STERILIZATION OF SOIL IN GLASSHOUSES.

New Zealand Journal of Agriculture, Volume XXV, Issue 1, 20 July 1922, Page 42

STERILIZATION OF SOIL IN GLASSHOUSES.

OPERATIONS WITH STEAM PLANT AT NELSON. Horticulture Division. The cultivation of tomatoes, both under glass and in the open ground, is attended by a good deal of risk of losses caused by insect and fungus pests. To how great an extent these troubles are due to mistakes on the part of glowers no one can say, but it is practically certain that this is a factor to be reckoned with. / Some years ago losses in under-glass tomatoes were very great, the trouble extending through practically the whole of the Dominion. The Horticulture Division was called upon to investigate the trouble, and in 1913 experiments were carried out in a number of houses lent for the purpose. From observations made the' opinion was formed that a too liberal use of organic manures and , artificial fertilizers and the maintenance of over-humid conditions of atmosphere weie the chief causes of trouble; further, that lack' of necessary ■ spraying' was responsible for preventable losses. The experimental work carried out proved these deductions to be correct, and the propaganda carried out by the Division at that time certainly had good results, wholesale losses now being very rare. There is, however, still a good deal of trouble with various blights that affect tomatoes and cucumbers, and very considerable losses from eelworm, which affects both plants. Experimental work in soil-sterilization has been cairied out for a good many years in England, America, and other countries, and in recent years in New Zealand. About six or eight years ago in England an apparatus named 'the “ Fumo ” was invented for sterilizing the soil by steam for ...the destruction of the spores of fungi which attack plants by means of the root, such as Fusarium lycopersici ' (sleeping-disease), Plasmodiophora brassicas (club-root), and also the resting spores of mildew, Cladosporium Julvum (leaf-mould), rust, blights, and other fungi which attack the foliage of tomatoes, cucumbers, &c. The high temperature of the steam, above 212 0 F. (boiling-point), also effectually destroys wireworms, grubs, and other insect pests which gnaw at the roots of plants. The absence of weeds growing in sterilized soil is very noticeable, showing that the great heat of the steam kills weeds and weed-seeds. ' It has also been found that the application of steam to soil has the effect of rendering some of the soil constituents more soluble, so that tomatoes and other plants thrive in a remarkable manner, and-the general result is equal to manuring the soil, while plants growing in sterilized soil lemain in a healthy condition. The patent “ Fumo ” apparatus, consists of a frame constructed. of steam-pipes in the form of a harrow or grid, with prongs or teeth about loin, in length, which are also hollow. The teeth.are pointed, and perforated to allow the steam to pass through into the soil. The parts, described are all covered with- a light metal cover, which effectually prevents a waste of steam from passing out of the soil. The apparatus

may be of any required size. A portable steam boiler or generator is placed at any convenient distance near the glasshouse, and this is connected with the sterilizer by. steam-pipes and a flexible tube. The method of using the apparatus is as follows After the soil has been loosened by forking over, the prongs of the harrow are thrust into the soil by pressing the apparatus downwardsgenerally by the weight of a man’s foot; steam is then turned on at the boiler, when pressure is at from 40 lb. to 50 lb. to the square inch, and instantly the steam impinges on every part of the soil covered by the sterilizer. . In this form a hundred jets of steam pass through the perforations in the prongs in such an effectual manner that the soil is steamed to a depth of 9 in. at least in about two to three minutes, according to the size and power of the steam-generator. The portion of the soil covered by the sterilizer is saturated by steam, the whole being brought to a temperature above boiling-point. Steam is then turned off at the generator, while the sterilizer is removed to a fresh place, this taking about two minutes to accomplish. Pressure of steam is then turned on again at the steam boiler and applied as before. The sterilizing-apparatus may be of any size, but the usual size, convenient for one man to work and lift about, is 5 ft. by 2| ft. or its equivalent. In this way a portion of soil slightly larger than the apparatus is sterilized at each shift. About 1,800 square feet of soil is usually sterilized in a day with one apparatus, at a cost of about 10s. for fuel and labour per. 1,000 square feet (this is under English conditions) ; but if the steam-generator is sufficiently powerful two or more sets of apparatus may be worked simultaneously from the same generator. This form of sterilizer is more economical to use than any other, because the soil has not to be removed and replaced, as is the case with other appliances used for steam sterilization. It is evident that the high temperature of the steam will, destroy the soil bacteria within its action. It is well known, however, that these organisms are more abundant below the top 9 in. of soil, and the . latter appears to become quickly reinoculated from the subsoil. If this was not the case the fertility of the soil would, of course, be injuriously affected. Experience has shown that sterilized soil is sweet and devoid of acidity. A fairly full description of this method of treating soil,' together with a tracing of the apparatus, was supplied by the Horticulture Division to inquirers in 1918. At Nelson Messrs. Vitetta Bros, have brought the method into practical use, as the following report from •the Orchard Instructor, Mr. W. C. Hyde, will show. . The accompanying illustrations are from photographs of the apparatus used by Vitetta. Bros. Mr. Hyde states : — “ Of very great interest is the work now being done here by the Vitetta Bros. They are busy carrying out contracts to steamsterilize the soil in about three dozen tomato-houses in Nelson City. In many cases the potting-soil is receiving similar attention.' This indicates the opinion of the best growers here regarding the matter. The plant has been considerably enlarged. A horizontal 6 h.p. boiler is used, working at a pressure of 75 lb. of steam. Two harrows are used' —a 2 ft. 8 in. by 2 ft. 8 in. and a 4 ft. by 2 ft. 8 in. The small one is found useful for working out the cornels. These keep two young

fellows busy manipulating them all day, while one of the Vitetta Bros, is attending to the boiler and general supervision.. A glasshouse 75 ft. long and 25 ft. wide is a good day’s work. The steam-plates (harrow is a very good descriptive term) are pressed into the ground, and after about five minutes’ saturation indicates that the steam has done its work. The plates are then lifted and moved to the next section, the treated area being covered with sacks. Tests showed a consistent temperature of 212 0 F.” It is too early as yet to say what degree of benefit this steaming process will be, but there is no reason to doubt that it will be considerable. It, will not, however, be safe for growers to become lax in other ways, for steaming the soil cannot give permanent immunity. The amount of good it will do in cases of eelworm

infection is also problematical. Tests have shown that the apparatus heats the soil to 212 0 to a depth of 9 in. How. far sufficient heat to kill eelworms extends below that depth we do not know. We do know that eelworms are found as deep as the depth penetrated by roots of the plants, and that in many cases is more than twice 9 in. We also know that if the subsoil remains infested the top soil will speedily be reinforced. In most of the experiments of which we have records disinfection for eelworm has been done with chemical substances. An account of experiments of this kind carried out for us, the Horticulture Division, by Mr. C. W. Howard, of Lower Hutt, appeared in the Journal for October last.