Effect of application of crop and licorice root residues and their biochars on potassium status of a calcareous soil

Application of different organic materials in organic agriculture may change the status of plant nutrients in deficient soils. Arid soils of Iran may have a considerable content of potassium (K); but their K content is decreasing due to intensive agriculture. This K deficiency may be alleviated by different organic materials application in organic agriculture.
In the current investigation, a completely randomized experiment was done with application of four plant residues and their produced biochars to a calcareous soil and their effect on different forms of K. Three grams of wheat straw, corn straw, rice bran and licorice root residue and their produced biochars was added to 100 g of a clay loam calcareous soil and incubated for 90 days at 22±2 °C and 50 % of saturation moisture content. The soil samples were air-dried and sieved and pH, electrical conductivity and contents of soluble, exchangeable, non-exchangeable, HNO3-extractable K and K release rate from soil minerals were determined.
Results indicated that plant residues had no effect on soil pH, but all biochars increased soil pH (mean of 0.07). Soil EC was increased with application of wheat and corn straws and conversion of plant residues to biochars had more effect on soil salinity. Licorice root residue and its biochar had no effect on the content of different K forms; but other plant residues and their biochars increased soluble, exchangeable and HNO3-extractable K in the order of wheat residue > corn residue > rice bran. On average, biochars had more effect than plant residues on the content of soluble, exchangeable and HNO3-extractable K (212, 269 and 286 mg kg-1, respectively). The content of HNO3-extractable K was not affected with plant residues and their biochars (except for corn straw). Wheat and corn straws and rice bran released 286, 217 and 146 mg K kg-1, respectively; and their biochars released 637, 429 and 290 mg K kg-1, respectively from K-bearing minerals and this may be due to the effect of organic molecules and non-organic cations of organic materials on mineral weathering and K release.
It is concluded that application of plant residues and their biochars may have significant effects on soil K status and alleviation of K deficiency and the role of biochar is more important than primary plant residues. On the other hand, increase in soil salinity and pH especially in calcareous soils of arid land should be take into consideration.