Modification of Soil Physical Properties by Maize Straw Biochar and Earthworm Manure to Enhance Hydraulic Characteristics under Greenhouse Condition

The deterioration of soil physical properties had led to a decrease in soil–water availability in facility agriculture. Thus, an experiment was set up with five soil treatments of 0% (CK, No additives), 3% biochar (BA 3 , Mass ratio), 3% earthworm manure (QA 3 ), 5% biochar (BA 5 ), and 5% earthworm manure (QA 5 ) to investigate the effects on soil physical properties and hydraulic characteristics under greenhouse conditions. The physical properties of soil including the soil bulk density ( BD ) and total porosity ( TP ) were measured; the results showed that BA 5 provided the lowest soil BD (1.24 g·cm −3 ) and the highest TP (53.09%) and was 13.8% higher than CK. More importantly, the saturated hydraulic conductivity ( K S ), field capacity ( FC ), permanent wilting point ( PWP ), and available water content ( AWC ) of the soils treated with biochar and earthworm manure were significantly higher than those of CK. At the same application rate, the effect of biochar on soil–water permeability and water-retention capacity was significantly higher than that of earthworm manure, in which the soil–water-characteristic curve ( SWCC ) showed that as BA 5 > BA 3 > QA 5 > QA 3 > CK, the FC and AWC increased from 28.90% and 14.13% under CK, respectively, to 40.73% and 21.91% under BA 5 , respectively; and the K S , FC , PWP and AWC of BA 5 increased by 45.93%, 40.91%, 27.46% and 54.96% compared with CK, respectively. The results revealed that the improvement of the soil TP was conducive to the enhancement of the soil K S and FC , enhanced the soil–water permeability and the water-retention capacity, and ultimately increased the AWC . From the perspective of improving the facility soil and economic benefits, the application of 5% biochar is considered to be the most beneficial.