Two vegetable soybean (Glycine max (L.) Merrill) cultivars, AGS 353 and Lightning, were hand- planted on 22/11/2012 and 21/11/2013 on the Cedara Research Station, KwaZulu-Natal, South Africa (latitude 29°32'S; longitude 30°16'E; altitude 1051 m), at 266 667 seeds ha-1.The Hutton soil had means of 44.5% clay, 57.9 mg L-1 K and 8.2 mg L-1 P before five potassium (K) rates of 0, 40, 80, 120 and 160 kg ha-1 and three phosphorus (P) rates of 0, 30 and 60 kg ha-1 were applied as potassium chloride (50% K) and single superphosphate (10.5% P), respectively. The fertilizers were hand-broadcast and incorporated with a tractor-drawn offset-disc harrow and konskilde. The trial had a randomized complete block design with three replicates. The plots were split for cultivar. The split-plots consisted of four rows of 5 m length, with an inter-row spacing of 0.75 m. Plant population was significantly higher in the 2012/13 season for both cultivars, but had no significant effect on mean pod and bean dry matter (DM) yields. Lightning produced significantly taller plants, more pods plant-1,a higherpercentage of export marketable pods and total plant, pod and bean DM yields than AGS 353. P application rate had no significant effect on plant and bottom pod height. Plant height increased significantly as a result of 0 to 120 kg K ha-1, whilst bottom pod height decreased significantly, due to significantly higher percentages of seedless pods as K application rate decreased. The number of pods plant-1 was significantly higher at 60 kg P ha-1, but K application rate had no significant effect on the number of pods plant-1. Pod and bean DM yields increased significantly from 0 to 60 kg P ha-1. Plant, pod and bean DM yields increased significantly from 0 to 40 kg K ha-1. Significant interactions were measured for pod and bean DM yields between the seasons, cultivars and K applications.
In the 2012/13 season Lightning produced significantly lower pod and bean DM yields at 0 kg K ha-1, but no significant differences were measured in the 2013/14 season. In the 2013/14 season, AGS 353 produced a significantly lower pod DM yield at 0 kg K ha-1 than at 40 and 120 kg K ha-1, whilst a significantly lower bean DM yield was produced at 0 kg K ha-1 than from 40 to 120 kg K ha-1. In the 2012/13 season no significant differences in pod DM yield were measured for AGS 353 among the K application rates. However, at 0 kg K ha-1, bean DM yield was significantly lower than at 120 kg K ha-1.
Key words: Vegetable soybean, phosphorus, potassium, yield
150 5. 2 INTRODUCTION
Soybean tolerates a wide range of soil conditions, but yields best on well-drained, fertile lands (Birch et al., 1990). They have a considerable macronutrient requirement, which varies according to soil and climatic conditions, cultivar, yield level, cropping system and management practices. With vegetable soybean, yield, flavour and quality are influenced by cultivar selection and soil fertility(Konovsky et al., 1994).
Research conducted in KwaZulu-Natal in the mid-1980s indicated that grain soybean responds strongly to direct fertilization, especially where the soil levels of phosphorus (P) and potassium (K) were medium to low (Birch et al. 1990). Imas et al. (2007) found that a 3 t ha-1 soybean crop is able to extract 240 kg of nitrogen (N), 45 kg of P and 100 kg of K. The withdrawal of nutrients per ton of soybean seed was approximately 60 kg of N, 5 - 6 kg of P and 18 - 19 kg of K.
Approximately 70% of the N and P, and 55% of the K taken up by the plant is removed from the land in the seed (Birch et al., 1990). Soybean can produce optimally at lower soil potassium levels than maize, but will remove five times the amount of potassium per ton of seed than maize. Therefore, soil potassium levels need to be monitored closely when maize is grown after soybean to prevent a yield reduction (Liebenberg, 2012).
