Improvement of planosol solum, 2

Part 1 Introduction Planosol solum is widely distributed in the Three-river plain of Black Dragon province of the People's Republic of China near the border with Russia, and is a low yield soil. The area of planosol solume is 17 square Mm (1/4 of Hokkaido Island in Japan) and 19 % of the Three-river plain of Black Dragon province. The cultivated fields ofplanosol solum are 8.6 square Mm and 29 % of the total cultivated area in the Three-river plain. The first horizon (Ap) is a humic soil which is suitable for plant growth and has a thickness of about 200 mm. The second horizon (Aw) is a lessivage soil which is dense and impermeable and has a thickness of about 200 mm. The third horizon (B) below about 400 mm depth is diluvial heavy clay. With the impermeable Aw horizon, plants suffer both from drought and excess water. The soil penetration resistance of the Aw horizon is more than 5.0 MPa (30 deg cone angle, 16 mm base diameter), and the roots of plants cannot penetrate the Aw horizon, while soil micro-organisms cannot live beneath it. Because of the shallow top soil of the Ap horizon, forests do not develop and only shrubs survive in the hills. In flat areas, field crops have low yields. Based on field cultivation tests by Zhao et al and soil investigation by Araya, the object of work reported in this paper was to determine a method of mixing the Aw and B horizons, in a one-to-one ratio underground, leaving the Ap horizon undisturbed. With the wide area of planosol solum, a plough system would be suitable for improvement because of high efficiency. Part 2 Mechanical properties of soils The planosol solum in China is a binary mixture of soil particles where silt forms the frame structure and clay fills the pore spaces. It is extremely hard and impermeable and has particular mechanical properties. This paper deals with the mechanical properties of the three horizons (Ap, Aw and B) of the planosol solum as an aid to understanding the draught requirement of a three-stage subsoil mixing plough for improvement of the planosol solum. Pseudogley soil which is a typical heavy clay soil in Japan was also tested for comparison. Tensile strength, compressive strength, shearing strength, soil-metal friction as static properties and soil brittleness as a dynamic property were determined. The results show that the cohesive strengths of all soils, except the B horizon, had maximums at particular soil water contents. These values were nearly the same as the plastic limits. The B horizon did not have a maximum value in the range of soil water content studied. The cohesion of the pseudogley soil was the smallest and that of the Aw horizonwas the largest. The Ap and Aw horizons had the same trend in brittleness; the impact energy required to fracture both soils increased with decreasing soil water content. The required impact energy of the B horizon showed a quite different trend from that of the other soils and was a maximum at 30 % d.b. soil water content. When the B horizon was dry, the required impact energy decreased and it became brittle. The commonly occurring soil water content in the actual planosol fields was more than 20 % d.b.. In mis range, the tensile strength of the Aw horizon was the largest, followed by the B horizon , Ap horizon and pseudogley soil. Comparing the cohesion at soil water contents in excess of20 % d.b., the tensile strength of the pseudogley soil was about one seventh of its cohesion but the tensile strengths of all planosols was about one-half of their cohesive values. Part 3 Analysis of draught of a three-stage subsoil mixing plough Based on earlier field cultivation tests and soil investigations, a one-to-one mixing of the second (Aw) and third (B) horizons was conducted to improve the planosol solum in China, leaving the first (Ap) horizon undisturbed by a three-stage subsoil mixing plough. The Ap and Aw horizons have a thickness of about 200 mm. The layer below about 400 mm depth is the B horizon. This part deals with the mechanism of draught production of the second and third plough bodies of the three-stage subsoil mixing plough. The results showed that in both the model and the field tests with the studied soils, the resistances caused by upheaving, tension, and cohesion were the largest contributors to the total draught, and the resistances caused by soil acceleration and adhesion were extremely small. In both the model and the field tests, measured and predicted draughts were in reasonable agreement. In the soil bin tests using a half-scale model plough in Japan where a Japanese pseudogley soil was used and the soil hardness in the soil bin was the same throughout, both predicted and measured combined draughts produced by the second and third plough bodies, where each plough body tilled a 100 mm depth, were smaller than that produced when a 200 mm depth soil was tilled by the third plough body alone, if the second plough body was removed and the three-stage subsoil mixing plough was used as a two-stage subsoil mixing plough. In the full-scale three-stage subsoil mixing plough tests in China, the draughts of the second plough body which tilled the Aw horizon and third plough body which tilled the B horizon, were nearly the same, despite the fact that the size of the third plough body was larger than that of the second plough body, because the cohesive and the tensile strengths of the Aw horizon, the second horizon, were larger than those of the B horizon, the third horizon. Here also, the combined draught produced by the second and third plough bodies, where each plough body tilled a 200 mm depth, was smaller than that produced when a 400 mm depth soil was tilled by the third plough body alone, if the second plough body was removed. If the second plough body was eliminated and the Aw and B horizons of the 400 mm depth were directly tilled by the third plough body alone, the draught requirement was steeply increased and large soil clods were formed by the tillage of the third plough body. Part 4 Fertilizer distributor for subsoil Based on field cultivation tests by Zhao and soil investigations by Araya, a one-to-one mixing of the second (Aw) and third (B) horizons was conducted to improve the planosol solum in China, leaving the first (Ap) horizon undisturbed by a three-stage subsoil mixing plough. This part deals with the development of a fertilizer distributor which applies chemical fertilizers into the subsoil, where the Aw and B horizons, which are lacking in phosphorus and calcium, are mixed. Additionally, based on root density distribution of plants, this paper deals with the development of a technique in which the fertilizer is placed more densely in the upper layer than in the lower layer within the 200-600 mm depth of subsoil. The results showed that when both granular and powder fertilizers were supplied at the rear of the first or second plough body, the fertilizers reached the bottom layer with soil mixing by the plough bodies, and the fertilizer distribution density was greatest in the lower layer of the subsoil. When the fertilizers were supplied at the side of the third plough body, an improved distribution was obtained. However, the fertilizer was not distributed over the entire operating width (300 mm), but was concentrated more around the side of the third plough body. As a result, when the fertilizer was supplied both at the rear of the third plough and the side of the third plough body, the distribution sought was obtained. Here, the distribution density of the fertilizer decreased linearly to the tilled soil depth in the subsoil. Part 5 Mixing of wheat straw and maize stalk into subsoil Based on field cultivation tests by Zhao and soil investigations by Araya, a one-to-one mixing of the second (Aw) and third (B) horizons was conducted to improve the planosol solum in China, leaving the first (Ap) horizon undisturbed by a three-stage subsoil mixing plough. This part deals with the collection of wheat straws and corn stalks which are left on fields after harvesting by combines, how to mix them into the subsoil and the determination of the limiting length of wheat straw and corn stalk to mix into the subsoil. It was postulated that, if organic matter such as the wheat straw or corn stalk is mixed into the subsoil, where the Aw and B horizons are mixed, low soil hardness may be sustainable because of increased pores within the aggregated formation. The results showed that when a curved plate and a finger-wheel rake were combined, the required straw and stalk collection was obtained. The wheat straws or corn stalks on the soil surface were first collected by the finger-wheel rake and then, dropped by the curved plate, into the furrow at the rear of the third plough body. The collection rates of wheat straws decreased when their lengths were less than 200 mm and corn stalks when less than 300 mm. For good mixing with the subsoil, the lengths of both wheat straws and corn stalks should be less than 300 mm. Cutting of the straws and stalks may be necessary to achieve this.