Legume Inoculation Discussed

Press, Volume CIII, Issue 30580, 24 October 1964, Page 10

Legume Inoculation Discussed

In initiating a symposium on problems of inoculating legumes at the conference of the New Zealand Grassland Association last week, Professor T. W. Walker, professor of soil science at Lincoln College and president of the association, said that in a country which depended so much on nitrogen fixation by legumes for its grasslands it would have been expected that there would have been a more vigorous research programme in this field. It was surprising that, in fact, so little had been done.

Professor Walker recalled that it was just over 75 years since rhizobia bad been discovered and the function of nodules on the roots of legumes in fixing nitrogen had been understood.

In a paper read by Dr. A. T. Johns, director of the Plant Chemistry Division of the Department of Scientific and Industrial Research, Mr R. M. Greenwood, of his division, pointed out that separate groups of rhizobia worked with each of the three main groups of legumes—the clovers, lucerne and other medics and lotus—but within each of these groups of rhizobia there were many strains differing in the range of species that they were effective with. Thus among clover rhizobia one strain might be effective on white and red clovers but not on subterranean, and a second strain effective on subterranean, weakly effective on red and ineffective on white, and so on. From Overseas All the main species of legumes now in use in New Zealand had been introduced from overseas, he said, and it was probable that their associated rhizobia had also been introduced by chance during the early days of settlement. Since then they had been spread by a number of agencies such as contaminants on seed, by the feet of animals, agricultural implements, in dust, floodwaters and agricultural lime, and while they were how widespread in New Zealand soils, sometimes beyond the range of their host plants, they were still absent from many areas. Greenwood said that quite high populations of clover rhizobia occurred in soils under pasture, even where they were recently developed from the virgin state, but in drier areas where annual clovers were more plentiful or dominant a larger proportion of strains were effective or partially effective on subterranean and other annual clovers, but ineffective or weakly effective on white and red clovers. In unimproved hill country pastures, clover rhizobia tended to have a very patchy distribution and it was possible for clover seedlings from oversown uninoculated seed to die from lack of nodulation in these situations. In South Island tussock country, clover rhizobia, not necessarily very effective strains, were present in small numbers over wide areas where clover plants could not be found. Oversowing of uninoculated seed might be quite successful, espcially if lime was applied as this stimulated the multiplication of those rhizobia already in the soil, but inoculation was obviously desirable. Wise Course Lotus rhizobia were plentiful wherever lotus was growing, but they were absent or very sparse over large areas where clover rhizobia might be plentiful, and it was wise to inoculate lotus seed when sowing new areas, especially with lotus seed at its present price. Lucerne rhizobia might be quite plentiful in some neutral soils, such as in the Hakataramea valley under lucerne, and in a coastal sand north of Foxton, under black medic, and they woujd be

present in limestone soils under black medic and in neutral soils in dry climates under various medic species, but over the greater part of New Zealand lucerne rhizobia were not free-living in the soil, hence the standard procedure of always inoculating lucerne seed.

Greenwood noted that lime might well be of significance as a source of rhizobia when acid soils were limed, though samples of lime even from the same works varied markedly in the number of rhizobia that they contained. Lucerne rhizobia seemed to be particularly at home with agricultural lime.

Discussing the advantages of pelleting legume seeds to protect the nodule bacteria on the seed by covering them with a layer of finely powdered non-acid material, Mr A. Hastings, a technical officer of the Plant Diseases Division of the D.5.1.R., said that the seed treated in this way could be sown in contact With superphosphate and other fertilisers without dam-

aging the nodule bacteria. The nodule bacteria on inoculated pelleted seed survived for a longer period and enough seed might be inoculated and pelleted at the one time to last for several days. The bacteria were also protected by pelleting when sowing of inoculated seed into dry ground was unavoidable. In soils where the acidity was marginal for effective nodulation, pelleting could make the difference between success and failure and pelleted seed facilitated legume establishment on soils where conventional inoculation failed.

Of the materials that could be used for coating seed after inoculation, Mr Hastings said that the best appeared to be a mixture of Gafsa phosphate and dolomite, followed by Nauru phosphate and dolomite. While on the majority of the 148,000 acres of country taken over by the Lands and Survey Department in the Te Anau area, pastures were successfully established by traditional cultivation methods, Mr N. A. Cullen, a principal scientific officer of the Department of Agriculture’s Invermay station, said that on about 30,000 acres of glacial moranic soils, clovers had failed to establish and pastures had as a result been very poor and unproductive. These soils were light in tex-

ture and liable to suffer from drought, and much of the area was broken in terrain and its bouldery state made cultivation difficult and expensive. The vegetation in its natural state consisted largely of stunted bracken fern with some fescue tussock, low fertility grasses and native species. Mr Cullen said that detailed investigations had been begun by the department in 1960 and they had been continued and expanded until the present time. Results had shown that clover inoculation was essential for successful establishment Where clovers were not inoculated they were mainly stunted and few survived more than a few months. In some of the earlier sowings where the clovers were not inoculated discing in lime gave superior results. This could be attributed to the lime aiding nodulation. Early investigations had also indicated that this soil was extremely deficient in phosphate, and as a result high initial applications of superphosphate had been shown to be necessary for establishment Spectacular Summarising the work in this area Mr Cullen said that results from oversowing clovers and grasses in the stunted fern had been spectacular and this method offered an economical means of developing this country. Large areas had already been developed by oversowing and topdressing. Key factors in success were the inoculation of clovers, use of a high initial application of phosphate, sowing in winter and early spring, ensuring that some cover was present for seedling establishment, and inclusion of suitable grass species. Liming was not necessary on this soil provided clovers were inoculated.

Although pelleting did not give superior results, this method enabled the seed to be sown directly with super and ensured a more even spread when sown from the air. Results with pelleted seed on large scale sowings on cultivated ground had not been completely satisfactory. In spite of extreme care in manufacturing, a large percentage of the pellets had on occasions proved ineffective, resulting in a thin clover establishment. The alternative of mixing peat culture, lime and seed directly in the drill was now being tried. Rash In presenting a paper on behalf of Mr A. F. R. Adams, a senior lecturer in the soil science department at Lincoln College, Professor Walker said that he believed it would be rash for anyone to say that a given legume should be pelletted for sowing in a given soil. It was safe to pellet lucerne, but for other legumes there was more doubt, and he suggested that it would be a matter of waiting and seeing.

In reviewing the papers, Professor Walker said that most farmers adopted the recommendations that lucerne should always be inoculated for sowing, but there was less certainty about white clover. The late Dr. P. D. Sears had told him that clover should also always be inoculated, even when it was sown where clover had been before, but Dr. Walker said that he did not think that there was much experimental evidence for this and very few farmers did, except where they were sowing clover where it had not been sown before. One would like to see more work done on whether white clover seed should be inoculated where it had been sown before, he said.