Ezenwa, I. V. and J. Cobbina. International Livestock Centre for Africa, c /o 1.1. T. A., P.M.B. 5320, Ibadan, Nigeria.

RELATIVE IMPORTANCE OF DIFFERENT NUTRIENT ELEMENTS AND RHIZOBIUM INOCULATION FOR EARLY GROWTH OF LEUCAENA

Introduction. One of the major factors limiting the usefulness of leucaena (Leucaena leucocephala (Lam.) de Wit. as a forage species is its slow growth rate during establishment (Atta-Krah and Francis 1987). This has been attributed to deficiency of essential nutrients in the soil (Cobbina et al. 1989), delay in nodulation (Jones and Bray 1983, Ezenwa and Atta-Krah1990) and lack of appropriate Rhizobium strains in the soil. A pot trial was carried out using soil collected from an area in southwest Nigeria where poor initial growth of leucaena {Leucaena leucocephala cv. Cunningham) was observed to identify the most limiting nutrient elements and the effect of Rhizobium inoculation on early root and shoot growth of leucaena.

Materials and methods. Four leucaena seeds, scarified by nicking the broad ends of each seed, were sown into each pot. Pots contained four kilogram of sieved surface soil (0 -15 cm depth) collected from an area in the derived savanna zone where poor initial growth of leucaena was observed. The soil is a sandy loam with the following chemical properties: pH, 5.6 (1:1 Soil:H20 ; Organic C, 0.80%; total N, 0.07%; Bray No. 1 available P, 3.70 ppm; exchangeable cations: Ca, 2.80 me/100 g; Mg, 0.97 me/100 g; K, 0.10 me/100 g; and Mn, 0.09 ppm. The treatments consisted of two levels of Rhizobium treatments, with and without Rhizobium, as main plots and nine levels of nutrient treatments as subplots in a split plot design with four replications. In each of the subplot treatments one specific nutrient at a time was subtracted from a mixture that made up the treatment of all nutrients (N, P, K, Ca, Mg, S, and micronutrients). One day before planting the seeds, the nutrients were added to the soil in the form of solutions at the rates given in Table 1.

Table 1. Rates of application of the nutrients and the compounds used.

TreatmentRate of nutrient applied (ppm) Chemical compound1

1 All nutrients All compounds below2 Nitrogen 50 n h 4n o 33 Phosphorus 50 NaH2P 0 4 • H204 Potassium 50 KC15 Calcium 5 CaC036 Magnesium 15 MgCl2 • 6H207 Sulphur 15 Na2S 0 48 Micronutrients:

(a) Cu 2 CuCl2 • 2H20(b) n 2 Zn powder(c) B 2 Na2B40 7 • 10H2O(d) Mo 2 Na2M o04 • 2H20(e) Co 2 CoC12 • 6H20

1 All chemical salts used were of "Analar" grade laboratory compounds.

The seedlings were thinned to one per pot three weeks after sowing. The seeds for the inoculated treatments were treated with a single strain of Rhizobium, Lx 206, in commercial grade granular peat (LiphaTech, Milwaukee, WI, USA). The pots were watered daily with deionized water in the greenhouse.

Plant height was recorded at twelve weeks after planting, when the plants were harvested by cutting at soil surface level. Roots were washed with water, and the nodules were carefully removed. The shoots and roots were dried to a constant weight at 70° C, then weighed to determine dry weight.

Results and discussion. Data on all parameters measured are provided in Table 2. Averaged over nutrient treatments, there was no significant difference between inoculated and uninoculated plants for all the parameters measured. Height, root, and shoot dry weights of inoculated and uninoculated plants without applied phosphorus or sulphur were significantly lower than those of the other treatments, but not significantly different from the control. The soil phosphorus content of 3.7 ppm is low compared to the critical level of 15 ppm (Dr. G. Adeoye, pers. comm.). Low soil phosphorus level is a major nutritional constraint to the rapid growth of leucaena (Sanginga et al. 1988). Good responses to phosphorus application by leucaena have been reported elsewhere (Sanginga et al. 1988, Cobbina, 1991).

Table 2. Height, root, and shoot dry weights of plants after 12 weeks’ growth as influencedby Rhizobium inoculation and varying nutrient subtractions.

Shoot height Inoc. No

Shoot dry weight Inoc. No

Root dry weight Inoc. No

Nutrient treatment -cm- -cm- -cm- -cm- -cm- -cm-All nutrients 41 44 8.7 10.6 5.1 6.2- Nitrogen 47 47 8.8 8.7 5.6 5.5- Phosphorus 31 28 5.7 4.3 2.3 2.0- Potassium 50 44 11.0 8.9 5.1 4.8- Calcium 67 58 14.4 13.5 5.7 6.7- Magnesium 52 63 12.6 13.4 5.8 6.3- Sulphur 32 41 6.7 6.5 4.1 3.2- Micronutrient 68 52 14.6 13.3 5.0 5.6Control 30 36 5.2 5.5 2.9 2.8LSD (0.05) 12 2.9 2.0Inoc. = with Rhizobium inoculant; No = without.

