The Degradation Features Of Farmland Soil Physical And Its Harmfuless In Guanzhong Area

There are favourable natural and ecological conditions in Shaanxi Guanzhongregion, it is suitable for a variety of crops because of the flat terrain, deep and loose soil, raindrenched. It has been the producing areas of major crops such as grain, cotton, oil, fruit andvegetable since the ancient times, and now it is the important producing areas of rainfed grain.However soil properties appeared the problems that recessive degradation with constantlyimprove in the land management with the implementation of land contract system, it showedcharacteristics with a poor performance of farmland drought resistance, the constantlyincreasing reliance of crops in the irrigation and fertilization, lower water and fertilizer usingefficiency, the high production mainly relied on the large amount of fertilizer, pesticides, seedand irrigation amount and frequency which increasing year by year. Soils’ ability to coordinatethe moisture, temperature was decreased. The degradation of farmland soil quality hasbecome the main bottleneck of agricultural sustainable development in Guanzhong area.There had been given enough attention on the degradation of chemical quality whichcharacteristic with soil acidification, however the degradation of soil physical gotunrecognized ever. The evolution of the physical state of soil profile, degradation mechanismand degradation effect had been studied through the dynamic monitoring of soil physical statein guanzhong plain and a combination with field test, analyzing the inherent relationshipbetween the modern farmland management measures (cultivation, fertilization, cultivation)and soil physical quality evolution, revealing the effects of activities of the human productionand business operation on the direction and and process of soil quality evolution, providing ascientific basis for the reform of the modern unreasonable implementation of soil using andmanagement, providing technical support for the new technology and products that beneficialto soil health,the results show that:1、The physical quality degradation characteristics of farmland soil profile inguanzhong plain was apparent with a clear performance of serious compactness for thesubsurface at the depth20~40cm. The bulk density in the0~60cm layer showed an initialincrease followed by a steady decrease with increasing soil depth. The soil bulk density waslowest in0~10cm layer (a mean of1.34g/cm3) with favourable porosity and the highest in 20~40cm layer (a mean of1.67g/cm3) which close to the limit bulk density (1.70g/cm3) ofthe land status. In the0~60cm layer the soil compactness increased with increasing soildepth, the average compactness in the0~15cm,15~25cm and25~45cm layer was482KPa,1647KPa,2268KPa, respectively. In conclusion, only the soil in0~10cm layer was fit forplanting, and subsurface layer was universally compacted, which posted severe problems inroot extending, gas exchange and water infiltration. The source of this problem was deepcompaction and downward-moving clay particle. What’s more, the problem under deep soillayers was invisible, thus we must pay great attention to it.2、Soil saturated hydraulic conductivity determines the ability of the acceptance ofrainfall and irrigation. The soil saturated hydraulic conductivity (Ksat) showed a sharpdecrease with increasing soil depth and there was a statistically significant difference in Ksatamong soil layers. the Ksatin the0~10,10~20,20~30,30~40,40~50,50~60cm layer was0.968,0.291,0.095,0.079,0.071and0.063mm/min, respectively. The differences of soilprofile conductivity performance restricted the farmland water infiltration in some degree. Atthe same time, studies have shown that the composition of soil texture and organic matterwere the major effect factors in the change of soil saturated hydraulic conductivity. Test alsoproved that the soil saturated hydraulic conductivity was not a constant during the period ofsaturated conductivity, the hydraulic conductivity had been decreasing along with themoisture conduction process, and there was significant difference at the depth0~20cm whichdecreasing rate was42.93%after the eight hours measuring time, the stability of saturatedhydraulic conductivity was low, exacerbating the farmland soil water repellency in somedegree.3、The farming management mode had a profound effect on the dynamic change ofsoil bulk density and compactness, and the modern shallow rotary tillage was the major causeof soil physical degradation. Results show that compared with before treating，subsoiling(SNT) and deep ploughing (PNT) treatments significantly (p<0.05) decreased soil bulkdensity at0-20cm depth, but it increased with rotary treatment (CT) The bulk density withPNT and SNT at0~10,10~20cm were1.35,1.47g cm-3and1.33g cm-3、1.45g cm-3.Compared with treatments SNT and PNT, CT showed the maximum amplitude of seasonalvariation during crop period, which had a low soil structure, and the value was more than theoptimum growth range most of the time. Treatments SNT and PNT were more effective inincreasing soil physical quality (in0-40cm soil layer) than CT. Obviously, treatments SNTand PNT were batter than CT in alleviating soil physical degeneration, improving soilpermeability properties.4、The soil structure stability performance with rotary treatment at the depth0~20 cm was poorer which had a harmful effect on crop root growth. There was a significantnegative correlation between compaction and water content with CT at the depth0~10and10~20cm, the correlation coefficient were0.886and0.854, respectively, while the SNT andPNT treatments had no significant correlation which were0.605,0.622and0.464,0.672respectively. It showed that the compactness with rotary treatment at0~20cm was sensitiveto the change of water content, and soil saturated hydraulic conductivity was lower with largervariation range. These showed the soil structure performance was poor and weak resistance tothe environment with the rotary tillage.5、Farmland crops root growth condition was bad with rotary tillage, it was mainlymanifested in the smaller root density and vigour. Timely using the subsoiling and deeploosening tillage could ameliorate it in the process of wheat grains. It would effectivelyimprove the wheat root vigor at the same time, and had a positive effect on delaying wheatroot senescence.