The phosphorus and nitrogen nutrition of bambara groundnut (Vigna subterranea (L.) Verdc.) in Botswana soils : an exploratory study

Bambara groundnut (Vigna subterranea (L.) Verdec.) is a legume crop grown especially by small farmers mainly in semi-arid parts of Africa both in mixed cultivation and pure stands. It is considered as a hardy crop because of its drought tolerance, resistance to pests and diseases and ability to yield on chemically poor soils. The crop produces edible seeds which can be eaten unripe or stored as dried pulse for later consumption. In Botswana it is grown under semi-arid conditions by small farmers as a minor crop. Attempts have been made to increase the bambara groundnut yield through application of nutrients and improvement of the nitrogen-fixing capacity.

This research has not been undertaken systematically and especially the responses to P and N fertilizers did not show a clear pattern. Given the low available P levels and the limited N supply in Botswana soils, a research programme was undertaken to investigate the P and N nutrition of bambara groundnut. The main objectives of the study were:(i) to investigate the effects of applied P and N on growth and development of bambara groundnut in Botswana soils, and (ii) to investigate the P and N uptake, shoot concentrations at different growth and development stages of bambara groundnut and (iii) how uptake, internal concentration and growth are related. The experimental programme consisted of five pot experiments and a large field experiment with P and irrigation as treatments in which bambara groundnut was grown on a representative low P soil.

In addition a sand culture experiment was conducted to establish the critical P concentration in bambara groundnut shoots. The critical P levels were subsequently used to determine the P status of bambara groundnut plants in farmers' fields.

In Chapter 2 the question was whether bambara groundnut will respond to P fertilization in a P-poor soil in the absence and presence of added Rhizobium inoculum, whether bambara groundnut selections are different with respect to P nutrition, and whether soil moisture level under field conditions limits P uptake. In a preliminary pot experiment, which was terminated at flowering (51 days after sowing; DAS), the response of bambara groundnut shoot dry matter (DM) to P fertilization (fertilizer ground and thoroughly mixed with the soil) was positive and linear. There was no effect of P fertilization on nodulation both in the absence and presence of added inoculum. Leaf blade P and N concentrations were not affected by P fertilization.

The two selections showed basically the same response to P fertilization. The indigenous rhizobia seemed to be sufficiently effective in nodulating bambara groundnut. In the field, P fertilization (fertilizer broadcast after ploughing and worked into the soil with a digging fork) had no effect on bambara groundnut plants, while irrigation increased all plant growth parameters except root DM. Total seed yield was 2.8 t and 4.2 t ha ^-1for rainfed and irrigated treatments, respectively. Leaf blade P and N concentrations were not affected neither by P fertilization nor by soil moisture content.

However, irrigation increased the uptake of P. Evidently, part of the P pool present in the soil becomes available under improved soil moisture conditions, probably due to improved P diffusion to the roots. The lack of response to applied P in the field experiment with a low P soil might imply that when root growth is not restricted by soil volume as is the case in pot experiments, available soil P was enough to meet the P requirements of bambara groundnut. This could mean that compared to other crops like cereals bambara groundnut has a low P requirement possibly combined with a high P efficiency.

This hypothesis was tested in a pot experiment (reported in Chapter 3.1) in which the P uptake and the internal P use efficiency of bambara groundnut were compared to those of maize and pigeon pea. Maize is a crop with a high P requirement and pigeon pea has the capacity to use soil P normally not available to other crops. The response of shoot dry matter to added P was highest for maize, intermediate for bambara groundnut and lowest for pigeon pea. Compared to the other two crops bambara groundnut had a very low root-shoot ratio. The P uptake of maize was two and five times higher than that of bambara groundnut and pigeon pea, respectively.

Contrary to maize, most of the P requirement of the other two crops could be met with P originating from soil. Maize had the highest internal P use efficiency, on average producing 1.89 g DM per mg P taken up, against values of 0.79 for bambara groundnut and 0.56 for pigeon pea. Therefore, lack of response by bambara groundnut to P fertilization in the field cannot be attributed to an exceptionally high P efficiency, but probably to its low P requirement.

Another reason for the difference in response of bambara groundnut to applied P as found in pot- and field experiment could be the mode of fertilizer application. In the pot experiment, P fertilizer was thoroughly mixed with the soil before planting while in the field experiment P was broadcast and worked into the top-soil with a digging fork. This means that in pots the P fertilizer is directly after germination available to the plant and in the field at a later stage or not at all during the growing period.

This idea led to a pot experiment described in Chapter 3.2, in which different timing of P application was studied. The effects of application at sowing, two weeks and four weeks after sowing were studied. It was found that large significant DM responses are only found when P is applied within two weeks after sowing and that this early application is also needed for a high seed yield. This indicates that under field conditions broadcasting of fertilizer at sowing is not an effective way to supply bambara groundnut plants in time (e.g. directly after germination) with additional P.

