[ 220 ] A MODIFIED FORM OF AUTO-IRRIGATOR BY G, REDINGTON, M.SC. (With 7 figures in the text.) THIS short paper describes a method by which the roots of potted plants can be assured of a regular and practically unvarying supply of water over a considerable period of their life. The apparatus used is simple, cheap, and easy to construct. In the first place acknowledgements must be made to Professor J. H. Priestley, with whom the idea of this modified form of autoirrigator originated. It was decided to use the apparatus in connection with experiments on the growth of plants in continuous light, in order that the plants under investigation could be supphed with water by this means. The general practice of watering plants grown in pots is to withhold water until the soil becomes dry, as indicated either by the appearance of the soil surface, the weight of the pot, or the houow sound emitted when tapped. The pot is then filled with water and the process repeated. This method obviously has many disadvantages both for ordinary and special circumstances, and the supply of water to the roots in this way is far from being regular. The modification of this method sometimes adopted, where measured quantities of water are supplied at regular intervals, also has disadvantages and can have little relation to the actual needs of the plant. In the growth of plants in continuous light it was important that all other external conditions should be kept as uniform as possible in order that no factor should operate tending to introduce any periodicity into the life of the plant. The auto-irrigator, it is claimed, is a method by which the supply of water to the soil is controlled by the rate at which it is being lost by evaporation from the surface and sides of the pot and by transpiration. Thus the supply to the plant is regulated primarily by its needs, and the soil moisture content remains practically constant. If this end is to be attained it follows that the water-supplying power of the apparatus must be greater than any demand which is to be made upon it. The idea of the auto-irrigator was apparently first developed in America, and an early apparatus was described by Livingston (4) in 1908 whose short paper was fouowed by one from Hawkins (2) with
A Modified Form of Auto-Irrigator 221 a further paper from Livingston(5) in 1918. In this case the water supply to the roots of the plant was provided by one or more porous porcelain cups buried in the soil of the plant pot. Originally these cups were cylindrical in shape and similar to the atmometer cup, but later conical cups were used in order to maintain better contact between the soil and the surface of the cup. The cup is filled with water and connected by a rubber stopper and a tube to a water reservoir at a lower level. As water is taken from the soil by plant roots or by evaporation, more water moves from the wall of the cup into the soil, the cup being replenished by water passing up to it from the reservoir. Holmes (3) showed that moisture equilibrium in the soil of pots equipped with auto-irrigators of this type was attained in 75 to 90 days, according to whether a clayey or a sandy soil was used. Livingston and Hawkins (0) found that in this way a fairly constant soil water content is maintained over a period of 24 hours, the maximum plus and minus variation from the mean in six pots being 4-6 per cent, of the mean moisture content of the soil on the basis of weight of dry soil. The modified form of apparatus to be described is hke the porous cup auto-irrigator only in that the soil of the pot is in contact with a water supplying surface which is itself automatically kept supplied with water. DESCRIPTION OF APPARATUS Suitable sized pots are selected (a little larger than would normally be used) and the drainage hole enlarged to a circular hole, varying in size with the size of the pot. Usually a hole in. in diameter is sufificient for a 3-inch pot^, i in. for a 4f to 5-inch pot and l in. for a 6-inch pot. A piece of ordinary grey flannel is cut to correct size and sewn up to form a cylinder which will fit closely inside the pot. A thread is run through the flannel, drawing it up at the point where it will pass through the enlarged drainage hole, leaving the top of the flannel level with the rim of the plant pot. The pot is thus lined with the flannel, which is continued down through the drainage hole and out in the form of a ' wick' several inches long. The drainage hole is then stopped up by screwing a rubber stopper tightly into the flannel from below. The pot is then ready for the reception of the soil and plant or seeds. After planting, the pot is stood in the neck of a large glass jar which is kept filled with water to within an inch or two of the bottom of the pot. Flannel 1 Sizes of pots expressed as internal diameter in inches.
222 G. REDINGTON was used because it is cheap and has considerable powers of water absorption when once wetted thoroughly. It is essential that the flannel and the pot be moist before being filled with soil. Stoppage Plant Pot Rannel Lihina. Rubber Stopper Extension of- Lining. Fig. I. of the drainage hole with a rubber bung was found to be the only way of preventing the roots from growing out through the hole and into the water below, and it was found that this did not interfere with the supply of water to the pot.
