Soil Characteristics and Root Development in Young Oil Palm (Elaeis guineensis Jacq.) Planted in Sites Affected by Bud Rots (Pudrición del cogollo)

Hector Albertazzi1, Carlos Chinchilla, Carlos Ramírez

ASD Oil Palm Papers, N°33, 1-32. 2009

Abstract

Some physical characteristics of soils were associated with root development in young oil palms and the presence of bud rots or 'pudrición del cogollo'. The study was conducted in two sites in a commercial oil palm plantation located on the central Pacific coast of Costa Rica. The previous planting had been seriously affected by the disorder locally known as 'flecha seca' (dry spear), which is similar in nature and symptoms to bud rot or 'pudrición del cogollo' as known in other countries of tropical America.

Treatments were arranged as a split plot design, where large plots had two types of soils (differing in chemical and physical properties) and small plots differed by the type of tillage done (with or without subsoiling)

Root development was followed by using the profile wall method, excavating a trench facing the plants. The type of root and its spatial arrangement were drawn on a matrix. Data were complemented by extracting soil samples to determine root dry weight and appearance (health)

The period following planting was characterized by the formation of mainly primary and secondary roots, which may be an indication that the main impulse of the plant is to get a good anchorage in the soil and send some deep roots to prepare itself for any dry period to come.

The development of fine roots (tertiary and quaternary) during the first months after planting was scarce, however, after a year all plants had developed an important root system comprised of both large and fine roots.

Root development occurred in patches, possibly due to a heterogeneous condition of the soil, both in terms of physical and chemical composition. When palms were two years old, most of their fine root systems were located within the first 20 cm of the soil profile. Most of the total root system (large and fine roots) was found within the first 20-40 cm.

The presence of a high and fluctuating water table was associated with a lower density of roots of all categories along the soil profile. A higher resistance to penetration (compaction) in the plot without subsoiling, was also an important impediment for root proliferation. The presence of soil layers with a resistance to penetration larger than 1.2 MPa resulted negative for root development.

A lower fertility, particularly low potassium and a high Mg/K ration in some soil pockets was also associated with less root proliferation and eventually with a higher incidence of the bud rot condition.

The use of 30 plants per treatment was considered enough to detect significant differences (5%) for variable root density when samples were taken with a 'Dutch' auger, but no less than 12 plants should be used in any study of the root system with any method, considering the large variation found between plants.

The use of the profile wall method to monitor root development in the soil profile is an effective and reliable methodology for studying the dynamics of oil palm root system.

Keywords: oil palm, root system, soil physics, profile wall, water table, resistance to penetration, pudrición del cogollo.

Introduction

Most studies on oil palm roots focus on their general spatial distribution, and some of them have shown the positive effects on root health of some tillage practices, the irrigation and a balanced nutrition (Alvarado and Sterling 1993, Forde 1972, Tinker 1976, Tan 1979, Agamuthu and Broughton 1986, Jourdan and Rey 1997). The positive effect of these practices was accentuated if realized during the first five years after field planting.

Root dynamics in time and space depend on the interaction of climate, agronomic management and the quality of the planting site. Site quality is the final result of the interaction between physical, chemical and biological soil characteristics.

The root system of the oil palm is formed of primary (I), secondary (II), tertiary (III) and quaternary (IV) roots. The function of the first two types is basically anchorage and transport of water and solutes. Basically, fine roots and also the growing points of roots I and II are responsible for nutrient uptake.

Fine roots (III and IV) reach no more than a few centimeters in length and can be considered the colonizers of a substrate, but larger roots (I and II) are pioneers, since they precede the formation of absorbing roots and reach greater lengths (Jourdan and Rey 1997, Ruer 1967). Soil conditions in a particular site will modify these characteristics of the roots (pioneering and colonizing capacity).

The condition known as 'flecha seca' (similar in characteristic to what is known as 'pudri- ción del cogollo' or PC in South America) was prevalent in oil palm plantations located on the central Pacific region of Costa Rica. The disorder was non-lethal and most plants recovered after several months or years of showing symptoms (Chinchilla and Durán 1999).

