Aspects of green manuring with special reference to soil carbon dioxide evolution

Early investigations into the effects of the incorporation of a green crop into the soil in tropical areas produced conflicting results. There was evidence in Nigeria particularly to show that green manures benefited the succeeding crops. In Uganda, however, no lasting effects were found and the practice was abandoned as a means of maintaining soil fertility. The objective of the experiments to be described was to re-examine the practice of incorporating green manure in terms of yield from a succeeding crop and effects on the soil. The work was carried out at Makerere University College Farm, ten miles from Kampala, Uganda. In part I the green manuring experiment, chemical analysis of the soil and test cropping results are described. The crops grown as green manures were sunn hemp (Crotalaria juncea) and maize and they were compared to weed fallow. The crops were incorporated by rotary cultivation and four crops were grown in one year. Supplementary water was applied to two of the treatments to determine whether water was a factor limiting growth, and nitrogen fertilizer was applied to two of the maize green manure treatments. The total dry matter incorporated in the year from the weed fallow was 3,400 1bs/acre (3,800 kg/ha) and from the sunn hemp 11,000 ibs/acre (12, 400 kg/ha). The maize green manure without nitrogen produced 20,800 1bs/acre (23, 300 kg/ha) and with nitrogen 29,000 1bs/acre (32, 550 kg/ha). A test crop of maize was then planted to determine if in fact there was any effect on soil fertility as measured by yield. The original green manure plots were split for fertilizer in the ratio of 3:2:1 of Nitrogen, Phosphate and Potash respectively, the other none. The growth of the test crop was followed by height measurements. The analysis of the data obtained showed that there was a significant inter-action between the fertilizer applied and the green manure treatments. When the test crop was harvested, the only significant response was to the application of the fertilizers, for the test crop did not respond to the intensive green manuring. Soil chemical analysis showed that the incorporation of green manures significantly increased soil carbon, potassium and calcium plus magnesium. Foliar analysis of the test crop showed that nitrogen, phosphorus and potassium levels were all above the accepted critical levels. Also the analysis showed that greatest response of the leaves was to nitrogen in the fertilizers applied to the test crop. In Part II some aspects of soil carbon dioxide evolution are discussed. The objectives of this series of experiments were to measure the soil carbon dioxide flux, to relate the loss of carbon dioxide to the breakdown of organic matter, to examine the effects of temperature and moisture and to compare an accepted laboratory method with results obtained in the field. In Chapter 3 the field experiments are described where a method using soda lime as a carbon dioxide absorbent was employed. Measurements of carbon dioxide flux were made on the green manuring experiment, where it was found that the soil in the maize green manure treatments produced significantly larger quantities of carbon dioxide than either the sunn hemp or weed fallow treatments. An equation of the type dx/dt = A –yx, where A = kg carbon returned per year, x = total carbon in the active soil layer and y = fraction of total carbon lost annually by decomposition, was used to relate the carbon loss with time. On the weed fallow treatment the half-life of organic matter was found to be 6.4 years. The moisture content of the soil was found to be the most important factor governing the production of soil carbon dioxide. When the soil was dry and saturated with water the carbon dioxide. When the soil was dry and saturated with water the carbon dioxide flux was severely reduced. Soil temperatures at 5cm depth did not have any effect on carbon dioxide flux. An experiment was carried out to determine the effect of cultivation on carbon dioxide flux and also whether maize decomposed more quickly than sunn hemp. After rotary cultivation of both maize and sunn hemp, carbon flux rose significantly but after two weeks fell back to its original level. There similar quantities of maize and sunn hemp were incorporated it was found that firstly there was a linear relationship between carbon dioxide flux and quantity of matrial incorporated and secondly sunn hemp decomposed more quickly than maize. Maize and sunn hemp were grown in hydroponic beds to determine the influence of root respiration on total carbon dioxide flux. The roots of the maize and sunn hemp were found to produce about 3.5 gm carbon dioxide/m^2/day; as the root weights were found to be higher than in the field the figure was probably rather lower under examined the use of the macro-respirometer was found to be unsatisfactory. A barium peroxide method was used, showed that sunn hemp decomposed more quickly than maize and that when the quantity of material was increased, so the carbon recovered rose. A comparison was made between the laboratory and field methods, and it was found that where similar quantities of dry matter were incorposition by about 400%. Measurements of the heats of combustion and carbon content of the green manures were made, so that an estimate of the accuracy of the field method of measuring carbon dioxide flux could be obtained. When equations relating energy content of the organic matter with energy liberated in the evolution of carbon dioxide were used, the calculated and measured carbon dioxide fluxes compared very favourably, indicating that the field method was reasonably accurate. Part III is a discussion of some aspects of the experiments. Soil moisture and temperature records from the green manuring experiment are described in the Appendix, together with methods of soil and foliar analysis, and rainfall confidence limits at the experimental area.