Farmland Soil Fatigue Status In Guanzhong Area

The returns of soil were decreasing year by year due to the intensive agricultural production, over using of soil and the increasing of chemical matter input to soils, making it a truth that the mechanism of resistance was damaged. Thus, the studies in terms of the mechanism of soil fatigue and preventing measures, specific indicators for soil fatigue and the measures for recover soil from the status of fatigue under modern conditions, have become one of the most important problems in soil science nowadays.1. There was a compact layer in the subsurface and subsoil in Guanzhong farmland universally, becoming one of the most remarkable characteristics of soil fatigue, which was invisible, fraudulent and people always neglected it. Soil bulk density was increased significantly in the subsoil than in the topsoil when the soil bulk density was much stable after the seeds were sown in autumn. More specifically, the soil bulk density was 1.00 g?cm-3 in topsoil, 1.40 g?cm-3 in 10-20cm, 1.61 g?cm-3 in 20-30cm, 1.60 g?cm-3 in 30-40cm. There was a significant difference between each layer. In the 10-20cm soil layer, the samples counted for 88% over 1.30 g?cm-3, exceeding the normal level. In the 20-30cm soil layer, the samples counted for 96% over 1.40 g?cm-3, exceeding abnormal level, counted for 52%, exceeding the limit value of 1.60 g?cm-3 for plants in this kind of soil. In the 30-40cm soil layer, the samples counted for 44%, exceeding the limit value for plants. The hard layer in subsoil and subsurface soil become the obstacle for plant roots extending and water infiltrating.Compared with the subsoil, the water content in top soil was relatively decreased. But, the soil compactness increased sharply in the 0-25cm soil layer, and it kept 1600KPa or so under 25cm. according to existing material, the soil compactness had affected the roots extending. Further study shown that, the cause of the increase was soil bulk density.2. The stability of topsoil aggregates in Guanzhong Area was weak universally. The results of dry-sieving method shown, the amount of aggregates with a diameter of 10-0.25mm counted for 60% or so, which standed for soil fertility. What is more, the amount of the aggregates decreased from the west to the east, and there were significant differences between different districts. However, the wet-sieving method shown the amount of aggregates with a diameter of more than 0.25mm counted for 30% or so, and without any differences between different districts. Compared with the dry-sieving method's result, the amount of the aggregates with a diameter of more than 0.25mm decreased by 1/2 or so, which mean the water-stability of the aggregates in Guanzhong was universally weak and it is prone to become hard, since the soil had a weak body.3. The organic matter content in Guanzhong Area ranged from 6.87 to 15.79 g?kg-1, following a Gaussian distribution. 28.95% of the samples had contents between 10.69 to 11.97 g?kg-1, the dominant interval. Compared with the data of the second general detailed soil survey, the organic matter content increased. The reason should be that the plants amount increased in soils under intensive production of soil, in the way of abating organic matter by inorganic matter. Further more, the organic matter content in 0-20cm increased by 40% than that in the 20-40cm layer, and there was a significant difference. Under 40cm the organic matter content maintained at 5 g?kg-1 or so. In Guanzhong Area, the organic matter content decreased from the west to the east, and this was due to the water and heat conditions in these districts.4. Labile organic matter could coordinate the soil nutrients and resisted plant disease and insect pests. The content of labile organic matter ranged from 2.23 to 5.53 g?kg-1, counting for 28.77 to 42.03% of the organic matter content. The labile organic matter in the 20-40cm decreased by 50% compared with the content in topsoil, and there was a significant difference between them. Under 40cm layer, the labile organic matter content just decreased slightly. In Guanzhong Area, the labile organic matter content decreased from the west to the east, having the same trend with the organic matter content. Further study shown, the correlation between the labile organic matter content and organic matter content reached a highly significant level.5. The calcium carbonate content in the 0-20cm layer was lower than that in the 20-40cm layer. That was to say, there was a phenomenon of calcium carbonate leaching in topsoil, due to the less use of compost, the less turn over of the subsurface soil and dispearation of calcium accumulation. And there was no significant difference between the districts.6. There was no difference of the exchangeable calcium content between the layers in Guanzhong Area. Concretely, the content in Baoji in the west was the highest, followed by Yangling and Xingping in the middle, and Linwei Qu was the minimum. Taking Yangling and Linwei Qu for example, the saturation degrees of calcium were 56.21% and 45.40% respectively, lower than the standard of 60-70% for health soils. Though, the content of exchangeable calcium could satisfy the plants, the availability would decrease as the saturation degree of calcium decreased, affecting the resistance of plants directly.