Variations In Soil Moisture Content Research Articles

Spatial variability of an agricultural field: Geostatistical analysis of soil texture, soil moisture and yield components of two rainfed crops

Summary Spatial variations of soil moisture content, textural components and yield components of two rainfed crops (vetch-oat and durum wheat) are studied by using classical statistical and geostatistical tools. The measurements were made at 81 nodes of a15×10m grid superimposed on a 1.21 ha plot selected in a 13.5 ha-field. The variability of each variable is characterized by a coefficient of variation ranging from 11% to more than 100% and a range varying from a distance smaller than the sampling interval to about 80 m. The concepts of dispersion and estimation variances are used to estimate the accuracy of the calculated mean values and to derive the relation between the dimensions of the sampled plot and the number of measurements needed to estimate average values with a given prespecified error. The results show that the total relative errors affecting the mean values calculated from 81 observations are in the range between 3 and 26% according to the variables, the major contribution arising from the finite character of the sampled plot. They also suggest that for the case where variables have spatial structure, decreasing the error on mean values cannot be achieved without increasing the size of the sampling surface.

Helium in soil gas: a method of mapping groundwater circulation systems in fractured plutonic rock

The Carnmenellis granite is one of seven Variscan plutons exposed in SW England which intrude a folded and thrusted succession of metasedimentary and metavolcanic rocks of Devonian and Carboniferous age. The granite contains higher than average radionuclide concentrations which give rise to enhanced heat flow and fracture-controlled, low enthalpy hydrothermal circulation. Measurements of 4He in soil gas have been made at 91 sites over the pluton and the surrounding host rocks, in order to assess variation of 4He emanation in relation to the distribution of radionuclides and the groundwater circulation system. The pattern of variation obtained shows little correlation with the results of an airborne total gamma-ray activity survey, but agrees well with the distribution of 4He in spring waters. Because the occurrence of zones of high 4He concentration in these spring waters shows a significant correlation with local variations of heat flow within the pluton, it is believed that the distribution of upwelling limbs of groundwater circulation cells controls the groundwater 4He flux. The spatial association of high values of 4He in soil gas with high values in spring waters thus suggests that fracture-controlled groundwater circulation systems can be mapped using the soil gas method, even in the absence of springs. However, the variation of 4He concentrations in soil gas is more complex than that given by spring waters. This is due to variation in soil moisture content affecting the pore space concentration of 4He. Nevertheless, resolution of this effect is possible by statistical analysis of the 4He distribution.

Effects of November–April grazing pressure on hill country pastures 3. Interrelationship with soil and pasture variation

Abstract Variations in soil moisture content, nutrient levels, and temperature were measured for a range of micro-sites located on a steep, north-facing hillside. Species distribution and plant responses to changing moisture conditions and different grazing treatments were related to these physical conditions. Soil nutrient levels (organic carbon; total, inorganic, and mineralised nitrogen; Olsen phosphorus; potassium) were highest in camp/old track areas reflecting nutrient transfer by grazing animals. Variation in nutrient level between other flat and sloping areas was low. Soil moisture holding capacities were similar for most areas, but sloping areas were consistently drier in the surface horizon (0–50 mm) because of less effective re-wetting compared with flatter areas. Maximum soil surface temperatures of 45–50°C were reached on clear days where low pasture cover existed on sloping areas. The effects of grazing treatments on ryegrass and browntop tiller number, white clover leaf number, and seedling establishment were similar across a wide range of micro-sites. However, pasture species content varied between micro-sites: ryegrass was dominant on camps and tracks; browntop increased with steepness of slope; white clover peaked on rolling and easy sloping areas; grass seedlings were most numerous on camps and tracks; and legume seedlings were most numerous on steeper slopes. Pasture type would appear to be determined more by nutrient fluxes (e.g., mineralisation rate, excreta return) and summer rewetting of the soil surface than by nutrient pool size and inherent water-holding capacity of the soil.

Seedbed tilth and nitrogen response of wheat

The effects of seedbed tilth (coarse and fine) and different levels of nitrogen application (0–150 kg/ha N) on wheat response were studied in loam and sandy loam alluvial soils ( Typic Camborthids) at three sites under irrigated conditions. The percentage of dry aggregates > 4 mm was 36.7 and 50.8 at site 2 and > 10 mm was 44.3 and 70.2 at site 3 under fine- and coarse-tilth treatments, respectively. The variations in soil tilth were obtained either by use of different types of tillage implements, their combinations and sequence, or by variation in soil moisture content at the time of cultivation. Crop growth was better and grain yields under coarse-tilth were from 0.3 (0 N) to 1.2 t/ha (150 N) higher than under fine-tilth. Total mineral nitrogen (NO 3-N and NH 4-N) content was larger in coarse-tilth treatments than in fine-tilth treatments.

