Soils Modified with Emulsions for Polyethylene Sheet

Observed properties imparted to soils at economical levels by a wide range of different polymer emulsions are discussed. The results of pot experin~ents are summarised indicating how specifically treated sand can reduce soil moisture evaporation, increase temperatures in wet soils and reduce leaching losses of fertilizers. Early shoot counts from four separate replicated field experiments planted in autumn and winter with modified soil barriers at or below the soil surface showed that less than 25 kg of dry polymer per hectare increased the numbers of shoots by 268%, 47% and 39% in dry land conditions, and by 21% in irrigated cane when compared with the respective control plots. Introduction The possibility of using synthetic polymers as soil conditioners is not new. During the late 1940's and early 1950's structure stabilisation of soils had great appeal and at one time more than 100 widely different chemicals were comme.rcially available. By the early 1960's most of these products had disappeared and published research papers decreased from 32 in 1954 to one in 1962. In 1972 a symposium at Ghentl re-stimulated interest in this topic and 23 papers 'were presented, including two which summarised the reasons, for theprevious failure ~ f -~o l~rners to find commercially usefi~l applications. The failure was financial in that returns failed to justify expenditure, as was the case for using clear polythene film as a mulch in the South African sugar industry. Millard3 ,and Rau and Millard4 showed that a clear polyethylene sheet mulch over the rows of sugar cane planted under cool conditions gave earlier tillering and significant yield increases, as high as 25 a 3,9 tons cane per hectare. Their conclusions were th~at he polyethylene mulch prevented moisture loss and increased soil temperatures. It was stated4 that PE film could have a use in the sugar cane industry if and when the price of polyethylene became more competitive and that a liquid mulch appli~zd as a spray would be ideal. These conclusions added impetus to the local programme of work then in progress on the use of polymer emulsions as soil conditioners. Matnials and methods Experiment 1 After some preliminary laboratory tests, the value of a thin continuous film of a poly~ner emulsion as a substitute for clear polyethylene sheeting, was assessed in a randomized block experiment in the field on Natal Estates property. The trial was planted in mid-June, 1975, on a heavy alluvium soil. .About 500 kg per hectare of 276 solids emulsion was mechanically foamed to a density of 500 g/litre and sprayed in a band about 20 cm wide over the cane row. As expected, the foam collapsed in about 30 minutes, but disappointingly did not form a continuous clear film on top of the soil, and it did not cause soil temperatures to rise. Other treatments comprised 50 tons of filter cake per hectare on the cane setts, a 5 cm depth of water applied in the furrow at planting, and a 5 cm depth of water applied over the row after planting. Shoot counts were taken 10 weeks after planting, ,and virtually no rain fell during that period. The trial was subsequently destroyed by fire and therefore no yield results wr:rt: obtained. SUBSTITUTES