Polysulfides as Soil Conditioners Item Type text; Book Authors McGeorge, W. T.; Breazeale, E. L.; Abbott, J. L. Publisher College of Agriculture, University of Arizona (Tucson, AZ) Rights Copyright © Arizona Board of Regents. The University of Arizona. Download date 28/09/2021 11:31:45 Link to Item http://hdl.handle.net/10150/602182 POLYSULFIDES AS SOIL CONDITIONERS . W. T. McGeorge, E. L. Breazeale and ' J. L. Abbott INTRODUCTION The soil performs three itiajor functions in crop productions: it must have the pore space to supply the air needed for root respiration; ft must have the capacity to meet the water reqiairement of the crop; and finally it must serve as the main source-to.f nutrients. More than any other single factor, soil struc- ture determines the ability of the soil tó fully meet these requirements. In other words, soil particle aggregation, water movement, and plant food avail- ability are integrated characteristics all closely related to the productive capacity of the soil. Tillage, crop sequence, salinity and alkalinity, and the quality of irrigation water Are prominent among the factors which contribute to changes in the physical condition of the soil in the Southwest. Tillage. Baver (I) gives four essential forms of consistency for soils as influenced by moisture content and in decreasing order wet to dry: I. sticky; 2. plastiças manifested b'y the property of toughness; 3. soft as characterized by friability; 4. hard. Research (4) has 'shown that the most severe structural loss occurs when the soil is tilled or si)therwise disturbed at a moisture con- tent represented by the moisture equivalent - plastic consistency. Friability represents the physical condition for tillage which is optimum. Tillage is a very important operation here in the Southwest because in an arid climate the surface soil dries so much more rapidly than the subsurface. This means that often when the surface soil is at optimum moisture content for tillage the subsoil may be wet enough to pack and puddle from the weight and vibra- tion of heavy tillage equipmeht. Over cultivation is also not advisable if the soil is dry, because the soil takes water more slowly if the lumps are reduced to a dust. Salinity and Alkalinity. The'minerais 'which compose the clay fraction of the soil are quite active chemically, particularly in the adsorption and exchahge of bases. When sodium is the dominant base in the irrigation water or the soil solution, it usually becomes the dominant base in the clay mineral. When an excess of adsorbed sodium is present in the clay mineral, the soil becomes highly dispersed and water penetration and soil breathing are seriously re- stricted. Hard waters are best for irrigation and the word hard, as used in irrigation terminology, refers to the ratio between the sodium and calcium content of the water. Crop Sequence. Plant roots, particularly fibrous roots, will help to improve and maintain aggregation of soil particles if the crop sequence best suited to TECHNICALT BUL LErlN NO. the land is used as a rotation. The protective covering provided by the crop also influences particle aggregation y protecting th e surface from the pu d dling effect of rain drops. Organic Matter and Calcium. The essential role of organic matter and cal - cium salts in promoting and maintaining good soil structure has been recog ' nized for many years. Practically all the materials which possess soil condi- tioning properties for alkaline -calcareous soils are direct or indirect sources of organic matter or soluble calium. The fundamental reactions between the clay minerals, calcium salts, and the oxidation products of elemental sulfur are well known. In recent years an increasing number of new materials have been proposed as soil conditioners. Among these are the polysulfides. POLYStILFIDES When aqueous solutions of the hydroxides or sulfides of calcium, ammo- nium, potassium, and sodium are heated with sulfur, the so called polysul- fides are formed. The chemical composition of-polysulfides is quite complex and precise information on their composition and properties is lacking. In addition-to sulfide salts the polysulfide solutions. contain sulfates, thiosulfates, and molecular sulfur. All the commercial polysulfides which have been or are being sold in Arizona are strongly alkaline and have ph values of 10.0 or higher. There is a serious misunderstanding and confusion among farmers con- cerning the value of the various forms of combined sulfur and elemental sulfur as soil conditioners. Sulfuric acid and, calcium sulfate react quickly with the clay minerals to produce the type of reaction which is conducive of aggregation in alkali- soils. Elemental sulfur and polysulfides can function only during and after oxidation to'active forms and not necessarily because of the sulfate