The use of rockdust and composted materials as soilfertility amendments

This thesis aims to investigate the use of two materials: rockdust and greenwaste compost for use as soil fertility amendments. A field trial was conducted over three years to investigate the impact these materials had on plant yield, plant nutrient content, soil chemistry and soil microbial communities in direct comparison with chemical fertilizer and farmyard manure. There were annual applications of compost, manure and chemical fertilizer in spring with one rockdust application in the autumn prior to the first year of the trial. Two harvests were carried out each year in summer and autumn to determine differences in plant yield. The soil was analysed prior to applications to determine the baseline chemical status then was analysed at two more points through the trial. Results from the field trial showed clear effects of organic amendments on plant yield that were attributed to nitrogen addition by compost and manure. NPK chemical fertilizer produced a yield effect at an earlier point in the trial compared to manure and compost. This, and the chemical analysis of the materials showed that the organic materials required a period of mineralization of organic-N in order to replenish soil available nitrogen. The results from the field trial also showed differences in plant nutrient content (as a measure of plant quality). The organic treatments were shown to generally produce higher plant nutrient content than the NPK fertilizer showing that the increased yields of the inorganic fertilizer treatment impacted negatively on the nutrient content. No yield effects due to rockdust addition were apparent after 3 years of the field trial. In addition, rockdust did not impact on plant nutrient content nor did it affect the soil chemistry despite 3 years of weathering that was considered sufficient time to release nutrients to the soil. Samples of soil were taken in the summer after the final year of the field trial to determine long-term changes in the soil microbial communities between the treatments. Results showed that there were fewer long-term changes than were initially expected in soil microbial communites at the end of a 3 year trial of these materials. Short-term greenhouse pot trials were also conducted using 3 different test crops to investigate the use of greenwaste compost and various types of rockdust for use as growth media in comparison with a peat based control. Results showed that while greenwaste compost supported plant growth as well as the peat based media, no rockdust type increased plant growth beyond the yield in a greenwaste compost control. The short-term supply of nitrogen in greenwaste compost was as good as that of the peat based control and while the phosphate supplied by the peat based control was shown to give excessively high plant P content. Results from the pot trials showed that greenwaste compost could wholly or partially replace peat in plant growth media without negatively impacting on plant yield or quality. The likely effects of various types of rockdust on soil chemistry were investigated by carrying out nutrient extractions using increasing extractant ‘strength’. Nutrient extractions showed that a high degree of rock weathering was required to release small quantities of trace elements from rockdust samples. Sodium, calcium and to a lesser degree potassium, magnesium, iron and phosphate were supplied in greatest quantities from most rockdust samples with basic rocktypes releasing highest quantities of nutrients. As a result of the work carried out in this thesis, it is concluded that composted greenwaste could be a valuable addition to agricultural soil and that it could replace peat in some plant growth media. There are some implications to the use of composted materials – transport and application costs and the potential addition of potentially toxic elements to soil; however the potential nutrient addition and improved plant quality could make it an attractive fertility amendment in some organic farming techniques. It was concluded that rockdust was not shown to influence plant yield or quality in the agricultural setting of the field trial, nor was it shown to be a useful addition to plant growth media. Therefore rockdust could not be proven to be a useful soil fertility amendment.