Poor growth in the savanna soils of Nigeria was attributed to annual burning of the vegetation which leads to loss of sulphur from the soil (Kang et al. 1981). The absence of nodulation may be due, in part, to sulphur deficiency in the soil, which according to Norris and Date (1976) prevents the host plant from protein synthesis.

Height and shoot dry weight of plants without applied N or K were significantly higher than those of the control treatment for inoculated plants, but not significantly so for uninoculated plants. Root dry weight of plants where K was not applied was also significantly higher than those of the control treatment in inoculated plants but not in uninoculated ones. These results indicate that N and K were limiting for early shoot growth and K for root growth in uninoculated plants but not in inoculated plants. The soil contents of these nutrients (N=0.07% and K=0.10 me/100 g soil) are low when compared to what are considered the critical values of N (0.15%) and K (0.20 me/100 g soil) (Dr. G. Adeoye, pers. comm.).

The adverse effects of N and K deficiency were ameliorated by Rhizobium inoculation, confirming the need for application of N to uninoculated leucaena plants during early growth. Sanginga et al. (1988) and Cobbina (1991) reported significant response to applied N during early growth when the plant had not started nodulating and fixing nitrogen. Delay in nodulation of up to 84 days has been reported in leucaena (Ezenwa and Atta-Krah 1990). The data on nodulation in this'trial were highly variable (coefficient of variation = 89%; data not presented). This might be because nodulation in leucaena normally starts about 12 weeks after planting

(Ezenwa and Atta-Krah 1990), the time at which the plants in this trial were examined for

: j ' ..»*■ r-r---... • ••vW. v v / l •••><’'--i$-* ■ .#• j \ i . ‘..r-

atest shoot heights and dry weights were obtained in treatments without applied calcium, or micronutrients. This agrees with results obtained by Sanzowicz and Couto

Adequate amounts of these nutrients were already present in the soil, and additional of these nutrients resulted in poor plant growth.

T .r ̂ X "!' " •? ' ~ "• ' ■' • "Conclusions. Phosphorus and sulphur were the most limiting elements for early growth of leucaena irrespective of Rhizobium inoculation. Nitrogen and potassium were only limiting in the absence of Rhizobium inoculation. Calcium, magnesium, and micronutrients were not limiting nutrients for shoot and root growth of leucaena. There was no direct benefit from Rhizobium inoculation. The beneficial effect of Rhizobium inoculation seemed to be related to an improvement in plant nitrogen and potassium nutrition in the absence of adequate amounts of the nutrients. The nutrients that will limit leucaena growth can be predicted from soil analysis.

References:

Atta-Krah, A.N. and P A . Francis. 1987. The role of on-farm trials in the evaluation of composite technologies: The case of alley farming in southern Nigeria. Agric. Systems 23(2): 133-152.

Cobbina, J. 1991. Nitrogen and phosphorus fertilization promotes rapid initial growth of leucaena on an Alfisol. Communications in Soil Science and Plant Analysis 22. (In press).

Cobbina, J., B.T. Kang and A.N. Atta-Krah. 1989. Effect of soil fertility on early growth of leucaena and gliricidia in alley farms. Agroforestry Systems 8:157-164.

Ezenwa, LV. and A.N. Atta-Krah. 1990. Initial growth and nodule development of leucaena and gliricidia. Leucaena Res. Reports 11:99-101.

Jones, R.J. and R A . Bray. 1983. Agronomic research in the development of leucaena as a pasture legume in Australia. In Proceedings of a Workshop on Ipil-Ipil Research in the Asian-Pacific Region, Los Banos, Philippines, p. 41-48.

Kangl B.T., E. Okoro, D. Acquaye and O A. Osiname. 1981. Sulphur status of some Nigerian soils from the savanna or forest zones. Soil Science 132:220-227.

Norris, D.O. and R A . Date. 1976. Legume bacteriology. In H.H. Shaw and W.W. Bryan (eds), Tropical Pasture Research Principles and Methods. Bull. 15., Commonwealth Bureau of Tropical Pasture and Field Crops, p. 134-174.

Sanginga, N., K. Mulongoy and A. Ayanaba. 1988. Response of Leucaena/Rhizobium symbiosis to mineral nutrients in Southwestern Nigeria. Plant and Soil 112:121-127.

Sanzowicz, C. and W. Couto. 1982. Effect of calcium, sulphur, and other nutrients on dry matter and nodulation of Leucaena leucocephala in a "cerrado" soil. Leucaena Res. Reports 3:4.