A METHOD OF MAINTAINING FRACTIONS OF FIELD CAPACITY IN POT EXPERIMENTS

Transcription

1 A METHOD OF MAINTAINING FRACTIONS OF FIELD CAPACITY IN POT EXPERIMENTS BY F. H. WHITEHEAD AND J. S. R. HOOD Department of Botany and Plant Technology, Imperial College, London {Received 20 October 1965) SUMMARY Difficulties in the maintenance of fractions of field capacity in pot experiments by existing methods are discussed. A new technique, only applicable at present to artificial soils, is described. It consists of blowing saturated air through the soil maintained at an appropriate temperature difference until the required field capacity is achieved. Results obtained are described briefly. INTRODUCTION AND METHOD Great difficulty is often encountered in maintaining moisture contents of less than field capacity in the experimental growth of plants in pots. During the course of growth, water is often lost by transpiration, evaporation, etc., from the soil. In itself this produces an uneven distribution of soil moisture. Results are often expressed in terms of fractions of field capacity, calculated from the loss of weight on drying the experimental soil, and the loss of weight on drying the same amount of soil at field capacity. When the desired fraction of field capacity is maintained by periodic surface addition of water even greater inequalities of moisture distribution arise. Since the root system under these conditions is seldom, if ever, in a soil moisture regime represented by the calculated value of 'fraction of field capacity' it is not true to consider the growth of such plants as being representative of the actual growth which would occur if a uniform soil moisture distribution had existed (Shantz, 1925; Humphreys and Roberts, 1965). To overcome this difficulty various expedients have been tried. One method is to bring the pots up to field capacity periodically and allow them to dry out again to the desired fraction of field capacity (e.g. Aspinall, Nicholls and May, 1964). This method is open, in part, to the same criticism as the first method described in that inequalities of distribution arise in the drying-out process such that the soil is much drier than average in the immediate vicinity of the roots. A further objection is that the final growth and developmental pattern represents a phasic repetition of growth under mesophytic conditions and varying degree of xeromorphic conditions. A second method adapted by F.H.W. in much of his earlier published work (Whitehead, 1962, 1963) consisted in introducing the required amount of water to maintain the theoretical fraction of field capacity by means of a veterinary hypodermic needle. The soil used was an artificial mixture of peat and sand and the needle was inserted at various points over the cross-section of the beakers used and to varying depths. This method does overcome the inequalities of distribution resulting from the other methods, to some extent, since the difficulties arise from the slow lateral and vertical movement of water in soils at less than field capacity. However, this method is itself open to many objections and is far from perfect so that many other systems for maintaining a soil at a fraction of 240

2 Experimental maintenance of soil moisture 241 field capacity were explored. Finally the technique about to be described was evolved and this has for some time been employed by us and our collaborators with very satisfactory results. In principle the method is simple. Warm, super-saturated air is blown through the soil which is at a lower temperature. Moisture tends to condense on the soil particles and, with practice and experiment, a rate of air flow can be found where the rate of passage of the air through the soil is such that not all the surplus water condenses in the lowest soil layers. To obtain a rate offlowso that the air at the soil surface is still super-saturated necessitates a soil of very open structure. So far it has not been found possible to adapt this technique to natural soil cores and only artificial soils have yielded satisfactory results. Thermostat at 25"C Glass wool Delivery tube 3 Glass wool Heating element Fig. I. Diagram of apparatus. The apparatus is shown in Fig. i. Essentially it consists of a reservoir of warm water in an aspirator (A) through which air is blown. This becomes both warm and saturated with moisture vapour. It passes through a water trap (B) to catch any droplets carried over and then, by means of glass tubes, to a pad of glass wool at the bottom of a beaker of soil. It escapes through the soil to the atmosphere. In practice, it was found advantageous with some soils, to replace the glass tubes with perforated polythene tubing. It was also found desirable to cover the soil surface with either a glass wool, or filter paper, pad. The exact rate of flow of air and the most desirable temperature difference between the soil and warm moist air must be determined empirically. Some typical results obtained using this technique are shown below. RESULTS Method I. Air was blown through an aspirator of warm water at a rate of 20 1/min. The resulting warm, saturated air at 28 C was passed through a water trap and into a 6 in. glass beaker (600 ml) through glass tubing reaching to a layer of glass wool at the bottom of the beaker. A pad of glass wool was placed on the soil surface. The soil used