Comparison of experimentally Determined and Calculated Hydraulic Conductivities for two alluvial sandy loam Profiles

AbstractThe dynamic water conductivity characteristics of two alluvial sandy loam profiles (Typic Ustochrepts) were determined following the „instantaneous profile method”︁ by monitoring the temporal variation in soil moisture content and potential at different depths in the profile, as the downward movement of water in the nearly saturated profile continued with evaporation prevented. The experimental sites differed in bulk density, moisture retention functions as well as dynamic water conductivity characteristics K(O). The unsaturated hydraulic conductivities were also calculated from moisture retention functions following the methods suggested by Campbell(1974) and Ghosh (1977). The calculated conductivity values agreed fairly well with the field data for the light‐textured soils studied and the calculated values can be used for all practical purposes.

Respiration in cropped and fallow soil

SummarySoil air was sampled daily at depths of 5, 10, 20 and 30 cm in fallow and cropped plots, which had either been rolled or cultivated. Carbon dioxide was measured by gas chromatography.Carbon dioxide concentrations were greater in the cropped than in the fallow plots. The rolled plots were wetter than the cultivated plots and carbon dioxide concentrations were also greater.In the fallow plots, carbon dioxide was correlated with soil temperature but variations in soil moisture content had little immediate effect.In the cropped plots, temperature effects were overshadowed by the response to irrigation which immediately increased the carbon dioxide concentration. These increases were attributed to enhanced root respiration, rather than to restricted diffusion or increased microbial activity.

Seasonal growth patterns in field beans (Vicia faba) as affected by population density, shading and its relationship with soil moisture

SUMMARYField experiments, usingVicia fabacvs Herz Freya and/or Maris Bead were made in 1972–5. Population densities were varied and shading treatments were applied. Soil moisture content was monitored throughout. Maximum dry-matter production per plant and per unit area were similar in 1973 and 1974, but lower in 1975. Variations in growth were associated with variations in soil moisture content. When pods began to fill, plants reached maximum total water content and root growth ceased.Increasing density enhanced, and shading retarded both leaf senescence and seed maturation. Maximum potential seed yield (5–6 t/ha) in 1974 was obtained at 32plants/m2. In 1975, yields of 7–9 t/ha were obtained at 66 plants/m2, probably as a result of retention of many pods by the plants. Since total dry-matter production was lower in 1975 than in 1974 we conclude that photosynthetic potential did not limit yield in the earlier year.From the patterns of rainfall, soil moisture and plant water content, we suggest that water supply may be a more important factor controlling yield than either solar radiation or plant competition, with the period following pod setting being especially vital. At this time plant water requirement may often be in excess of supply.

Phosphorus Uptake by Soybeans as Influenced by Moisture Stress in the Fertilized Zone1

AbstractReports on the effect of soil moisture stress on plant uptake of P have been conflicting. The effect of the dynamic variation in soil moisture on P uptake by soybeans (Glyfine max L. Merr.) was studied in vivo. A split‐root technique with plants grown in 3 liter containers in the greenhouse was used whereby moisture stress was localized in the zone having the only source of P for the roots, while water and P‐free nutrients were supplied through the other portion of the root system. The procedure minimized the effect of plant‐water deficit.Differences between moisture treatments accounted for 81.5% of plant P content when the effects of fertilizer treatments were removed, while variation in soil moisture content within each moisture treatment accounted for only 8%. The extent P uptake was impaired by high moisture stress was conditioned by the P status of the plant which in turn was determined by the previous soil moisture history of the zone supplying the P. After watering, plants exposed to a high moisture stress resumed P uptake at a rate exceeding that of plants absorbing P from the adequately watered compartment.The range of response to which increasing levels of fertilizer P was obtained increased as the period of moisture stress in the fertilized zone was prolonged. Temporary luxury consumption of P, induced by high levels of soil P and foliar application, benefited soybeans whose P supply was curtailed by high moisture stress.Moisture stress appeared to affect P uptake largely through its influence on diffusion rather than mass‐flow. It is suggested that the inconsistencies of field results reported in the literature studying the relationship of P uptake and moisture stress could be caused by the peculiar P uptake behavior found in these experiments.

