The Effects Of Soil Moisture Content On The Growth And Distribution Of Sorghum Root Systems

Project and Seminar Material for Agricultural Engineering AE

The Effects Of Soil Moisture Content On The Growth And Distribution Of Sorghum Root Systems


Abstract


Irrigation level is believed to influence root distribution along the soil profile. There is no known reason for the phenomena. The aim of this study was to see how water content at the soil surface and around the root crown of sorghum (Sorghum bicolor L. Moench) affects root distribution with depth. The work was done in an experiment where an array of soil surface and deeper soil layer water contents were established under the plant, and their effects on root and plant growth measured. Variable soil water profiles were achieved by growing plants in soil filled vertical PVC tubes of varied diameters, lengths and distances between the soil surface and a water table. It was found that when the top soil (top 3 cm) was wet (at least above 70% of field capacity) crown roots initiation and establishment proceeded at potential rates, resulting in a large number (14) of crown roots which grew to a depth of 30–40 cm, at 24 days after emergence. Irrespective of deeper soil layer water contents, when the soil surface water content was low, crown root numbers were markedly reduced (from 14 to 6) and existing crown roots increased in length (from 38 to 87 cm), at 24 days after emergence. Total crown roots’ length per plant was approximately the same, irrespective of the soil moisture regime. When soil surface moisture was low, plants had several thick distorted crown root initials that apparently did not penetrate the hard dry soil surface. It is concluded that by limiting crown root number per plant, a dry soil surface causes compensatory growth in existing roots, which subsequently reach deeper soil layers. Thus, the soil surface moisture content, as affected by irrigation (or rainfall) frequency, exerts control over sorghum root distribution along the soil profile.


Chapter One


1.0 Introduction

Sorghum (Sorghum bicolor L. Moench) root distribution along the soil profile has been shown to vary with soil water regimes (Nakayama and VanBavel, 1963; Plaut et al., 1969). Root distribution along the profile is a major factor affecting soil water extraction and water use (e.g., Gardner, 1964; Stone et al., 1973). Sorghum irrigated at high frequency develops a typically skewed (Merrill and Rawlings, 1979) and nearly a logarithmic (Gardner, 1964} root-distribution profile. Under conditions of less favorable water supply, root distribution is less skewed and a relatively larger proportion of the root mass is concentrated in deeper soil layers. No well-documented explanation for the difference exists. One common concept is that more roots are found at deeper soil layers in a drying soil because roots grow where water is available.

Sorghum roots, as in all Gramineae, consist of two systems the seminal and the crown roots. The growth potential and the viability of the seminal root of sorghum are limited (Blum et al., 1977) and a major part of the root volume consists of the crown roots. The crown roots are initiated from crown-node buds, beginning about a week after emergence. In aerated hydroponics, where the potential root development is expressed (Blum et al., 1977), crown roots are initiated in distinct, consecutive cycles. Rooting cycle amplitude is approximately 7–10 days and two to four roots are formed at each cycle.

Since crown-root initiation and initial growth occur at the soil surface, it was hypothesized that the water status of the soil surface affects their initiation or penetration into soil. If fewer crown roots are established due to soil surface desiccation, root distribution along the soft profile could be affected irrespective of the water status at deeper soil layers. The effect on root distribution would possibly be mediated by compensatory growth (Jordan et al., 1979) and extension of existing roots in the soft. This study was performed
in order to evaluate this hypothesis.


Background Of Study

Among the cereals and legumes, sorghum (Sorghum bicolor [L.] Moench) and soybean (Glycine max [L.] Merrill) seem to be very promising for many of the smallholder farmers in West Africa. Sorghum is the most important food crop in the semi-arid tropics (FAO, 1995a), where it constitutes the main grain food for over 750 million people in the region (Food Security Department, 2003). On the other hand, soybean is a major source of high quality protein and vegetable oil. The crop offers a variety of wholesome products that are affordable to many, and so, has great potential to reduce undernourishment. Although the bulk of sorghum and soybean is grown in the savanna region of Nigeria, there is growing evidence of impending less annual rainfall in the region which, coupled with temperature increases, would reduce soil moisture availability (Adejuwon, 2004). In the last few decades, there has been a steady decline in productivity of sorghum in the region, due mainly to the occurrence of drought (Chiroma et al., 2006). As for soybean, its cultivation is on the increase when compared to other crops of vegetable origin, as the crop has been introduced to some other parts of Nigeria owing to its economic importance (Lasisi and Aluko, 2009).
Both sorghum and soybean have been found to be successfully grown in Nigeria Agroecological Zone. Sorghum is such a resource-friendly crop that it could thrive even on marginal soils, whereas soybean has the inherent ability to nodulate freely; the two attributes of which are desirable under the low-input agriculture common in the zone. Most of the resource-poor farmers in the zone could take advantage of the comparatively low input requirement in the cultivation of these crops to maximize returns. However, even though the southern part of Nigeria is more humid than the northern part, management-responsive water deficits often occur in the area (Aina, 1993), due mainly to the erratic distribution of rainfall that contributes to suboptimal growth and reduced yield of key crops (Chukwu, 1999).

