Nitrogen Accretion Research Articles

The effect of phosphate supply on the growth and botanical composition of annual type pasture

The results of two long-term field experiments and two 1-year experiments are reported. In three of these, severe phosphate deficiency was present initially. At high phosphate supply, the annual total yield was not significantly related to age of pasture over periods of 10–13 years. At intermediate and low supply, yields relative to high phosphate supply increased significantly with time; these increases are believed to demonstrate residual effects of phosphate. Indirect evidence for nitrogen accretion from clover under severely phosphate-deficient conditions is presented. Sward components—in the long term—responded differentially to phosphate supply. With very low phosphate, erodium (Erodium botrys (Cav.) Bertol.) and flatweed (Hypochoeris glabra L.) were dominant; whereas with high phosphate, cape-weed (Cryptostemma calendula (L.) Druce) and ripgut brome grass (Bromus rigidus Roth)—or else barley grass (Hordeum leporinum Link)—were dominant. Subterranean clover (Trifolium subterraneum L.), though present under these extremes, was relatively more plentiful at intermediate levels of supply. However, at "steady state" conditions, the range in clover content was fairly narrow (from c. 20 to 40%). The significance of these findings to a sheep infertility problem ("clover disease") of subterranean clover-dominant pastures is discussed.

Nitrogen fixation in latosolic soil under grass

Four latosolic soils, varying in total nitrogen content from 0.050 to 0.098 per cent and in organic matter from 0.95 to 1.56 per cent, were collected from the farm of University College, Ibadan, Nigeria. No change in nitrogen content was observed when the soils were kept moist in the greenhouse for four months. Where a legume (Centrosema pubescens) was grown gains of nitrogen in the soil—plant system equivalent to from 100 to 200 pounds nitrogen per acre — 6 inches deep were obtained. Where a grass (Eleusine coracana) was grown gains of nitrogen in the soil—plant system equivalent to from 100 to 130 lb. nitrogen per acre — 6 inches deep occurred in the two soils with low initial nitrogen levels. The possible origin of this nitrogen accretion is discussed. The data support, but do not prove, the hypothesis that it is due to non-symbiotic fixation in the soil. In one of the soils showing a significant nitrogen gain under grass the presence of Azotobacter was not demonstrated and Beijerinckia occurred in only one of 24 samples examined. Thus, if non-symbiotic biological nitrogen fixation did occur, Clostridium and other micro-organisms not belonging to the Azotobacteriaceae must have been the agents of fixation involved. On the other hand, the presence of Azotobacter in the two soils in which no fixation occurred could easily be demonstrated. It is suggested that non-symbiotic nitrogen fixation by soil micro-organisms under grass cover in the humid tropics may be of greater significance than has been believed in the past.

Soil Nitrogen Accretion under Different Covers in a Tropical Rain-Forest Environment

IN the course of an investigation1 of soil fertility-levels in plots on the Faculty of Agriculture farm at University College, Ibadan, total (Kjeldahl) nitrogen accumulation in the upper 16 in. of soil under five different covers over a three-year period was measured.

A Comparison of Several Legumes with Respect to Nitrogen Accretion1

A Comparison of Several Legumes with Respect to Nitrogen Accretion¹