A Modified Form of Auto-Irrigator 223 The amount of water supplied to the soil is controlled mainly by the relation between the area of the water-supplying surface' the flannel lining to the pot and the volume of soil in the pot. As this volume increases, the proportion of surface to volume decreases, so that the smaller the pot the greater the supply of water. There will thus be a practical limit to the size of the plant pot that can be used, though what this limit is has not yet been determined. It will vary with the water requirements of the plant, the kind of soil used and the degree of consolidation of the soil in the pot. In practice the method has been used successfully for pots up to b^ in. in diameter. Usually for large-sized pots containing plants which make a heavy demand on the water supply, it is advisable to use a double thickness of flannel, whilst a single layer has been found sufficient for pots up to 4 in. in diameter. EXPERIMENTAL The chief tests of the apparatus were made with a series of maize plants grown in pots very much smaller than would ordinarily be used for the purpose. As it was desired to put the capacity of the apparatus to the most severe test, it was decided to grow half of the test plants under continuous illumination, as under these conditions the practically unceasing loss of water in transpiration would throw a great strain on the power of the apparatus to supply adequately the abnormal needs of the plant. Space was available in a room 10 ft. by 5 ft., lighted by four Ediswan gas-filled electric lamps, each of 500 watts. The temperature of the room was approximately 30 C. and the average relative humidity 35. A vigorously growing plant such as maize under these conditions would demand a large supply of water. Six young comparable maize plants (var. Giant Caragua) were selected from about 50 seedlings, and potted, using ordinary potting soil that had been passed through a J-inch sieve. The pots were 4I in. in diameter by 5 in. deep. The drainage hole was enlarged to a diameter of ij in. to accommodate the double thickness of flannel used for lining the pot, and stopped as usual with a rubber bung. The pots were then placed each in the neck of a glass jar which was filled with water to just below the bottom of the pot. Three of the plants were grown in continuous light (C i, C2 and C3), and three in intermittent light (L i, L2 and L 3). Four of the plants were grown under experimental conditions for 15 weeks (from February 15th to June 12th). At the end of that time they had reached a height of from 6 to 8 ft., had borne and matured
224. REDINGTON terminal staminate inflorescences and were bearing the young pistillate inflorescences. us uo = is I 3 30 ^^ 10 S ime in days. 3 ;3 (7 ZL 3i, AS i"? Fig. 2. Plant grown in continuous light. isa 35 O 30 /O Time in days 3 13 If ii. 3U li^ S"6 <Jh 100. Fig. 3. Plant grown in continuous light. DETERMINATIONS OF SOIL WATER CONTENT Many determinations of the water content of the soil in the pots were made during this period, the first samples being taken 8 days after potting up the seedlings. The sampling tool used was a cork borer of i-8 cm. diameter. All determinations were made in dupli-
A Modified Form of Auto-irrigator 225 cate, two samples being taken in each case from different parts of the pot. The holes were then filled up with soil taken from the bulk C.3. I J Ut, LT JO Tb CjU/OO Fig. 4. Plant grown in continuous light. LA 30 10 Time in days. 13 17 1U iw ur -JO rl, cj^. 100 Fig. 5. Plant grown in intermittent light reserved from potting, every care being taken to ensure as nearly as possible the same degree of consohdation. The method used for determining the moisture content was that adopted by the Agricultural Education Association(l).