Likewise, Babalola and Opara-Nadi (1993) attributed the incidence of drought in the West Africa savanna to the erratic nature of both the onset and cessation of rains. Rainfall in this region is also bimodally distributed, a phenomenon that contributes substantially to poor crop performance in the region (Odurukwe et al., 1995). There are, therefore, indications that the expected decrease in rainfall in the savanna region of Nigeria would most unlikely elude the Agroecological Zone (Igwe, 2004), which is in the derived savanna zone. In spite of the prevailing sub-humid climate, the zone is often characterized by high evaporative demand of the atmosphere. With the increasing competition for water from many sectors and the global scarcity of water resources, irrigation seems a less realistic and unsustainable resort.

In soils, the proportion of water retained in crops’ root zone could even be more crucial to increased productivity than the total rainfall (Payne et al., 1990). In that regard, water availability to crop plants in Nsukka area is further limited by the edaphological factor of weakness in structure of the soils. Poor water retention in these soils under field conditions stems more from the ensuing adverse pore size distribution, which results in high infiltration rates and conductivity. This structural constraint applies especially to all the fragile Ultisols of southeastern Nigeria (Mbagwu, 1987; 1990). The implication of these limitations is that future agriculture in this zone may be water-constrained, if no means are devised for coping with the situation. The soil resources of West Africa have great potentials for high productivity, but for the glaring soil and water management failures (Babalola and OparaNadi, 1993; FAO, 1995a). According to Lal (1997a) one of the key conditions for improving soil productivity in the sub-Saharan zone is to ensure effective infiltration and storage of water in the soil. Appropriate soil and water conservation practices that encourage soil moisture retention are, therefore, needed for maximizing rainwater resource and achieving the optimum yields of sorghum and soybean in Nsukka zone. Such measures as no-till, mulch application, and mixed cropping enhance moisture storage by providing groundcover or live vegetation on the soil (Obi and Nnabude, 1990), and appear more practicable.

Tillage systems modify soil structure, temperature, and water distribution; and, hence, they influence root distribution (Waddell and Weil, 1996) and ultimately crop yield.

However, they are characteristically inconsistent in their agronomic effects. As a result, the existing relationships among tillage systems and crop yields are neither strong (Hatfield et al., 2001) nor fully defined and understood (Agriculture and Food, 2004). Whereas the conventional tillage (CT) conserves soil moisture through some known mechanisms (Hillel, 1982; Smith 1993; Agele et al., 2000); the increasingly popular no-till (NT) system may, by
virtue of promoting organic matter build-up, likely improve soil aggregation and thus water infiltration and storage in the soil (Appropriate Technology Transfer for Rural Areas [ATTRA], 1999). Conversely, the effect of surface mulch is almost always predictable. It normally has positive effect on infiltration (Adekalu et al., 2007), soil hydrothermal regime and fertility, and crop yield (Thiagalingam et al., 1996). Combination of CT or NT with mulch modifies the soil surface and may have much greater impact on moisture status. Such a modification changes the soil water balance in terms of profile storage, drainage, and evaporation; and so would ultimately affect how efficiently crops use rainwater input (Hatfield et al., 2001). Generally, tillage systems and mulch practices employ modification in soil physical condition and/or formation of a physical barrier in conserving soil moisture.


1.2 Objective of Study

The purpose of this study is to examine the effects of soil moisture content on the growth and distribution of sorghum root systems.


1.3 Justification of Study

This study is significant as it compares facts and reveals the essence of soil moisture content on the growth, effective development and distribution of sorghum.

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