The effect of direct phosphorus and potassium fertilization on soybean (Glycine Max L.) yield and quality

Abstract

Soybean is a vital cash, oil and protein crop. To achieve good yields and quality, adequate amounts of essential nutrients are required. Therefore, application of P and K plus inoculation with Bradyrhizobium bacteria should be included in the general production of soybean. However, the practice in South Africa is to apply no P and K when producing soybean since the farmers rely on residual P and K from the previous cropping season. The objective of this study was to determine that direct P and K application to a soybean crop may have positive results in terms of production and quality. The research was conducted at the Hatfield Experimental Farm of the University of Pretoria under green-house and open field conditions during the 2010/2011 season. The field trial treatments consisted of combinations of 3 levels of P (0, 20 and 40 kg P ha-1) and 3 levels of K (0, 50 and 100 kg K ha-1)applied in factorial combination for a Completely Randomized Block design, replicated four times. The pot trial was also a factorial experiment using a Completely Randomized Design with the two factors each at five levels of application (P at 0, 10, 20, 30 and 40 kg P ha-1 and K at 0, 50, 100, 150 and 200 kg K ha-1). Each treatment combination was replicated four times. Phosphorus and K were applied as Superphosphate (10.5%) and KCl (50%) respectively. The cultivar LS 6161R was planted under rain-fed conditions while LS 6162R was used as test crop in the green-house. Seeds were inoculated at planting with Bradyrhizobium japonicum, with no additional N applied during the season. Composite soil samples were collected from each plot and pot before and after planting and analyzed for pH (H2O) and plant-available nutrients. During the growing season,the field trial plants were sampled for LAI while canopy closure and plant height were measured for plants in the middle rows of each plot. Harvesting commenced after leaves senesced and pods had turned brown. The data recorded was on the number of pods per plant, number of seeds per pod, number of nodes per plant, fresh and dry root, stem and pod mass, 100-seed mass, total seed yield as well as protein and oil content. The results for the field trial showed that K significantly improved plant height, canopy closure and 100-seed mass as compared to the control. The application of P and K revealed no significant impact on leaf area index. Although not significantly, pod number per plant was reduced by applying P, resulting in the control having the highest number of pods. A significant improvement in grain yield was observed through application of K. The highest grain yield (2.60 t ha-1) was observed at the highest K level (100 kg K ha-1). The lowest grain yield was observed where no K fertilizer was applied. Although grain yield was not significantly affected by P nor the P*K interaction, there was a trend of increased yield with increased levels of P and P*K.Phosphorus, irrespective of the application rate, increased protein content but decreased oil content, while increased K application rates resulted in increased oil content while it decreased the protein content as compared to the control. The green-house data showed that plant height was significantly and positively affected by P, K as well as the P*K interaction. Maximum mean plant height were recorded with low application of P and no K (10 kg P + 0 kg K ha-1) as well as medium application of K and no P(0kg P + 100 kg K ha-1) which were significantly higher than the measurements recorded at 20, 30 and 40 kg P ha-1 regardless of K applied. In general, the range in number of nodes per plant was very narrow (19 to 21) and node number was not affected by P and K application. The lower levels of P fertilizer (10 and 20 kg P ha-1) gave the greatest number of pods. P*K interaction effects were not significant. With two exceptions, plants receiving 40 kg P ha-1 regardless of K tended to have the highest number of nodules. Although there was no statistical significance recorded between the treatments, 30 kg P + 150 kg K ha-1 produced the highest root fresh mass which is higher than that of the control plants but on par with plants receiving 10kg P + 100 kg K ha-1. The data on dry root mass of soybean had shown that various rates of P had a negative effect on it. There was a gradual decrease in pod mass with increased application of P from 10 to 40 kg P ha-1with the latter having the lowest pod mass than even that of the control. Although K and P*K interaction were not significant, all K application rates resulted in increased fresh and dry stem mass. From the current study, medium to high levels (± 100-150 kg K ha-1) of K applied directly to the soybean crop can be recommended as it had a positive impact on soybean growth and yield. On the other hand, the plant’s reaction to P was very much dependent on the initial soil P level, resulting in varying reactions. Therefore the farmer’s practice of using residual P from the previous season could not be proven completely wrong.