3 242 F. H. WHITEHEAD AND J. S. R. HOOD was an artificial sand/peat mixture (1:1 v/v) oven dried before being placed in the beaker. To test the evenness of distribution of soil moisture small Terylene mesh bags containing a known weight of dry soil were distributed at various depths in the beaker before the start of the experiment. At full field capacity the moisture content of this soil was 36% by weight of the oven dry soil. Air was passed for a continuous period of 20 hours. After this period the moisture content of the bags of soil was determined. Table i shows the results. Table i. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker ("0) Mean 3o.8o o = 83% F.F.C. Range 3.45%. S.D. ±i.oo4 o. It can be seen that the soil has been moistened from oven dry to 83% field capacity in 20 hours. The resulting distribution is remarkably even and represents no more than that found in natural soils. Method II. Using the same soil with a more powerful pump (delivery 70 1/min) but delivering the air to the soil by plastic tubes with two rows of in. diameter holes 30 apart at i-cm intervals for a period of 2 hours only, gave the results shown in Table 2. Table 2. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker Co) Mean 3.76% = io o F.F.C. Range 2.38"o. S.D. ±o.864 o. When this soil was treated in the same way as above but for a longer time a more even distribution of water occurred. This can be seen from the results given in Table 3. These results were obtained by treating oven dry soil for 8 hours. Table 3. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker (%) Mean 21.28% = 58% F.F.C. Range 2.43%. S.D The experiments were repeated using a John Innes No. 2 potting compost. Using Method II (but replacing the plastic tubes with glass tubing) for i hour. The results are shown in Table 4. Table 4. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker (%) i.oo i.ii Mean 1.63% = 3.5% F.F.C. Range 1.49%. S.D. ±0.596%. Using Method II (plastic tubes) for 2 hours the results using John Innes No. 2 potting compost were as shown in Table 5. Table 5. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker ( o) Mean 3.76% =?.% F.F.C. Range 1.40%. S.D. ±0.658%.

4 Experimental maintenance of soil moisture 243 Tests on a derived soil In the results so far given the soils were of comparatively open texture offering little resistance to the passage of air. The experiments were repeated using an artificial soil derived from a natural clay soil. The clay was oven dried, then crushed and sieved. The artificial homogenous soil was made up from various sieve fractions as follows: 1 volume: 2-30 mm mesh particles. 2 volumes: mm mesh particles. I volume: mm mesh particles. Only Method II (glass tubes) gave satisfactory results. With Method II (plastic tubes) it was found difficult to retain the finer soil particles in the beaker as the air flow was too great. With a smaller rate of air flow a very uneven soil moisture distribution resulted. With Method II and glass tubes the results for i hour are given in Table 6. Table 6. Moisture content of bagged samples inserted in a beaker of soil at various depths Moisture from bottom to top of beaker ( o) i.o o.q Mean i.i9 u = 2.98"n F.F.C. Range o.39''u. S.D. ±o.i28 o- DISCUSSION It would appear from these results that in the case of over-dried soils, there is a more uneven distribution of moisture at the end of i hour's treatment than after 2 or more hours' treatment. Thus although the percentage field capacity increases greatly with time there is no obviously greater inequality of distribution as shown by the total range. Experience has shown that once the soil is moistened to some extent the initial inequalities tend to disappear. In practice it has been found that 'topping up' during the course of experiments lasting several weeks using fractions of field capacity from 100 (, to 12^% leads in time to a greatly reduced inequality of distribution. In practice using beakers of the size stated and growing plants of Helianthus annuus it was only found necessary to attach them for about i hour per day after the three internode stage. Up to this time, either shorter periods of attachment to the apparatus, or less frequent attachments, were adequate to maintain the desired fraction of fleld capacity. This method has the great advantage that root systems are always in a moisture regime which departs very little from that represented by the calculated fraction of field capacity. It is stressed that only by trial and error can the proportions and quantities involved be determined. As a rough guide, too slow a rate of air flow and/or too large a temperature difference results in a significantly wetter region at the bottom of the beaker. Too fast a rate of air flow blows the flne particles of soil out of the beaker. Too slow a rate of air flow and too small a temperature difference leads to too much time being required to achieve the transfer of the desired amount of water. ACKNOWLEDGMENT I am grateful to Humphreys and Roberts (1965) for their criticisms of earlier papers which stimulated work on the development of this technique.

5 244 F. H. WHITEHEAD AND J. S. R. HOOD REFERENCES Asi'iNALL, D., NiCHOLLS, P. B. & MAY, L. H. (1964). The effects of soil moisture stress on the growth of barley. I. Vegetative development and grain yield. Aust.J. agric. Res., 15, 729. HUMPHRIES, A. W. & ROBERTS, F. J. (1965). The effect of wind on plant growth and soil moisture relations; a re-assessment. Neiu PhytoL, 64, 315. SHANTZ, H. L. (1925). Soil moisture in relation to the growth of crop plants. Agron. J., 17, 705. WHITEHEAD, F. H. {1962). Experimental studies of the effect of wind on plant growth and anatomy. II. Helianthus annuus. Neii; PhytoL, 6l, 59. WHITEHEAD, F. H. (1963). Experimental studies of the effect of wind on plant growth and anatomy. III. Soil moisture relations. Neto PhytoL, 62, 80.

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