In the field and pot experiments (Chapters 2 and 3), P fertilization had no effect on the shoot P concentration of bambara groundnut plants irrespective of the treatment (P rate, time of P application and irrigation). This led to the question whether internal shoot P concentrations found in bambara groundnut plants in previous experiments were adequate or not. To answer this question, the critical shoot P concentration of bambara groundnut was determined in a sand culture experiment using 12 P rates varying between 0.21 and 159 mg P plant ^-1for the whole experimental period. The critical P value, determined graphically and associated with 10% reduction in maximum shoot dry weight, was about 0.15% with a critical range of 0.15 to 0.20%.

The critical P value showed that bambara groundnut plants in our previous experiments (pots and field) were growing under suboptimal P levels. Subsequently a farm survey was conducted on ten farms to assess the soil P status of bambara groundnut fields in Botswana under farmers' conditions, and to compare the P nutritional status of those field-grown plants with the critical shoot P concentration determined from our sand culture experiment (Chapter 4.2). Soil and plant samples from each of the ten farms selected for the survey were collected at about 78 DAS and analysed for P. The soil P levels from 80% of the farms were below five milligrams P kg ^-1(P-Bray). On two farms, shoot P concentrations were at the critical level and on the other farms below the critical level.

In all previous experiments, bambara groundnut plants were growing under marginal P conditions with shoot P concentrations around or below the critical level, irrespective of the P fertilization and irrigation treatments, while the N nutrition was adequate. This raises the question whether the plant N supply will still be adequate if bambara groundnut plants are grown under conditions where P will be no longer marginal or deficient. Under such conditions, higher DM production will not only increase the plant N requirement, but also the N ₂ fixation might behave differently.

To answer these questions, a pot experiment described in Chapter 5 was conducted to find out whether (i) the capacity of the N ₂ fixation process, possibly supplemented with some native soil mineral N is high enough to meet the plant N requirement also under non-limiting P conditions, and (ii) at internal P concentrations exceeding the critical level, the amount of fixed N increases with increasing internal P.

In the experiment, bambara groundnut plants were fertilized with different rates of P with or without additional mineral N. Phosphorus fertilization increased shoot, root, and nodule dry weights, seed yield, shoot P and shoot N concentrations and shoot P and shoot N contents at all sampling periods. Nitrogen fertilization on the other hand had no effect on the parameters measured but influenced the time to plant maturity. Under non-limiting P conditions bambara groundnut seems to be able to meet its N requirement from N ₂ fixation and soil mineral N. The results indicate that no mineral fertilizer N should be supplied at sowing, but this does not rule out the possibility that at a later stage additional N may be needed. One of those might be the onset of podding, when a major shift in assimilate partitioning from vegetative growth to pod filling takes place.

To investigate this, the effect of N fertilization at different development stages of bambara groundnut: at sowing (as in the previous experiment), at the early vegetative growth, at (50%) flowering and (50%) podding, was investigated in a pot experiment (Chapter 6). To separate the contributions of soil mineral N and N ₂ fixation to the N nutrition of bambara groundnut plants, treatments with sterilized and unsterilized soil (control) were incorporated in the experiment. Nitrogen fertilization did not affect shoot dry weight and seed yield. Only N fertilization at 50% flowering decreased nodule number and nodule dry weight, shoot N concentration and shoot N content. This can mean that the flowering stage is critical for nodule formation in bambara groundnut.

Sterilization of soil decreased nodule number and nodule dry weight. Shoot N concentration of plants grown in sterilized soil was decreased only at the 50% flowering stage, and poor N nutrition at early stages of growth delayed the flowering and podding stages by two weeks. Shoot N concentration of the sterilized soil treatment recovered despite the low nodule number and nodule dry weight, implying a high N ₂ fixation efficiency of nodules present. It was not possible to quantify the contribution of the soil mineral N to the N nutrition of bambara groundnut because nodules were present in the sterilized soil treatment throughout the experimental period. Nitrogen fertilization, irrespective of time of application or growth stage was not beneficial to plant growth and reproduction.

Finally in Chapter 7 a general discussion of the main results on the responses of bambara groundnut to P and N fertilization in Botswana soils and the implications to bambara groundnut farmers is presented. It is concluded that P is important for growth and seed yield of bambara groundnut, and the low P requirement may be responsible for its ability to thrive in chemically poor soils. But for a positive response to P fertilization to occur, the soil moisture content must be adequate and the fertilizer should be available to the seedling within two weeks after sowing. Bambara groundnut can meet its nitrogen requirement from N ₂ fixation and soil mineral N, and there is no need for supplementary mineral N fertilizer. The shoot P concentrations at the early podding stage seem to be a suitable guide for monitoring the P status and P requirement of bambara groundnut.