Erosion is the process by which materials such as soil or sediments are moved from one place and deposited in another place. Erosion usually occurs because of wind, water, ice or gravitational pull of sediments downhill, a process known as creep. Salinity is the measure of salt in a soil. Salinity is due to sudsidence of the water table and then salt water moving into the soil and because of capillary action, it is brought to the surface. Salinity also increases because of the use of fertilizers applied to crops. Compacted soil occurs when a stress is applied to soil and the pressure causes the air to be displaced from the pores between the grains of sediment in the soil. This squeezes or compacts the soil particles together. Compacted soil makes it difficult for water to permeate and thus is not optimal for the growth of plants. Soil becomes acidified when a proton donor is added to the soil. This can be due to the addition of an acid, or nitrogen compounds from fertilizers. Even aluminum sulfate in soil can release protons. Leaf litter can cause acidic soil because there may be organic acids in certain leaves. A build-up of dangerous chemicals is known as soil pollution. These can include salts, radioactive materials, toxic compounds and even disease causing agents. Heavy metal, pesticides, solvents and seepage from land-fills can cause soil pollution. Soil is a natural resource. It can take between 800-1000 years to make a 2.5 cm layer of fertile soil. Therefore, it must be conserved and protected.