— 83 — Bio-chemical Reaction of Tuti Soils.

(a) Fallow Soil.

Formation of NHg Vigorous after three days.

Formation of NO Positive after ten days. Formation of NOg Positive after ten days.

Denitrification Incomplete after one month.

The effect of fallowing on the number and kind of micro-organisms has already been mentioned. That a very real effect is produced by the partial sterilisation involved, is shown by the fact that after cultivation for a season the reactions obtained with the same soil were as follows : (b) Cultivated Soil.

Formation of NHg Vigorous after three days.

Formation of NO Positive after ten days.

Formation of NOg Positive after ten days.

Denitrification ... ... ... ... Complete after twenty days.

A sample taken from a spot just outside the cultivated area, where, as far as is known, arid conditions had prevailed for many years, gave the following results : (c) Desert Soil.

Formation of NHg Vigorous after ten days.

Formation of NO Positive after one month.

Formation of NOg ... ••• • Positive after one month.

Denitrification ... ... .• ... Complete after twenty,days.

In the case of the cultivated soil the decomposition of the NO 3 is believed to be due to an increase in the denitrifying bacteria, while in the desert soil it was obvious that the nitrogen-forming bacteria were deficient.

The samples were taken in every case from the first foot.

Generally speaking it may be stated that, as with soils in England, continuous cultivation is detrimental to the production of nitrogen, but the fallow period should not be too prolonged, or a reversion to desert conditions quickly ensues with a predominance of fungi and a deficiency of the useful organisms of the soil.

As distinct from the formation of nitrate, sketched above, there exists a means by which the supply may be replenished without the intervention of leguminous crops. An organism, Azotohacter chroococcum, is widely distributed throughout the Sudan soils, which has the power of directly fixing or assimilating the nitrogen of the atmosphere.

It is particularly abundant in light open soils, while clay soils are not so favourable as a result of deficient aeration.