Some water pollutaants are :

(i) Microorganisms e.g., protozoans, bacteria and viruses

(ii) Eggs or larvae of disease causing vectors like mosquitos.

(iii) Oil spills.

(iv) Fertilizers and pesticides.

(v) Radioactive wastes.

(vi) Industrial wastes.

(vii) Sewage disposals and domestic wastes.

The causes of soil erosion are :

(i) Deforestation - The roots of the plants hold the soil particles tightly and the plants slow down the speed of flowing water thus preventing the soil from erosion. 

(ii) Leaving land uncultivated for long time will cause the soil to erode. 

(iii) Excessive grazing will remove the vegetation cover of the land and cause soil erosion. 

(iv) Flood or heavy rains can wash away the soil and cause soil erosion. 

(v) High speed winds can also carry away the top layer of the soil and cause its erosion. 

Growing of  crops year after year results in the depletion of certain nutrients. The soil becomes deficient every year. If these nutrients are not replenished, the soil will become unfit for cultivation and land may become barren. Hence, replenishment of soil is essential.

The two natural ways of soil replenishment are:

(i) Crop rotation i.e., rotation of crops. In this practice, different crops having varied requirements of nutrients are grown on the same land during different seasons. So that the soil is not depleted of any particular nutrient all the time, often a legume crop e.g., pea, gram, pulses etc. are grown alternately to replenish the soil.

(ii) Leaving the agricultural land uncultivated for one or two seasons.This helps the soil to regain its richness.

Following are the measures that one should take for maintaining the fertility of the soil :

(i) Crop rotation - which ensures efficient use of nutrients by all the plants. 

(ii) Growing of leguminous plants as one of the crop which increases the nutrient content of the soil by fixing nitrogen. 

(iii) Leaving the soil uncultivated for one or two seasons.-  This helps the soil to regain its richness.

(iv) Using manure - Being organic in nature it impreoves the fertility and quality of the soil. 

The term water pollution is used when there is :

(i) Addition of undesirable substances to water bodies. For example, addition of industrial wastes containing poisonous salts like mercury salts, pesticides, insecticides, etc. Addition of disease causing organisms, like cholera causing bacteria through sewage.

(ii) The removal of desirable substances from water bodies. For example, depletion of dissolved oxygen and nutrients which adversely affects aquatic animals and plants.

(iii) A change in temperature of water in water bodies which adversely affects the organisms living in the water bodies.