CAB129-37 — Page 360

Page 360

The pH and Ca and Mg values for the three main groups of soils illustrate well the accumulation of bases in the soils at lower elevations, and the figures taken as a whole indicate the general richness in bases of the area, which is confirmed by the results of field experiments. The bulk of the P values are low, which is in accordance with the results of field experiments. It might be expected, however, that the more calcareous valley soils would give higher P values than they in fact do. This may reflect the inadequacy of acid extraction methods on such soils. An interesting further feature is the small group of high phosphate soils in each soil type, possibly due to the chance sampling of old habitation sites, defunct termite mounds, or cattle bomas, in all of which phosphate tends to accumulate. It is noteworthy, too, that extracted Mg values in the soils of higher elevations are of the same order as extracted Ca, and even in the valley soils represents 8 milli-equivalents of Mg as against 11m. eq. of Ca. The ratio of Mg to Ca is thus higher than is usual in British soils. K shows low values only on the valley soils; but this may be a defect of extraction and analytical procedures on high Ca soils.

Only the grey soils at Urambo have been examined in sufficient numbers to justify generalisation. These show the following features.

Of 16 samples whose pH was examined, 6 were between pH 4.6 and 5.5 and 9 between 5.6 and 6.5, which range covers the more typical soils in the small number examined. Of 14 soils examined for phosphate 12 were below 0.5 mgm. extracted P per 100 gm., the two remaining soils showing values over 1.0 mgm./100 gm. Ca was below 25 mg./100 gm. in 7 of 14 soils examined and below 100 mgm. in the remainder, with only one high exception, over 300 mgm./100 gm., from a habitation site. Mg values were mostly from 26 to 100 mgm./100 gm., and K,O lay between 5 and 12 mgm., no soils low in this element being encountered. The overall picture is .of a light leached soil, very deficient in P,O, and Ca., . but not acutely acid.

The pH figure has presumably been maintained at a fairly high level by the annual addition of potash to the top soil in the fires which sweep through the woods in the dry season.

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Four Urambo red soils examined show rather low pH values, three of them being below pH 5.5, with an extracted P below 0.5 mgm./100 gm., and both Ca and Mg below 50 mgm./100 gm. Potash values were all high, between 9 and 12 mgm.K/100 gm. The rather lower pH of these soils, as compared with the grey soils, may reflect their heavier texture.

The heavier red soils of Namanga, Southern province, give results very similar to the Urambo red soils, but here again the numbers analysed in 1947-48 were low. More attention was devoted to the brown silty soils of Block B. These are only slightly acid, all values being above pH 5.6, while Ca values are all above 25 mgm./100 gm., with a mean value of 65 mgm./100 gm. K2O may, however, be low, the mean value being 6 mgm./100 gm.. with 5 soils out of 11 below 4 mgm./100 gm.

Soil fertility experiments

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In the presentation of the results of these experiments, the yields have been adjusted so as to eliminate the effects of fortuitous variations in plant population from plot to plot. Where plant population was itself significantly affected by treatment such adjustment is not permissible and has accordingly not been made. The method followed was that of Finney (1946).

In the tables the following abbreviations have been used for varieties:-

Valencia Spanish Bunch Natal Common

Val. Sp. N.C.

Four types of experiments were designed to elucidate the main soil fertility requirements of the groundnut crop. Each of these types was repeated eight times in the production area at Kongwa, once on grey and once on red soils at Urambo and once on a heavy red soil at Namanga in the Southern Province (Block A). Page 360 of 1097

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