PC can be the final product of accumulated stresses on the plants that bring out some physiological responses that in the presence of a triggering phenomenon (such as prolonged drought or soil water saturation, usually associated with a previous heavy loads of bunches) may take the plant to a point where it can succumb to the attack of secondary opportunistic pathogens. This and other similar disorders have been associated with a clear deterioration of the root system (Alvarado et al. 1997, Chinchilla and Durán 1998, Chinchilla and Escobar 2004). In Colombia, for example, the PC incidence was higher in compacted soils (Acosta and Munevar 2002).

There are few studies on the development of the oil palm root system in soils affected by PC-related disorders. A particular study focused on root architecture and spatial distribution of roots and the relationship between aerial symptoms and root health (Albertazzi et al. 2005). The objective of this work was to document the dynamics of the root system of young oil palms planted in an area where the previous plantation had been severely affected by a PC-related disorder.

Materials and Methods

The study was done in the Quepos area (central Pacific coast of Costa Rica) during the period from May 1998 to June 2000. The area presents a distinctive dry season from January to April and there is an increase in rain intensity during the months of September and November ( Fig. 1). Soils are of alluvial origin and topography ranges from flat to gently sloping, and the soils are classified as Inceptisols.

A split plot design was used where larger plots corresponded to two soil types that differed in some chemical and physical characteristics, and small plots with or without tillage (subsoiling). Plants were of the Deli x Kigoma / Ekona (DxK/E) variety that were planted late April and early May 1998.

Soils from site two had been classified by Nuñez (1996) as Typic Eutropepts, and the site had a fluctuating water table affected by tides where the drainage work done did not completely solve this problem. This site also had a lower potassium content than site one (classified as Fluvaquentic Eutropet). Site one presented better soil aeration (better textures and no tide effect). Both sites are located at 8 masl. The origin of these soils is from alluvial sediments with an ocric epipedon and a cambric endopedon (Nuñez, 1996). Chemical and physical characteristics of these soils appear in Table 1. In both places, PC had affected more than 60% of the plants, so there was an effort to correct the drainage deficiencies found be-fore replanting.

Root development was followed in two ways: with a soil auger (Eijkelkamp, http://www.eijkelkamp.com) and by the profile wall method as described by Bhöm (1979). Soil samples with the auger were taken at a depth of 0-20 cm at one meter distance from the stems of 30 plants (Rapidel, 1988). The volume of soil with roots (750 cc) was put in a bucket with water for five minutes and then the content was flushed through a sieve with water under pressure. The recovered roots were classified into two groups according to size: large (primary and secondary roots) and fine (tertiary and quaternary). Each individual root within each group was evaluated according to its deterioration from a healthy appearance following a visual scale:

1. up to 25% root deterioration
2. between 25% and 50%
3. between 50% and 75%
4. up to 100% (Annex 1)

The type of deterioration observed on roots was called 'corky root' due to its appearance. The percentage of deterioration was then calculated:

where the observed value varied between 0 and 4.

Finally, the roots were placed into an oven (70°C during three days) to obtain their dry weights. Sampling was repeated four times at six-month intervals, starting one month after planting.

Root dynamics in situ were followed with the profile wall method described by Bhöm (1979). Trenches (one meter deep, one meter wide frontal to the plant, and 1.20 m. long, with steps for access, Fig. 2) were excavated next to the stem of each plant.

After taking the first data on roots exposed (20 cm along the soil profile) the trenches were prepared to reduce disturbances on root development for the information to be taken in the future. A one-square meter glass was adjusted to the wall facing the plant, which was first covered with a sheet of black polyethylene plastic (to prevent light entrance), and later with a white piece to reduce overheating of the soil profile. Finally, the trench was covered with a roof built with propylene sacs to prevent water accumulation and avoid direct light heating the surface where data was going to be taken.

The site was visited every six months starting six months after planting. During each visit (four in total), the plastics were put away and a transparent polycarbonate sheet was placed over the glass. A one-square meter matrix was drawn on the sheet with lines every five centimeters to form 25 cm² squares. Data were taken on this matrix: number and type of roots (I, II, III, IV) and distribution in space.

A handheld analogical penetrometer (Eijkelkamp, cone #2 of 15.96 mm diameter with an angle of 60 degrees) was used to determine resistance to penetration in the soil profile close to the plant during the second rainy season after planting. Data were taken after two days from a 50 mm rain. Three sets of data were taken from each point (every five centimeters down to one meter deep, on the side opposite the trench). All measures were taken the same day.

