The Role of Vermicompost on the Change of Physical, Chemical, Hydraulic and Leaching Components of a Sandy Loam Soil

 Vermicompost is a type of biological organic fertilizer obtained from earthworm activity. Vermicompost is used in sustainable agriculture due to its beneficial effects on diversity of plant nutrients and physical-hydraulic modification of soil. However, high presence of solutes in the structure of vermicompost causes soil salinity, increases soil sodium content and changes soil pH. Soil flushing is one of the well known strategies to minimize the mentioned disadvantages of vermicomposting. Although flushing can reduce the soil salinity and sodium content, it leads to transportation of some soil substances such as nitrate, dissolved organic carbon and colloids which their tracing is necessary because of soil quality monitoring and possibility of water resources pollution. The objective of the current study was to investigate the effects of vermicomposting on soil chemical, physical and hydraulic properties and its role on the amount of soil total dissolved salts (TDS), sodium, nitrate, dissolved organic carbon and leaching behavior of colloids.

To treat the soil, 1.45 weight percent of vermicompost (17.68 tones/hectare) was mixed with regular soil. Physical, chemical and hydraulic properties of soil were determined. PVC columns with length of 20 cm and internal diameter of  5.95 cm were used and filled with soil to perform leaching during 24 hrs in saturated condition  experiment. The effluent of columns were collected at various interval times, and their sodium, nitrate, dissolved organic carbon, TDS and colloid contents were measured and the cumulative amounts of them were calculated at 6 and 24 hrs. All experiments were carried out in three replications, and the mean comparison of leaching parameters was done according to Duncan's multiple range test at probability level of 5%.

Vermicompost increased the studied soil chemical properties i.e, organic matter, organic carbon, extractable nitrate, soluble sodium, soluble and exchangeable sodium, EC and TDS to 12.42, 12.9, 118.96, 80.43, 44.48, 109.4 and 109.4 %, respectively and decreased soil pH to 2.35 %. Soil bulk density reduction to 3.81 % and enhancement of soil porosity, saturated hydraulic conductivity and the pore water velocity to 1.38, 7.25 and 5.6 %, respectively are the other results of vermicompost application. The used vermicompost fertilizer caused displacement of soil water retention curve to more moisture around of saturated and permanent wilting points and reduction of air entry potential. In this regard, vermicomposting increased all of soil hydraulic coefficients of van Genuchten model including θr, θs, α and n, and its effect was specially more on θr and α. The result of leaching experiments showed that the amounts of leached TDS, sodium, nitrate, dissolved organic carbon and colloid in vermicompost-containing soil during 6 hrs were 491.4, 65.22, 116.71, 47.68 and 24.86, and during 24 hrs were 946.3, 72.16, 146.26, 95.11 and 41.97 mg/Kg, respectively. For the natural soil, these amounts during 6 hrs were 240.9, 11.84, 20.08, 23.2 and 15.11, and during 24 hrs were 665.6, 15.69, 44.48, 58.34 and 29.39 mg/Kg, respectively. Therefore, vermicompost significantly increased the amounts of leached TDS, sodium, nitrate, dissolved organic carbon and colloid, because of containing more contents of solute, sodium, nitrate and organic matter in its structure. It also increased the porosity and hydraulic conductivity of soil, and made changes in soil water retention curve (P<0.05). The presence of more sodium in vermicompost together with its effect on soil porosity enhancement increased the colloid dispersion and consequently its leaching. In addition, the leaching rate of all of parameters at 24 hrs in comparison to 6 hrs decreased significantly due to high amount of solute leaching through mass flow at initial time of leaching experiment and leaching residual solute by time-consuming process of diffusion.

Although vermicompost can enriched the soil due to increasing nitrate and organic matter contents, it leads to soil salinity and increases sodium contents. Flushing the soil treated by vermicompost removed the amounts of TDS, sodium, nitrate to 10.4, 76.2 and 44.6 % during 24 hrs. Therefore, leaching had a considerable effect on soil sodium reduction and a little effect on soil salinity reduction. Moreover, in comparison to chemical fertilizers, the high nitrate fraction of applied vermicompost resulted in sustainability of soil fertility. It is expected soil salinity and nitrate leaching fraction of vermicompost will be reduce by managing leaching methods, treating vermicompost before using and reducing fertilizer application rate. Thus, the results of current study warn the farmers who used vermicompost in soil to control the soil salinity, ground water pollution and vertical colloid migration.