1. Urban soil characteristics
1. 1 The soil has no layers.
Human activities produce all kinds of wastes. In the past, many times, underground construction invaded and turned over the soil in disorder, which destroyed the original soil surface layer or humus layer representing soil fertility and formed a layered and irregular soil structure.
1.2 The soil is dense and the structure is poor.
Urban soil is low in organic matter and organic colloid. Under the action of mechanical and artificial external forces, soil particles are squeezed, and the soil density is high, which destroys the aggregate structure with good water permeability and forms a compact, hardened, flaky or massive structure with poor physical and chemical properties.
1.3 There are many soil intrusions.
A large number of sundries, underground structures and pipelines are mixed into the soil, occupying underground space, which changes the three-phase composition, pore distribution, water, gas, heat and nutrients of the soil.
1.4 soil nutrient deficiency
Most of the fallen leaves of garden plants in urban areas are transported away or burned, which makes the soil unable to return to the roots to recycle nutrients like the natural soil in forest areas. Under the condition that there is basically no nutrient supply in the soil, a large number of invaders occupy a certain amount of soil, which leads to insufficient nutrient area for plant growth and reduces the absolute content of water, gas and nutrients in the soil. Plants grow on this kind of soil and absorb nutrients from the limited nutrient space every year, which is bound to make the urban soil more and more barren.
1.5 soil pollution
Laundry water, vegetable soup, grease, acid and alkali salts and other substances produced by urban human activities enter the soil, which exceeds the self-purification ability of the soil and causes soil pollution. In recent years, some cities use 10%-20% sodium chloride as the main road deicer, and the melted salt water has become a new pollution source that affects the survival of plants.
2. The influence of urban soil on the growth of garden plants
2. 1 Effect of soil compactness on the growth of garden plants
Soil compactness is also called compactness or soil compactness. The density of urban soil is obviously higher than that of suburban soil, which is a main feature of urban soil. In general, the impact depth of the tread area is 3- 10cm, the soil hardness is 14-28kg/ cm2, the impact depth of vehicle rolling is 30-35cm, and the soil hardness is 10-20kg/ cm2. In some special sections, the impact depth after mechanical multi-layer compaction can reach more than1m.
The permeability, drainage and water-holding capacity of soil are restricted by soil compactness. With the increase of soil compactness, the aeration porosity decreases, which leads to the decrease of soil permeability, gas exchange, poor tree growth and even suffocation of root tissue. In areas with dense soil, due to the difference of soil density inside and outside the tree planting pit caused by excavation and backfilling, the roots of trees often cannot penetrate through the dense soil layer outside the pit, forming abnormal distribution around the wall of the tree planting pit, and the growth status of trees also deteriorates. In addition, the vertical leakage and horizontal diffusion of water in the pit are blocked after irrigation or rainfall, resulting in water accumulation in the pit, which may lead to the death of rotten roots of trees. At the same time, with the increase of soil density, the mechanical impedance also increases, which hinders the extension of tree roots.
Dense soil will reduce the amount of mycorrhiza in some trees. Mycorrhiza produced together with tree roots can enlarge the root surface area of absorbing water and salt by 100- 1000 times, and can provide additional inorganic salts, especially increase available nitrogen, and improve the nutritional status of trees. The inhibition of urban soil slab on mycorrhiza makes the adaptability of trees with ectomycorrhiza such as pine, spruce, fir and larch and trees with endomycorrhiza such as Ginkgo, Cupressaceae, Taxodiaceae, Acer, Platanus and Ulmus decline.
The restriction of soil compactness on root growth often causes trees to change the root distribution characteristics, many deep-rooted tree species become shallow roots, and most tree species support the reduction of root system, thus weakening the stability of trees and easily lodging due to urban mechanical factors such as strong winds.
In a word, plant roots live in dense urban soil conditions, and their physiological activities are reduced and their life span is short, leading to rotten roots and dead roots. The aboveground part can't get enough water and nutrients, and there are dead branches and scorched leaves. In this way, they grow worse year by year until they die. On the contrary, when the soil hardness is less than 0.8 kg/cm ~ 2 and the soil bulk density is less than 0.9 g/cm ~ 3, the soil water and nutrients are scarce, the roots grow thin and weak, and dead roots appear. Only loose soil with moderate soil density, soil hardness of 0.8-8kg/ cm 2 and soil bulk density of 0.9- 1.45g/ cm 3 is suitable for water, fertilizer and gas, and trees are easy to take root, with developed roots and flourishing branches and leaves.
