Soil structure

Soil structure refers to aggregates. Aggregates are formed as a result of the arrangement and grouping of soil particles. Sand and gravel aggregates do not demonstrate cohesion very well. In most good agricultural soils, aggregates that stick together form readily. The structure of the soil aggregates forms when colloidal particles clump together. The clumping is usually a result of the activities of soil organisms and temperature changes resulting in freezing and thawing. Without a coating of organic matter, the individual granules will continue to clump until they are clods.

Good agricultural soils are granular with many pore spaces, occupying between 40% and 60% of the total soil volume. Recall how important pore spaces are to aeration and drainage. Air and water are trapped within these pores and if the pores are numerous but small and poorly connected, as in clay soils, the restricted movement does not allow for good drainage or aeration. Furthermore, when the pores are full of water and air is not able to move in, plant roots suffer as there is not enough oxygen for root growth. Sandy soils, on the other hand, have their problems as well. For one, the large pores allow gravity to drain water out of the soil as soon as the pores are filled. As the water drains, the pores fill with air, which in turn speeds up nitrogen released by microorganisms. Much of the nitrogen released is lost as plants cannot utilize nitrogen that quickly.

Contrary to popular habit, watering can severely damage plants. Mainly, too much water leaches minerals and slows mineralization processes with anaerobic conditions (due to the reduction or lack of free oxygen). Overwatering can slow the release of nitrogen, which interferes with plant growth and speed up the breakdown of nitrates.