The nutrient requirements of white clover on hill soils
This investigation was part of a larger study to understand the main factors which affect the establishment, growth and persistence of white clover (T. repens L.) in improved hill pastures. Precise knowledge of the nutrient requirements for both the establishment and maintenance of white clover on hill soils is needed to make pasture improvement more reliable and less costly.The specific aims of the study were:1. To identify the magnitude of response of white clover to the major nutrients and lime when grown on acid hill soils.2. To establish critical levels of nutrients in the shoot.3. To evaluate possible benefits to the phosphorus nutrition of the plant from inoculation with mycorrhizal fungi.Experiments were carried out in glasshouse, growth room and field, using soils representative of the main groups found in hill areas, although the emphasis was on deep peat. In laboratory experiments, white clover after inoculation with Rhizobium was not found to respond to nitrogen nor did this element interact with phosphorus or potassium. However, the plants responded markedly to phosphorus and potassium; the response to one element was greatly influenced by the level of application of the other. There was a positive response to lime up to a pH of 5.5; thereafter, further lime reduced growth, apparently through its effect on the phosphorus nutrition of the plant. There was little response to magnesium. Field experiments with an established pasture on deep peat soil confirmed that lack of phosphorus and potassium can severely limit growth of white clover. The value of results obtained from laboratory experiments for prediction of response to nutrients in the field is discussed.Critical concentrations of phosphorus, potassium and magnesium in the dry matter of the shoots were 0.20% P,0.9% K and 0.29% Mg. Although the critical concentration of calcium was not determined precisely, the data suggest that a concentration of 1% or less in whole shoots restricts growth. The use of shoot analysis and critical concentrations to determine the need for maintenance dressings of fertilizer is considered.Mycorrhizal fungi were successfully introduced into the roots of white clover in both laboratory and field experiments the responses to inoculation depended on soil type, introduced endophyte, the presence of indigenous endophytes and the environmental conditions. In laboratory experiments with a deep peat soil there were marked responses in dry matter production and nutrient uptake, coupled with beneficial effects on nodulation and nitrogen fixation. Low temperatures and the wetness of this soil were probably the major environmental factors which prevented growth responses in the field.With two brown earth soils in the laboratory there were no responses in dry matter production, possibly because the soils contained a high density of indigenous endophytes. However, Glomus caledonius did significantly increase yield in the field on one of the soils during the second year of growth. By contrast, on the other brown earth soil, Glomus mosseae (LI) significantly depressed yield in the year of sowing. It is concluded that mycorrhizas should be collected from well established white clover pastures throughout Britain and screened in the laboratory, care being taken to match the mycorrhizal endophytes with both cultivar of white clover and strain of Rhizobium.The study has laid the basis for further nutrient work with other hill soil types and with mixed swards of white clover and companion grasses in the field. It has also shown that, although mycorrhizas have varying effects on clover growth in the field, some can be very beneficial and further investigation is justified.