226 G. REDINGTON Possible variations in soil water content would be between (a) samples from different pots; h Lo K N o ^30 to h O20 10 s - Time II1 da\ ' Jjs Fig. 5. Plant grown in intermittent light. ^ _. L3. ^3f r \ 510 Time in davs. 3 HI] iu ZL Fig. 7. Plant grown in intermittent light. loo (b) samples from different parts of the same pot; (c) samples taken at different periods during the growth of the plants;
A Modified Form of Auto-Irrigator 227 (d) samples taken with the water in the reservoirs at lower levels; (e) samples taken after the intermittent light plants had been in the light for 16 hours, and others taken after they had been in darkness for 8 hours. Possible variations from these causes were investigated, with the following results. (a) and (c). In the accompanying figures the soil water content for the six pots is shown graphically^ over the whole of the experimental period of 15 weeks in four cases and 9 weeks in the other two. These latter plants were removed at the end of this time as they had grown to the roof of the experimental room, a height of 8 ft. It will be seen that in four cases there was a close agreement between the different pots over the greater part of the growth period and it was not until the plants had reached fuu height and were beginning to flower that the soil water content began to fall. Plant C I made the most rapid growth and so was the first in which the demand for water exceeded the supply. It is only to be expected that such vigorous plants growing in small pots will in time reach the stage of requiring more water than the apparatus could supply. The breaks in the curve of plant C 3 are due to the fact that on these two occasions the pots were purposely left without water. The dozens of samples from which these curves were plotted were taken on different dates at different times of the day between 8 a.m. and 9 p.m., the six pots generally being sampled together. (6). The general distribution of water in the soil of the pots was investigated by comparing samples taken from different depths, and also samples taken at varying distances from the centre of the pot. These showed that there was a slight increase in water content from the surface to a depth of 9 cm., and also a slightly higher water content near to the flannel lining. Samples 0-3 cm. depth 3-6 6-9 close to flannel lining near to centre of pot Average water content of 6 pots (%) 42-7 44-8 45-8 44-8 42-4 These small differences cannot be of significance in considering the general supply of water to the plant roots. 1 Soil water content throughout is calculated on wet soil.
228 G. REDINGTON {d). On two occasions the water in three of the containers was allowed to fall from 6-9 cm. below its usual level. The average water content for the pots supplied from the normal level was 44-6 per cent., and from the low level, 43-9 per cent. (e). With the three plants in intermittent light, samples taken after the 8 hours' darkness showed practically the same water content as those taken after the 16 hours' light period. After several weeks' growth the soil water content of one pot was twice allowed to drop, once to 24-6 per cent, and again to 10-7 per cent. On filling up the containers again, the normal water content was attained within three or four days. GENERAL This apparatus has been used with success for the growth of many species of plants, including such diverse tj'pes as Zea mats, Gossypinni herbaceum, Boehmeria nivea, Kleinia artieulata, Maranta arundinacea, Canna indica, Vicia faha and Lapageria spp., in pots varying in diameter from 2J to 6 in. The chief precautions that have been found necessary in the use of this method of auto-irrigation are due to the fact that in the early stages of the life of a slowly-growing plant, there is a tendency for water to be supplied to the soil more freely than the plant requires, and in young, newly potted plants of this type this had resulted in the loss of 5 plants out of 45 grown. In view of this tendency it is advisable to observe the following precautions in the case of young, newly potted plants of slow growth. 1. A single thickness of flannel should be sufficient for all sizes of pots up to 5 in. in diameter. 2. The soil used for potting should contain a fairly high proportion of sand. 3. The soil should not be rammed too firmly about the roots, but should be left rather looser than is usual. 4. After potting up such young plants, allow two or three days for the soil moisture content to reach equilibrium, and then empty the containers and just keep the flannel moist until the plants have rooted freely in the fresh soil. When active new growth of leaves indicates that this stage has been reached, the containers can be kept fllled with water in the usual way. It win of course be remembered that there exists no free drainage in the pots, so all water entering has to be removed by evaporation and transpiration. Hence it is inadvisable and quite unnecessary
A Modified Form of Auto-Irrigator 229 to "water in" newly potted plants or newly sown seed, as is usually done. The apparatus wiu remain in working order as long as will be usually required for the growth of plants under either ordinary or experimental conditions, and one is now in use which has supplied a 63-inch pot for over a year. The cost of the electric light used in the experiments on the growth of these plants was met by a Research Grant from the Carnegie Trust for the Universities of Scotland. REFERENCES (1) The Official Method fur the Mechanical Analysis of Soils. AgyicuUuyal Pyogress, 3. 1926.. (2) HAWKINS, LON. A. The porous clay cup for the automatic watering of plants. Plant IVoyld, 13. pp. 220-227. 1910. (3) HOLMES, E. S. Moisture equilibrium in pots of soil equipped with autoirrigators. Johns Hopkins University Ciyculay, pp. 208-210. March 1917. (4) LIVINGSTON, B. E. A method for controlling plant moisture. Plant IVoyld, 11, pp. 39-40. 1908. (5) Porous clay cones for the auto-irrigation of potted plants. Plant Woyld, 21, p. 202. 1918. (6) LIVINGSTON, B. E. and HAWKINS, LON A. The water relation between plant and soil. Cayn. Inst. Wash. Pub. 204, pp. 1-48. 1915. PHYT. XXVII 4.