Results and discussion

Root evaluation using the Eijkelkamp auger

Root density. There was a significant difference for the interaction between sites and soil tillage (P=0.002, DMS 5%): root density was the poorest in site two (poor drainage) with no subsoiling ( Fig. 3). Nevertheless, the development of tertiary and quaternary roots was relatively poor during the first month after planting in all treatments: total root density was 0.39 grams per liter, where 36% corresponded to fine roots

Approximately six months after planting, root density had increased 4.3 times in the site with better aeration. Total root density at that moment was 1.69 g/l, where fine roots accounted for 49%. This behavior supports the practice of applying the fertilizer close to the base of the plant during the first months after planting. After approximately six months fertilizers could be applied farther from the stem.

Root health. There were no statistical differences that could be attributed to tillage (subsoiling), but to soil type. There were also no differences due to interaction between sites and tillage. A higher percentage of root deterioration was found at site one (38% root deterioration), but from an agronomical point of view these differences were probably not very important since 33% of the radical system was also deteriorated in the other site, and this was considered too high to guarantee a good performance of plants at either site.

The progressive deterioration of the root system was evident even before PC-like symptoms appeared in some palms at both sites; which happened be-fore the evaluation carried out one year and a half from the first evaluation ( Fig. 4). A similar behavior had been observed in other experiments where root deterioration (initially in quantity and later in quality) preceded the presence of PC-like symptoms (Albertazzi et al. 2005). See Annex 1

A low incidence of PC in site one (< 5%) was associated with an apparent better ability by the palms to maintain a higher density of fine roots (compensating in part for those that were deteriorated). This response was not observed in plots that were not tillage and sites with poorer drainage, where PC eventually reached a higher incidence.

Root dynamics: profile wall method

It was apparent that soon after planting, the young palms favored the growth of larger roots (primary and secondary types), which help the plant to anchor in the soil and explore deeper layers of soil in search of water ( Fig. 5, Fig.6, Fig.7 and Fig.8 ).

When plants reached approximately two years of age, most large roots were concentrated within the first 20-40 cm of the soil profile, and the fine system (tertiary and quaternary root types) was concentrated within the first 20 cm. This type of behavior seems to be normal for the oil palm and also other palm species (Barrios 1998, Armas et al. 2005). For adult oil palms, Cristancho et al. 2007 found most roots within the first 30 cm and concentrated within a radius of 4.5 meters from the stem.

The evaluation done six months after planting coincided with the first rainy season ( Fig. 5, Fig.6, Fig.7 and Fig.8 ), but the development of new roots was still poor. Most of the large roots were concentrated within the first 20-30 cm, and fine roots tended to proliferate near the surface, particularly in the poorly aerated site. The number of fine roots located deeper in the soil profile increased during the second evaluation done once the dry season had already become established ( Fig. 5, Fig.6, Fig.7 and Fig.8 ).

The presence of many fine roots growing superficially was the most notorious effect observed during the third evaluation done 18 months after planting (second rainy season). Most primary and secondary roots were found between 20-50 cm, and the numbers were larger in the better aerated sites. During the last evaluation done (two years after planting) the negative effect of the dry season was noted, particularly in the soil with more problems with aeration, where roots of all orders decreased ( Fig. 7 and Fig.8).

It was evident from the beginning that root growth was patchy, probably following sites with better chemical and physical conditions, but root distribution changed in time and space ( Fig. 5, Fig.6, Fig.7 and Fig.8 ). This type of behavior or plasticity of the root system has been documented in many crops (Arnone 1997, Van Noorwijk et al. 1996, Grime et al. 1993).

Most roots (80-90%) were concentrated within the first 50 cm of the soil profile ( Fig. 9), and a similar behavior had been documented by Barrios (1998) in Venezuela. Considering this fact, a statistical analysis was done on root density at two depths: 0-50 cm and 55-100 cm.

More large roots were found along the soil profile in site one (better aerated but without subsoiling). The opposite was observed in site two without subsoiling ( Fig. 10), P<0.05%, MSD 5%).

