Soybean tolerates a wide range of soil conditions, but yield 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 demonstrated that grain soybean responded 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) stated 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). Tables 1.1 and 1.2 provide guideline
13 recommendations of K and P fertilization rates for various soil K and P levels and potential yields under conditions in KwaZulu-Natal, South Africa.
TABLE 1.1 Guidelines for K-fertilization of soybeans
Soil K K-application for yield potential (t ha-1)
1 2 3
mg kg-1 (kg ha-1)
20 20 30 60
40 16 23 47
60 13 19 39
80 11 17 34
100 10 15 31
120* 0 0 0
(Fertilizer Society of South Africa, 2007. Adapted for a lower removal figure)
* No K reaction expected
TABLE 1.2 Guidelines for P-fertilization of soybeans
Soil P (Bray1) P-recommendation for yield potential (t ha-1)
1 2 3
mg kg-1 (kg ha-1)
5 20 40 60
10 17 31 45
15 15 25 35
20 10 20 30
25 11 (6)* 19 (12)* 28 (18)*
30 10 (5)* 18 (10)* 26 (15)*
(Fertilizer Society of South Africa, 2007. Adapted for a lower removal figure)
* Maintenance fertilization if removal of 6 kg P ha-1 by soybeans is accepted
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 – 4 t ha-1 can be produced. N fixation is, however, 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 provides the roots with 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).
14 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 soybeans (Rajcan et al., 2000; Chauhan, 2007).
The critical level of soil K for soybean has been found to be 80 mg L-1 (Birch et al., 1990). 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 90 mg kg-1, but where a K-deficiency occurs, soybeans react well to K-fertilization (Liebenberg, 2012). Soils which 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 soybeans are generally used in a rotation with maize and remove fairly large quantities of K, the Fertilizer Advisory Services (Fertrec) of the KwaZulu-Natal Department of Agriculture and Rural Development‟s recommendations use a critical soil K level of 100 mg L-1 to ensure that the maize crop receives sufficient K.
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). Waluyo et al. (2004) reported that P increased the number of nodule primordial 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 plant-1 increased with the application of P. Zarrin et al. (2007) reported that the combination of Rhizobia and P increased nodulation and seed yield. 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.,
15 2008; Win et al., 2010; Liebenberg, 2012), although Nedić (2005) found no effect. Zheng et al.
(2010) found that soybean yields improved with P application under drought stress conditions.
Shahid et al. (2009) reported that plant height and the number of branches plant-1 were significantly higher at 75 and 100 kg ha-1 P than at 0 kg, 25 kg and 50 kg ha-1. The number of pods plant-1, pod length, number of seeds pod-1, seed yield and oil yield were significantly higher at 100 kg P ha-1 than at all the other P rates. Sharma et al. (2011) found significant increases in plant height, number of pods plant-1 and grain yield with increased P application rates (0 kg, 30 kg and 60 kg ha-1), but the response varied with soybean variety.
In KwaZulu-Natal soybean is widely grown in soils where P is strongly adsorbed by clay in the soil, making it unavailable to the crop (Birch et al., 1990 and Liebenberg, 2012). Approximately 6 kg P is removed per ton of soybean seed (Liebenberg, 2012). Even where target P levels are met, it is recommended that 20 kg ha-1 P be applied at planting, preferably band-placed along the row, although Mallarino (2006) found no response in yield to band or broadcast application.
Soils with medium to low available P levels should receive a minimum of 20 to 40 kg P ha-1 (Birch et al., 1990). However, the combination of N, P and K must be balanced for optimum yield to be achieved.
Birch et al. (1990), Mahamood et al. (2009) and Sharma et al. (2011) reported that certain grain soybean cultivars respond differently to inadequate P levels. Nwoke et al. (2009) and Wang et al. (2010) suggested that P-efficient cultivars could, therefore, play a major role in increasing soybean yield. Similarly, edamame cultivars may yield optimally at different levels of fertility.
Soybean is more tolerant of aluminium than maize, provided the seed contains sufficient molybdenum. Soybean roots are therefore able to penetrate highly acidic subsoils and utilize the subsoil moisture present. Consequently soybean can survive drought periods better than maize in South Africa (Liebenberg, 2012).
Soybean leaf analyses are effective in identifying nutritional problems. Leaf samples must consist of leaf blades without the petioles. The uppermost, mature (fully expanded) trifoliate leaves should be picked. Birch et al. (1990) recommended that the leaves be picked after flowering when the upper part of the plant bears young pods and the lower pods are fully elongated. However, Liebenberg (2012) recommended that the leaves be picked during the late
16 flowering stage and early pod fill stage. Approximately 50 to 100 randomly selected leaves should be collected and dried as rapidly as possible soon after collection. Leaves covered with dust should be washed or dusted. Table 1.3 provides sufficiency ranges and excessive levels for various micronutrients in soybean leaves.
TABLE 1.3 Sufficiency ranges for soybean leaves sampled prior to pod set (Small and Ohlrogge, 1973) and preliminary Cedara sufficiency ranges using leaves sampled after flowering as prescribed and excessive (toxicity) levels (Ohlrogge and Kamprath, 1968)
Element Sufficiency range Excessive (>) USA Ohio, USA Cedara
Micronutrient (%)
Nitrogen 4.26 – 5.50 4.00 – 5.50 7.0
Phosphorus 0.26 – 0.50 0.26 – 0.50 0.8
Potassium 1.71 – 2.50 1.40 – 2.50 2.7
Calcium 0.36 – 2.00 0.36 – 2.00 3.0
Magnesium 0.26 – 1.00 0.22 – 1.00 1.5
Sulphur 0.30 – 0.60 0.20 – 0.60
Micronutrient (mg kg-1)
Manganese 21 - 100 21 - 100 250
Iron 51 - 350 51 - 350 500
Boron 21 - 55 21 - 55 80
Copper 10 - 30 10 - 30 50
Zinc 21 - 50 21 - 50 75
Molybdenum 1 - 5 1 – 5 10