shows an unusual accumulation at depth; though cotton roots can reach the zone of accumulation, the variability of nitrate at these depths renders it impossible to relate changes to uptake. The figures show that under growing cotton the whole of the upper 4 ft. of soil can have low nitrates (of the order of 1 p.p.m.). Unless the roots obtain nitrate from the fifth and sixth feet accumulations the plants must be taking up nitrate as fast as it is formed, and the nitrate-supplying power of the soil must be of first importance. Figures are given showing differences in nitrification in soil from the different rotations.The effect of heavy nitrogenous manuring on soil nitrate is shown to be largely confined to the year of application, and no accumulations could be demonstrated. The average recovery of added nitrogen in this experiment is only of the order of 10%. The yields of the heavily manured treatments show the same violent fluctuations as do the unmanured treatments.In the three-course rotation experiment surface nitrates in August and September in the wider rotations are strongly negatively correlated with the rainfall of the period 1 July to 15 August. Over the period July to September the distribution of nitrates in the upper 2 ft. of the profile changes; there are differences between the rotations in this respect.
Large expanses of vertisols currently used, especially in Africa, for low-intensity live-stock or rain-fed agriculture could beneficially be brought under irrigation. They generally have favourable chemical characteristics with relatively few instances of salinity or alkali hazard. Whilst slow rates of water movement through the soils may limit water application and restrict artificial drainage, these vertisols are capable of sustained productivity, under irrigation, of a wide range of crops.
The results of two sets of soil nitrate measurements on the three-course rotation plots at the Gezira Research Farm are described. The first set is a long-continued series taken over 21 years; it shows for the first foot samples that there is a distinct tendency for nitrates to be depressed during the rainy season, this tendency being more marked in the more open rotations. This is ascribed to washing down of nitrates. The second set of measurements shows accumulations in the second foot and a distribution consistent with downward movement.In the Sudan Gezira the yields of irrigated cotton are positively correlated with the amount of rainfall in the 6 weeks before sowing. It follows from the work described here that this is not due to increased nitrate in the top foot of soil at sowing time.The sampling of this experiment for nitrate was started by my predecessor, H. Greene, and continued by O. W. Snow. The field experiment itself has been conducted by the Plant Physiology Section of the Research Division of the Ministry of Agriculture.
A laboratory study has been made of nitrification rates of the soil's own nitrogen, ammonium sulphate and certain organic nitrogen compounds. It is shown that nitrification in this soil is substantially normal, with no striking differences from that in soils of other arid regions.I am indebted to H. Greene and E. M. Crowther for access to their earlier unpublished work on nitrification in Gezira soil, and to O. W. Snow of this Service for advice during the course of this work.
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