Soybean, if effectively inoculated with Rhizobium bacteria at planting and grown in soils with a satisfactory pH (6.0), can supply its own N requirements. If well nodulated, yields as high as 3 to 4 t ha-1 can be produced in South Africa. However, N fixation is inhibited by high levels of mineral N in the soil, by drought stress and by poor soil aeration. K is very important to N fixation, because it stimulates early root growth, thus ensuring early nodulation. In addition, K ensures that the roots have the necessary carbohydrates for optimum nodule functioning.
Studies have shown that nodule number and weight, and total N accumulation in the plant increased as the supply of K increased (Imas et al., 2007).
K not only improves yields and water use efficiency, but also benefits various quality aspects. Oil and protein content are improved and larger seeds are produced (Chauhan, 2007), which are essential factors in the production of quality edamame. Drought tolerance and the plants‟
resistance to pests and diseases are also improved (Imas et al., 2007; Tomar et al., 2007).
Isoflavones, which are associated with the prevention and treatment of cancer, diabetes, hypertension and heart disease, have been found to increase with increased levels of K fertilization in soybean (Rajcan et al., 2000; Chauhan, 2007).
151 The critical level of soil K for soybean has been found to be 80 mg L-1 (Birch et al., 1990).
However, Rhem et al. (2001) recommended that K applications were necessary at soil test levels of below 120 ppm. K is not immobilized in most South African soils and soybean is able to utilize K-reserves well. In KwaZulu-Natal soybean does not react to K fertilizer applications when the soil K-status is above 80 - 90 mg kg-1, but where a K-deficiency occurs, soybeans react well to K-fertilization (Farina, 1992; Liebenberg, 2012). Soils that have been analyzed to show medium to low levels of available K should receive 30 to 60 kg ha-1 of K, respectively (Birch et al, 1990). However, as soybean is generally used in a rotation with maize and removes fairly large quantities of K, the fertilizer recommendations provided by the Fertilizer Advisory Services (Fertrec) of the KwaZulu-Natal Department of Agriculture and Rural Development are based on a critical soil K level of 100 mg L-1 to ensure that the subsequent maize crop receives sufficient K. Farina (1992) reported that maize required a soil test of 110 mg L-1 to yield optimally.
K-deficiency symptoms are characterized by yellowing of the leaf margins. These generally appear between late flowering and early pod-fill (Liebenberg, 2012). As with maize, these deficiency symptoms first appear on the lower leaves. With maturity, the deficiency symptoms expand to leaves closer to the top of the canopy.
P is essential for soybean growth, pod development, yield and seed quality and can be absorbed until late in the pod-fill stage (Liebenberg, 2012). A lack of this element may prevent other nutrients from being absorbed by the plants (Sharma et al., 2011). In KwaZulu-Natal soybean is widely grown in soils where P is strongly adsorbed by the soil, making it unavailable to the crop (Birch et al., 1990 and Liebenberg, 2012). At soil P test levels below 10 ppm (Bray 1), P applications would be required to achieve the expected soybean yield (Rhem et al., 2001).
Barbagelata et al. (2002) reported significant responses in yield to P application when the soil P levels were below 9.5 ppm.
Waluyo et al. (2004) reported that P increased the number of soybean nodule primordia and therefore had an important role in the initiation of nodule formation. Kumaga et al. (2004) and Bekere and Hailemariam (2012) reported that the number of nodules per plant increased with the application of P. Zarrin et al. (2007) reported that the combination of Rhizobia and P increased nodulation and seed yield. Zheng et al. (2010) found that soybean yields improved with P application under drought stress conditions. Shahid et al. (2009) and Sharma et al. (2011) reported that plant height, the number of branches plant-1 and pods plant-1, and yield increased
152 as P applications increased. Birch et al. (1990), Mahamood et al. (2009) and Sharma et al.
(2011) reported that grain soybean cultivars responded differently to inadequate soil P levels.
P deficiency is characterized by paler and smaller leaves, shorter plants and premature defoliation of the lower leaves (Liebenberg, 2012). Oil and protein content may also be affected (Yu et al, 2008; Win et al., 2010; Liebenberg, 2012), although Nedić (2005) found no effect.