Rock-phosphate mobilization induced by the alkaline uptake pattern of legumes utilizing symbiotically fixed nitrogen

The ability of plants to exert a certain degree of selection when absorbing nutrients often has as a consequence that on an equivalence basis unequal quantities of cationic and anionic nutrients are absorbed. To maintain electroneutrality inside and outside its tissues, the plant absorbs or extrudes ions not considered as nutritive ones. As a result of such regulatory actions of the plant, the pH in the root environment can be affected which in turn can exert an i nfluence on the solubility and availability of nutrients.In this dissertation, attention is paid to the implications of utilization of symbiotically fixed N 2 by legumes for the usefulness of rock phosphates as phosphatic fertilizers. The chain of thoughts is, as follows: 1. originally present soil phosphate and soil nitrate enable legumes to form nodules in which symbiotic N 2 fixation can take place which for its further growth will make the host plant independent of any form of combined N (NH4+and/or NO3-) ; 2. when the legume has exhausted the initial NO3-supply, but can continue to grow because of the availability of symbiotically fixed N 2 , it will absorb more cationic than anionic nutrients; 3. on account of this cationic uptake pattern, the growth medium in the vicinity of absorbing roots will acidify; 4. as a result of this acidification alkaline rock phosphates, when added as fertilizer, might be partially solubilized; 5. because of this solubilization induced by N 2 fixation, rock phosphates might be more useful P sources for legumes than for other crops such as cereals, and they might also be more useful for legumes utilizing symbiotically fixed N 2 than for the saw legumes under conditions of an ample supply of NO 3 -N.In a series of experiments, it was investigated whether this chain of thoughts could be verified and, if so, which limiting factors could manifest themselves and how these might be eliminated. These experiments were conducted in greenhouses with the use of two growth media, namely pure quartz sand and a sandy loam low in P which was located at a depth of a few meters in a sand quarry near Lunteren. The experimental crops used were soybean, alfalfa, peanut, and maize. The phosphate fertilizers employed were triple superphosphate and alkaline rock phosphates from Mali and Morocco.In experiment 1, use was made of sand as growth medium, soybean as test crop and superphosphate as P fertilizer. It could be shown that, as long as the crop could make use of NO 3 -N, soybean exerted a pH-raising effect on the sand. Shortly after the stage at which the NO 3 -N was exhausted and the plants transferred to symbiotically fixed N 2 as N source, a decline in pH could be observed. In the first weeks of growth, the pH-values of rhizosphere- and non-rhizosphere sand were found to differ. Probably as a result of intensification of the root system, later on these differences disappeared.In experiment 2, an evaluation was made of the ability of soybean, grown m sand, to make use of P supplied in the form of the two rock phosphates. With an ample supply of NO 3 -N, the plants appeared to be unable to utilize these P sources, but this was different for plants utilizing symbiotically fixed N 2 , on account of the fact that these latter plants succeeded in acidifying their root environment. With the use of superphosphate, the growth of plants utilizing fixed N 2 was very poor, which could be ascribed to P toxicity. To compensate for the absence of NO3-as major anionic nutrient, these plants appeared to have absorbed excessive quantities of phosphate. In the case of NO 3 -fed plants, P uptake was less extreme.Experiment 3 was comparable to experiment 2, except that the sandy loam was used as growth medium. It appeared that in this case the acidifying effect of the plants utilizing fixed N 2 was insufficient to solubilize the rock phosphates used. The short duration of the experiment, the P-fixing capacity and the pH-buffering capacity of the soil, and a shortage of available soil phosphate were factors likely to be responsible for the negative results obtained in this experiment.Since peanut is often grown m light-textured soils having low pH- buffering capacities, this crop was chosen as test crop in experiment 4 to examine its ability to utilize P applied in the 3 different fertilizer form to sand. Even when the superphosphate quantity applied was reduced to me-half of that applied to soybean in a previous experiment, P toxicity was still noticeable. For unknown reasons, symbiosis between host plant and Rhizobium strain was insufficiently effective to bring about an acidification of the sand. As a result, solubilization of rock phosphates