The term "soil improver" includes true soil improvers, which are organic materials incorporated into the soil to improve its fertility/structure, tree planting composts (where localised improvement in the soil is needed), surface mulches (used to suppress weeds and conserve soil moisture) and specialist products such as turf dressings. True soil improvers are materials which are added to improve a soil's physical condition, either by improving soil structure and/or increasing water holding capacity. To achieve this they should have a high organic matter content. pH and nutrient content are relatively unimportant although some soil-improvers, such as farmyard manure, do supply plant significant nutrients in addition to their benefits to the physical nature of the soil. There should ideally be a wide range of particle sizes from dust up to about 25 mm. Tree planting composts are a specialist form of soil improver used where localised improvement of the soil is needed. Again the main component is organic matter but a significant nutrient content is also desirable, and particle size should be slightly smaller. Turf dressings are similar in properties to tree planting composts, but a lower organic matter content can be tolerated. The most stringent requirement is a fine particle size. Much chipped bark is used as a surface mulch but UMS is less suitable because it degrades too quickly and will support weed growth so does not control weeds in the same way as a bulk mulch would.
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