Fine root density varied between sites (P<0.05%, MSD 5%) at both depths, where the better aerated soil was superior. Root density was 4.6 times lower in the second layer of the soil profile (> 55 cm) in the better aerated site, and 11 times lower in the soil with more drainage problems ( Fig. 11), and as a consequence, fine roots were essentially confined within the first 50 cm in the soil with poorer aeration that had no tillage.

Resistance to penetration and root development

A handheld penetrometer can be used to detect a compacted layer along a soil profile (De León et al. 1998, Rooney and Lowe 2000, Duiker 2002). The information obtained can be used to diagnose compaction problems and to select alternative methods to alleviate the problem.

Tillage (subsoiling) was associated with a decrease in compaction levels, (P=0.004, MSD 5%) ( Fig. 12), but there was no significant interaction between sites and tillage treatments.

Root development was negatively affected when resistance to penetration was higher than 1.20 MPa: the probability of finding any roots where resistance was higher than this value was less than 5%, and nil with values higher than 1.60 MPa. According to this, any site with values near 1.2 MPa along the soil profile could have conditions favorable to PC, since such values are limiting to root development (Coder 2000) and eventually health. Similar results were documented by Acosta and Munevar (2002), where compacted soils (>1.07 MPa) were associated with higher incidences of PC.

Root growth and PC incidence

The root system grew actively up to the third sampling date. However, in plants showing PC-like symptoms during the fourth sampling date, growth was more vigorous. A contrasting behavior was observed in plants that remained healthy, which maintained moderate growth of the root system throughout the duration of the experiment ( Fig. 13).

PC incidence and aerial growth

Palms affected by PC (typical symptoms on the crown) appeared approximately one year after planting in both sites, but incidence was lower in the area with better drainage conditions where incidence only reached 0.05% when plants were two years old. On the contrary, at site two with more drainage problems, incidence reached 58% at that time. On site one, most plants entered a recovery phase soon after symptoms appeared, but on site two this process took much longer.

On site one, the water table remained below one meter deep most of the time, but in the other site, a combination of high precipitation and the tide effect caused the water table to remain near 50 cm for prolonged periods of time. The effect was so evident it was necessary to scoop water out of the trenches before data could be taken. The negative effects of a high water table on root development have been well documented (Purvis 1956, Hartley 1977).

In addition to the effect of poor drainage, the low fertility of site two could also negatively affect root development. Potassium was particularly low where PC incidence was higher (0.11 vs. 0.17 (cmol(+)/l). Nevertheless, potassium was considered low in both sites, since the reference level for the plantation was 0.2 cmol(+)/l). K saturation was 0.44% in the site with poor drainage and 0.67% in the other site (reference level was near 2.5%). Another factor that accentuated the effect of low K levels was the high concentration of magnesium in the soil, which caused very high Mg/K ratios (77 vs. 27 in the site with lower PC incidence). The reference level was 2.5-15.

Aerial growth of plants showed differences between sites and tillage, but no interaction of these variables. Growth was better where soil physical conditions and fertility were better. At the moment data were taken, bunch load was higher in the better site ( Table 2).

Conclusions

The presence of a high and fluctuating water table was associated with reduced root density (both large and fine roots) throughout the soil profile in young oil palms. Soil compaction and low fertility (particularly low K and a high Mg/K ratio) were also shown to be associated with less root proliferation in the soil profile.

A less developed root system corresponded with reduced aerial growth and less bunch load. It was apparent that under these circumstances (causing impediments to root development) the plant made an attempt to produce new roots to compensate those that had died prematurely; however this was not done without a waste of energy. Repeating this cycle may cause a progressive deterioration of the whole plant and the possible appearance of disorders such as spear rots or PC. Incidence and severity of PC seem to depend on the degree to which the root system has been suffering progressive deterioration.

It is advisable to sample at least 30 plants with the Eijkelkamp soil auger to declare statistical differences (5%) in root density between treatments, since there is a large variation between plants. On the other hand, the use of the profile wall method is a good alternative for studying root dynamics in time according to any treatment applied, but time between samplings cannot be too long since root distribution and quantity varies greatly in time.

It was shown that the root system grew in patches (particularly fine roots), apparently looking for sites with better physical and chemical conditions. Soil compaction, for example, clearly affected root distribution along the soil profile, where there was no root growth where resistance to penetration was higher than 1.2 Mpa.

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