Next+to+Soil+Fertility

=**Soil Fertility Is Determined by Soil Structure and Nutrient Retention**= ====Soil is either sand, silt, or clay. Sand particles are the largest, and clay particles are the smallest. Soil often occurs as a series of layers called **soil horizons.** The deepest horizon, which is just above the solid rock, is known as __//substratum//__. Substratum is when rock is just beginning to disintegrate into the soil by water that has seeped down to that level. In this level, there is no growing plant life. Above the substratum is the //__subsoil__,// which is mostly clay. Only the deepest roots reach this level, which can be up to one meter thick. Above the subsoil is the __//topsoil//__, which is on the surface and varies in thickness. This level usually contains sand, silt, and clay in equal amounts. This level also is where plants absorb most of their nutrients.====



====In order to topsoil to be fertile, it needs to be a mixture of at least four components--mineral particles, water, air, and organic matter. Many of the nutrients plants need are released as the particles are formed from the erosion of rock. The particle size affects the soil fertility. The larger the particles, the more porous it is which creates pockets of space that collects water and air. Up to 25 percent of the volume of fertile topsoil consists of pockets. The pockets allow the roots to absorb water and oxygen. The small particles pack tightly together, which results in none or very few air pockets and that means that there is little or no oxygen or water available for the roots. This is why plants do not grow well in clay.====



====The organic matter in topsoil is a mixture of fallen plant material, the remains of dead animals, and bacteria and fungi. This organic matter is called **humus,** and it is rich in a variety or nutrients. Humus is porus, which gives plants access to water and oxygen. It also helps to prevent erosion by binding the soil. The water flow is called **percolation.** The more porus the soil, the greater rate of percolation. If there is excessive percolation, flowing water removes many water-soluble nutrients needed to make the soil productive. This is known as **leaching**. With too little percolation, topsoil becomes water logged, choking off a plants supply of oxygen. Soils with optimal percolation drain water from all but the smallest air pockets.====



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