A Study on the Effects of Gravel Processing on Soil Improvement
-Test on the Physical Properties of Soil Processed by Gravel Crushing-

  Gravel found on farmlands often interferes with farm operations (Photo-1). Therefore, through farmland consolidation conducted by the national and prefectural governments to support farming, gravel processing has been conducted to improve soil, including the improvement of its grain size distribution. Gravel removal has been conducted as a soil processing method in Hokkaido. Stones and gravel in surface soil are sifted out and carried away from the agricultural field (Photo-2). In recent years, gravel crushing (Photo-3) has been conducted as well. Under this method, stones and gravel are crushed by machine. It is economically excellent, since the operating costs are low.

  In gravel crushing, crushed stones and gravel are returned to the surface soil. To determine whether there were any changes in the physical properties of the surface soil (drainage, water retentivity) after gravel crushing, surveys were conducted over the course of several years: before gravel crushing, after such crushing, and 1, 2 and 3 years after such crushing.

  The surface soil was found to have become soft and porous after gravel crushing. Therefore, the compaction of the soil through ground leveling after gravel crushing was effective for securing the drivability of farming vehicles.

  It was confirmed that, up to 3 years after processing, for both the soil processed by gravel removal and the soil processed by gravel crushing, the gravel content (volume percent) of surface soil was kept to within 2%, which is the operation management standard. Changes in the physical properties of soil after gravel processing were also examined. For both gravel processing methods, the processed soil was found to have appropriate hardness, to maintain favorable permeability, and to not form plow sole layers (compacted soil layers resulting from tillage or compaction by farming vehicles) that would hinder farm work at 3 years after processing.

  The chemical properties of the surface soil (total carbon content, total nitrogen content, and fertilizer retentivity) were also examined for both gravel processing methods. No significant differences between the methods with respect to changes in the chemical properties of the surface soil over the years were discerned. The physical properties and chemical properties of surface soil was not found to have deteriorated over the years regardless of the gravel processing method.

  For agricultural fields that meet the requirements for gravel crushing, such as maximum grain size of stone and gravel, compressive strength, and gravel content, gravel crushing can reduce the cost of gravel processing. Gravel crushing can ensure inexpensive and appropriate soil improvement. The examination findings have been used to encourage farm households to introduce gravel crushing. In this way, the findings have been utilized to promote efficient farmland development.

(Contact: Rural Resources Conservation Research Team, Civil Engineering Research Institute for Cold Region)

On a Way of Managing the Shapes of Street Trees So That They Contribute to the Streetscape

Introduction

  Street trees have various functions, including improvement of the roadside environment and landscape, the ensuring of road safety through light shielding and delineation, and moderating of cold/hot temperatures and dry/humid air by creating leafy shade. However, for the purpose of cost reduction through less frequent pruning and in response to complaints from roadside residents about fallen leaves, cases of excessive pruning, including those of excessive branch-shortening by pruning, have been observed. Not only this has deprived street trees of their functionality, but it has also undermined the health of street trees and has caused plant death.

  In light of this, our research institute conducted a study on an appropriate pruning method that allows street trees to maintain their landscape function, which is a major function.

Outline of the research results

  The Handbook for the Pruning of Street Trees (Japan Landscape Contractors Association, 2001) shows "desirable tree height to branch spread ratio by tree form (guide)" (Figure-1) and "method for calculating the tree height to branch spread ratio" (Figure-2) for species that are currently used as street trees. Assigning values in the table of Figure-1 to the equation in Figure-2 generates "desirable tree heights". The values for "desirable tree height to branch spread ratio", however, extend over a wide range, and so do the values for "desirable tree height" obtained from the calculation. Therefore such values cannot be used as standards for pruning.

  To obtain "the desirable tree height to branch spread ratio" at which the landscape function of street trees is most enhanced, a study was conducted on how the appearance of tree height and branch spread of street trees affect the landscape. Composite photos with various trees species were produced by altering tree heights and branch spread (Figure-3). Subjects gave their evaluations of the photos (on a scale of like/dislike). In addition, "green coverage rates" (the area of trees (including trunks and branches) as a share of each image) were calculated. The relationship between impression evaluation and green coverage rate (Figure-4) shows that the higher the green coverage rate is, the higher the "like" evaluation is. However, the "like" evaluation peaks at the green coverage rates of 15-20%. When the green coverage rate exceeds this range, the "like" evaluation tends to decrease.

  Composite photographs whose green coverage rates are near the peak had tree height to branch spread ratios that were around the median values of the range indicated as "the desirable tree height to branch spread ratio." With these findings, we proposed that, in managing street trees, the median value of "the desirable tree height to branch spread ratio" and "the desirable tree height" obtained from the median value should be used to maintain the landscape function of street trees.

  When the Guidelines for Planting Alongside Roads in Hokkaido (Draft) was revised in 2016, this outcome was reflected in "Chapter 5 Execution/Management".

(Contact: Scenic Landscape Research Unit, Civil Engineering Research Institute for Cold Region)