which they -form but because of the calcium and hydrogen with which the sulfate is linked. An important property of sulfur is a neutraliza- tion of hydroxyl ions and carbonates in the soil during oxidation. The four polysulfides mentioned contain salts of the respective bases which may enter into base exchange reactions and elemental sulfur which is only active during and after oxidation. The alkalinity of the ppolysulfide solutions is a property which must be reckoned with in a consideration of these materials as soil conditioning agents. s:. OBJECTIVES AND MODE Or ATTACK Soil micro- organisms are instrumental in the oxidation of sulfur and sulfide compounds in the soil and therefore incubation procedures are essen- tial for a study of the effect of polysulfides on soil properties. The oxidation of the pentasulfide has been represented as follows (5) for soils containing . alkaline -earth carbonates. CaS5 + 8 02 ± 4 H2O _ CaSO4 ± 4 H2SO4 H ,SO4 + CaCO3 = CaSO, f CO2 + H2O caching studies are also essential in a study of the effect of polysulfides on soil properties because in alkali' soil reclamation the sodium replaced from the clay minerals, during and after oxidation, must be removed in order to get full benefit from the conditioner. POLYSULFIDES AS SOIL CONDITIONERS One objective of this investigation was to determine whether the poly- sulfides will oxidize readily in alkaline -calcareous soils and to learn if gypsum is one of the products of oxidation. Where oxidation only was studied, the incubations were . conducted with 100 gram portions of soil. Where both oxidation and the effect of soil properties were to be studied, the incubations were made with 500 gram portions of soil in order to have sufficient soil for tests which might measure the conditioning value of the Another objective was to determine the effect of the products of the oxida- tion on the soil, particularly the soil properties which conditioning agents are supposed to alleviate or correct. Five hundred gram portions of soil . were used in these studies. Capillary rise and percolation tests were selected for measuring the effect on the soil because the-'irrigation_ farmer has a major interest in the way his soil "takes water. -.Experiment I The fir experiment compared calcium polysulfide, gypsum, and sulfur. The elemental sulfur was mixed with the soil and subjected to a period of incubation before examining the effect on - soil properties. Calcium poly - sulfide contains soluble calcium and combined and molecular sulfur which functions as a conditioner -during and after, oxidation. In view of the claim that calcium polysulfide,, applied in the irrigation water produces an imme- diate increase in water penetration, this material was not subjected to a pro- longed period of incubation before examining the effect on the soil. In other words the calcium ,polysulfide was used in a way that would simulate its present use in the field, namely, .a direct effect and an indirect effect of the limited oxidation -obtained during the lea." period. Gypsum is imme- diately effective and therefore there was no period of incubation for this treatment.: This incubation and Ieaching experiment was conducted in one- liter - capacity glazed clay pots using 500 grams of air -dry soil. The soil was pro- tected from evaporation Ioss during the incubation of the sulfur treated soil and the incubation was conducted at laboratory temperature. ,For the leach- ing operation, one acre- foot -equivalent per 500 grams of soil was calculated and all water applications were made on an acre -foot basis. Laboratory tap water was used This water contains 400 p.p.rn.soluble salts of which 47 p.p.m. is sodium and 56 p.p.m. is calcium-. 'This- is a favorable Na to Ca ratio for an irrigation water. One of the soils used in this experiment was a Cajon- silt loam from an experimental area near Gilbert and the other a silt blanket which represents the surface soil at the Safford Experimental Farm. A partial analysis of each of these soils is given in Table 1. The difference in texture is evident in the exchange capacity. Other important differences are in sodium percentage and organic matter content. Sulfur. The soils to which sulfur were added were incubated for 21 days at a moisture content represented by 60 percent of the water holding capacity. The sulfur was mixed well with the soil at the rates of 0.5, 1, 2, and 4 tons per acre -foot of soil. The sulfur used was 100 mesh fineness. Gypsum. The gypsum was mixed with the soil at the rates of 1 and 5 tons per acre -foot for the Gilbert soil and 5 and 10 tons for the Safford soil. TA.B tial analysis of soils used in peximent GiIhert pI-1, paste 7.7 8..0 pH 1:10 - . 8.8 9.0 Exchange capacity, m.e./100 gms. " 18.4 37.1 Exchange Na, m.e./100 gms. 2.3- 8.5 Exchange K, m.e./100 gms.