2.2 Effects of soil nutrients on the growth of garden plants
The fallen leaves and branches of plants in cities are often removed as garbage, which makes it difficult to return to the soil, which interrupts the nutrient cycle of the soil and has a low organic matter content. According to the determination, the content of organic matter in most urban soils is below 65438 0%, which is the main source of soil nitrogen, and the decrease of organic matter directly leads to the decrease of nitrogen. In most soils, the available nitrogen is less than 30ppm, and the available phosphorus is less than 65,438+05 ppm. Compared with suburban soil, nitrogen.
Plants need more than 16 kinds of essential nutrients, most of which are provided by soil. Water-soluble inorganic salts absorbed by plant roots from soil are the basic substances for the formation of chlorophyll, various enzymes and pigments, and also the activator of photosynthesis, especially low nitrogen will hinder photosynthesis. The lack of nutrients in urban soil greatly reduces the carbon growth of urban plants. Coupled with factors such as poor ventilation and lack of water, the growth of urban plants is lower than that of similar plants in suburbs, and their life span is shortened accordingly.
2.3 the impact of urban soil moisture on the growth of garden plants
The water content in plants is 60-80%, which becomes an important part of plants. The water needed by plants mainly comes from soil. However, soil moisture mainly comes from atmospheric precipitation and artificial water replenishment, and is stored in soil pores. Among them, available water exists in capillary holes and moves up and down in the pipeline under the action of capillary force, rising to the field capacity and falling to wilting coefficient. The water content suitable for plant growth should be 60-80% of soil field capacity. Soil moisture content is related to soil debris content, soil compactness, ground pavement, distance from surface water and groundwater level. Urban soil has a high density, contains more impurities and other inclusions, and the road surface is closed, so it is difficult for natural precipitation to penetrate into the soil, and most of it is discharged into sewers, resulting in that natural precipitation cannot fully supply trees to meet their growth needs, while underground buildings go deep underground, which makes it difficult for roots to approach and absorb groundwater, so the soil water content is low, and the water balance of urban plants is often negative, which in turn leads to poor growth, early defoliation and even death.
2.4 the influence of urban soil air on the growth of garden plants
The oxygen in soil air comes from the atmosphere. Due to the closed pavement and pavement, urban soil hinders gas exchange, the soil is dense, the pores of non-capillary used for gas storage are reduced, and the soil oxygen content is low. Plant roots generate energy by breathing soil oxygen to maintain physiological activities. Due to insufficient oxygen supply in soil, root respiration is weakened, which has a negative impact on root growth. According to the investigation, when the porosity of soil aeration is reduced to 15%, the growth of root system is hindered. When the porosity of soil aeration is reduced to below 9%, the root system is seriously deprived of oxygen, and alcohol accumulates during anaerobic respiration, resulting in the death of root system poisoning. At the same time, due to the lack of oxygen in the soil, the reproduction of microorganisms in the soil will be inhibited, and the nutrients released by microorganisms will be reduced, thus reducing the effective nutrient content in the soil and the utilization of nutrients by plants, which will directly affect the growth of plants.
2.5 the influence of urban soil temperature on the growth of garden plants
Soil temperature mainly comes from solar radiation and heat generated by coal combustion. Soil temperature fluctuates due to the daily and annual changes of climate. In the urban environment, because of the different orientation of buildings and different soil temperatures, it has an impact on plant growth. In northern cities, the soil temperature in the north of the building is lower than that in the south of the building, and the freezing period in winter is longer. The growth and phenology of trees in spring and autumn are obviously different from those in other places. In spring, the temperature gradually warms up, and the vegetative organs on the ground begin to move, while the underground roots are still in the frozen soil layer, resulting in the loss of water and peeling of the branches and leaves of trees on the ground; In autumn, due to the decrease of air temperature and soil temperature, trees shed their leaves in advance. In addition, when the pavement is paved under direct sunlight, the surface soil temperature is as high as 50℃ in summer, which causes the surface roots to burn and lose their vitality.