Non-symbiotic nitrogen fixation in some Hawaiian pasture soils

Evaluation of rates of non-symbiotic acetylene reduction in some pasture soils indicates the presence of certain bacteria capable of providing as much as 32 kg/ha/yr nitrogen to these soils. Variations in soil moisture content, especially as related to season, appear to have a profound effect on rates of acetylene reduction in soils tested. The facultative bacteria responsible for this fixation have been identified as species of the genera Klebsiella, Enterobacter and Achromobacter.

Effect of Environmental Factors on Pyrazon Action in Sugar Beets

The effect of temperature and soil moisture content on the toxicity of soil-applied 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) to sugar beets (Beta vulgaris L. ‘U.S. H-8’) was studied under controlled environmental conditions. High temperatures during or after germination increased the susceptibility of sugar beets to pyrazon while variations in soil moisture content did not have a significant effect. Sugar beet seeds absorbed three times more pyrazon at 35 C than at 18.3 C. During imbibition more than 90% of the pyrazon taken up by sugar beet fruits was concentrated in the pericarps surrounding the seeds. Furthermore, the herbicide which had been accumulated in the pericarp during imbibition did not move into the tissues of the developing seedling during or after germination. Comparative studies showed that there was a lag period in absorption of pyrazon by sugar beet seeds enclosed within their pericarps. This lag period did not occur in sugar beet seeds from which the pericarps had been removed, or in seeds of common lambsquarters (Chenopodium album L.). It is concluded, therefore, that the pericarp contributes to a physical mechanism of selectivity which enables sugar beets to avoid great accumulation of pyrazon when the mechanism of biochemical inactivation of the herbicide is not yet operative.

Stomatal Opening as a Practical Indicator of Moisture Stress in Cotton

AbstractThe response of stomata to varying soil moisture content was studied in cotton by means of a new type of porometer suitable for field work. The results showed that maximum stomatal aperture was only attained when soil moisture was near the upper limit of the so called “available range”. These results indicate the possibility of making practical use of the new type of porometer for the purpose of determining soil moisture deficiency under field conditions, thus aiding in irrigation practices.

Losses, through Denitrification, from Soil of Applied Inorganic Nitrogen even at Low Moisture Contents

THE losses of nitrogen in gaseous form (oxides of nitrogen) through denitrification of inorganic nitrogen, particularly nitrate, added to water-logged soils is well known1–3. Other workers4–6 showed that nitrogen was lost by denitrification even from drained soils, that is, under non-water-logged conditions. These workers did not consider the effect of varying soil moisture content on the extent of denitrification, apart from ensuring that the soils were not water-logged. However, in drained soils water content may vary from, saturation (maximum water-holding capacity) down to the wilting percentage, in so far as plants are concerned, or even lower. Bremner and Shaw7 showed in incubation tests that a moisture content of approximately 60 per cent of the maximum water-holding capacity was critical, in that denitrification occurred only when soil moisture content exceeded this figure.

Flow of Soil Moisture in the Unsaturated State

Conclusions and SummaryIt was assumed when this problem was undertaken that there existed a function f of the moisture content that could be multiplied by the permeability constant k of the Darcy equation to make it applicable to movement of moisture in unsaturated soil. It would appear, however, from the experimental data obtained for a typical soil, that in addition to a pressure gradient due to variation of soil moisture content from point to point in the soil, there may be a gradient due to the dynamic process itself. It is perhaps unfortunate that f is not a function of ρ alone, but this should not constitute an insurmountable obstacle in applications. Modifications may be made to simplify the analysis at the expense of some precision. The experimental data do indicate that the permeability decreases rapidly with the moisture content, and this is in agreement with the observed phenomena that irrigation water moves very rapidly when first applied but slows down very appreciably as it moves out from the source. Even though the problem is difficult, requiring a rather complicated mathematical approach, it is an important one and deserves further consideration. An attempt should be made to eliminate the geometrical variable R from equation (41) by use of a relationship such as that of equation (25) from which R can be obtained as a function of r, ρ, and ∂ρ/∂r. This will permit the expression of f in terms of ρ with the gradient of ρ as a parameter, both of which may be measured in the soil. Other soils should be studied and attempts should be made to find approximations that will render the principles involved useful to irrigation engineers.