Ms. 2283. THE GROWTH OF SOYBEAN (GLYCINE MAX). ROOTS IN ... Mississippi Agricultural and Forestry .... Soil moisture was maintained at one third bar.
Ms. 2283
P l a n t and Soil 41, 81-89 (1974)
THE
GROWTH
OF SOYBEAN
(GLYCINE MAX)
ROOTS IN RELATION TO SOIL MICROMORPHOLOGY b y V. E. NASH and V. C. B A L I G A R Mississippi Agricultural and Forestry Experiment Station
SUMMARY Greenhouse studies were c o n d u c t e d in which t h e pore size and g e o m e t r y of t h e g r o w t h m e d i a were varied s y s t e m a t i c a l l y b y v a r y i n g ped size or t h e a m o u n t of sand, silt, or clay. This s t u d y showed t h a t one can relate t h e • m i c r o m o r p h o l o g i c a l properties of soils to t h e i r physical properties and to root growth. This p r e l i m i n a r y w o r k also suggests t h a t it is feasible to use this greenhouse t e c h n i q u e to s t u d y t h e root p e n e t r a t i o n of natural soil layers under controlled greenhouse conditions. INTRODUCTION
The health and vigor of the entire plant are conditioned by the distribution and activity of the roots. In order to study the growth and development of the root system in soil it is essential to give consideration to the characterization of the pore system. For this purpose, soil porosity values are somewhat less useful than the pore size distribution which express the effective size distribution of the void space. The rigidity of the pore structure as well as pore size distribution influence root penetration. W i e r s u m 5 reported that in a rigid system a root is only able to penetrate a pore which has a diameter exceeding that of the young root. He also concluded that rigidity of pore structure is an important factor in altering root growth. M i l l e r and M a z u r a k 4 reported on the relationship between particle and pore sizes for the growth of the sunflowers. They observed that roots were able to penetrate pores smaller than the
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V. E. NASH AND V. C. BALIGAR
smallest root diameter by enlarging some of the pores as they forced their passage through the soil. A u b e r t i n and K a r d o s 2 studied the effect of rigid and non-rigid glass bead systems in which pore size ranged from 46 ~zm to 412 ~m in diameter. Maize roots did not grow into rigid porous systems which had pore diameters smaller than approximately 138 ~m, whereas in non-rigid systems, roots could grow equally in all pore sizes studied. It can be concluded from the work of W i e r s u m 5 and A u b e r t i n and K a r d o s 2 that in most soils roots penetrate partly by growing through existing voids and partly by moving aside soil particles. In this study we measured the effect of varying the pore size in a systematic manner by changing (1) the ped size or (2) the relative amount of sand, silt, and clay on the root growth of soybeans. In the latter case a layer of varying characteristics was intercalated between layers of sandy loam soil. Micromorphological features of these central layers were then related to root penetration. MATERIALS AND METHODS
Preparation o] soil aggregates (peds) Naturally occurring peds were separated from a Houston clay soil by dry sieving for use as a growth medium. These peds varied in shape from granular to sub-angular blocky. They were hard when dry and were resistant to dispersion by wetting or mechanical manipulation. Tile Houston clay is composed of 51.0 per cent clay, 44.8 per cent silt an d '4 .2 per cent sand. Montmorillonite is tile dominant clay mineral although appreciable amounts of mica and kaolinite are present. The size range of tile peds used were: 4-6.36 mm, 2-4 ram, 1-2 ram, 0.5-1 ram, and < 0.5 turn. Tile physical properties of three ped sizes are shown ill Table 1.
Preparation o/sand, silt and clay mixtures Kaolinite (Hydrite t~) was used as the clay source in these mixtures to minimize the problems of volume change wittl wetting and drying. This has a median size of 0.77 ~m. The silt fraction (2-50/,m) was separated by sieving and sedimentation from a Longview silt loam soil. Commercial Ottawa sand was separated into the 100-250 ~m range and used in t h e preparation of mixtures for tile central-resistant layer. The various ratios of sand, silt, and clay were mixed in a twin shell dry blender for 5 hours to obtain uniform mixing. In Table 2 ratios of sand, silt, and clay are shown as well as other pertinent physical data.
SOIL MICROMORPHOLOGICAL EFFECT ON ROOT GROWTH
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Preparations o] the plant growth pots Soybean plants were grown in quart size paper cylinders, 15 cm height and 8.5 cm diameter and coated on the inside with 2 per cent lucite dissolved in acetone solution. Four cm of sandy loam soil were placed in the cylinder followed by a 3-cm layer of variable composition. This layer is referred to as the central layer and plant growth is related to its characteristics. This layer was sealed to the cylinder with paraffin. Six cm of sandy loam and 1 cm of sawdust was placed above the layer. Phosphorus and potassium were applied at the rate of 26 kg P and 42 kg K per hectare, Five seeds were planted in each cylinder. After germination these plants were thinned to 2 plants per pot. Soil moisture was maintained at one third bar tension throughout the growth period by adding water to the desired weight every 48 hours. The treatments were arranged in a randomized block design with three replications. After 21 days the roots were separated on screens by washing out the soil with a gentle spray of water. The cylinders were cut at the desired depth in order to separate the root growth above, below and within the central layer. The central layer of one cylinder from each t r e a t m e n t was embedded in Castolite (acrylic polyester resin). The sample was impregnated under partial vacuum and allowed to harden overnight. Thin-sections were prepared from these Castolite impregnated blocks by standard methods.
RESULTS AND DISCUSSION
Ped-size effect on root growth The initial experiment was carried out with natural peds of various size in an attempt to change the pore size while maintaining a constant chemical and mineralogical substrate. Surface soil from a Houston clay was used for this, since peds of different size had similar composition. Previous experience had also shown that these peds were also quite stable in water and during slight mechanical manipulation. The period required for seed germination was related to ped size as shown in Table 1. Emergence occurred 4 days earlier in pots containing ped sizes 2-4 mm and 1-2 mm than for the smaller or larger peds. In the coarse peds (4.0-6.3 ram) the delay in germination was likely due to poor water relations. Water movement to the seed is slow because of the limited number of contact points between peds as shown by microscopic examination. For the smallest peds we suspect poor aeration to have limited germination. It is more difficult to explain the results of the 0.5-1 mm size, since measurement on capillary and non-capillary porosity would indicate a satisfac-
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V. E. NASH AND V. C. BALIGAR
tory medium. Also, microscopic examination showed pores of 2001500 ~m in the large dimensions. This should be sufficient for proper aeration as may be seen in Fig. 1. Soybean root growth data shown in Table 1 indicate that optimum growth occurred in pots with peds greater than 0.5 mm. Even plants in the pot with the largest peds grew well after overcoming TABLE 1 Some physical characteristics of soil aggregates in relation to growth and development of soybean seedlings Aggregate size mm
Bulk density g/co"
Pore space * cap- nonillary cap% illary
Hydraulic conductivity em]h
Moisture retention
Root growth
.04 bar %
1/3 bar %
15 bar %
40.5 41.3 39.1
30.3 29.7 29.1
19.7 19.9 20.0
Germination time verti- later- days eal al mm mm
%