did not take place. It was furthermore observed that peanut belongs to the group of plants exhibiting a neutral uptake pattern (equivalent quantities of cationic and anionic nutrients absorbed) when N is absorbed as NO3-.In experiment 5, alfalfa was grown m the sandy loam. With this perennial crop, the acidification induced by plants utilizing fixed N 2 appeared large enough to enable alfalfa to make use of P applied in rock phosphate form. In the case of NO3-nutrition, the soil pH became high enough to render the sparsely soluble Mali rock phosphate largely unavailable to alfalfa. It could be concluded that the influence exerted by perennial legumes m the soil pH can be large enough to overcome a buffering capacity the soil may have.The possibility exists that in extremely poor soils, lack of available P prevents the development of nodules so that symbiotic N 2 fixation cannot take place. In such cases, application of a small quantity of soluble P fertilizer may be sufficient to put into motion the chain of reactions eventually resulting in solubilization of rock phosphate that was also applied. In experiment 6, a small quantity of 32P-labeled KH 2 PO 4 was added to sand or sandy loam which was placed in pots m top of a larger quantity of sand or soil to which either me of the two rock phosphates was added. With the use of sand as growth medium, it was found that with the aid of the small quantity of KH 2 PO 4 applied, soybean could indeed make better use of the rock phosphates. With the soil, such a priming effect of the starter-KH 2 PO 4 was not observed. More so than in other experiments, low light intensity constituted an obstacle to a normal development of the N 2 -fixing mechanism, thus reducing the ability of the root system of soybean to acidify, its root environment.It is known that vesicular- arbuscular mycorrhiza (VAM) can make a contribution to the P nutrition of plants growing m P-depleted soil or m soil to which sparsely soluble rock phosphate was applied. In experiment 7, it was examined whether the joint actions of Rhizobium and VAM in mobilizing soil P and P added as rock phosphate would be more effective than the single actions of each of the micro-organisms . The experimental crop was again alfalfa grown m the sandy loam. It was found that for the control treatment (no fertilizerP added) and for the Mali rock phosphate treatment the quantities of P absorbed by the plants were indeed largest when both Rhizobium and VAM were active. In the superphosphate- and Morocco rock phosphate treatments, VAM did not make a contribution to the P nutrition of the plants, which can be seen as an indication that the availability of these fertilizers was high enough to meet the requirements of the alfalfa plants without any aid rendered by VAM.In experiment 8, it was shown that maize, as an example of a crop that in the case of NO3-nutrition raises the pH of its root environment, lacks the ability to utilize P applied in rock phosphate form. In this respect, the behavior of maize is similar to that of leguminous crops, like soybean and alfalfa, when these crops are amply supplied with NO 3-N and do not utilize symbiotically fixed N 2 .The conclusion to be drawn from the results obtained is that alkaline rock phosphates are relatively useful as P sources to c-raps which are capable of acidifying their root environment. Legumes possess this ability, provided that they utilize symbiotically fixed N 2 . In such cases, they withdraw from the soil more cationic than anionic nutrients, thereby exerting an acidifying effect m their root environment.Such an ability to solubilize rock phosphates is, therefore, dependent m the extent to which N 2 fixation can take place. Such a fixation can be hampered by low light intensities, but also by lack of a small quantity of readily available phosphate needed by the young plants to build up an N 2 -fixation mechanism. Once such a mechanism is operative, it can indirectly make a contribution to the mobilization of alkaline rock phosphates when these have been applied to the soil. From the results obtained it appeared that the initiation of a chain of reactions eventually leading to the solubilization of rock phosphate can be brought about by adding a small quantity of superphosphate or by establishing a symbiosis of legume and VA mycorrhiza.The results obtained provide opportunities for those countries which possess alkaline rock phosphates that do not lend themselves to being used as basic materials for the manufacturing of superphosphates. Application of these rock phosphates creates a possibility to grow legumes with only a slight investment in the form of rock phosphate needed to supply the plants with two highly important and expensive nutrients, namely nitrogen and phosphate.