Phosphorus (P) is another key member of the “big three” primary nutrients. Phosphorus is involved in many processes critical to plant development. Key among them is photosynthesis, where plants convert sunlight to energy. Phosphorus is also important to respiration, cell enlargement, cell division, energy storage, and energy transfer. This makes it a principal to the quality of the plant and the fruit it produces. P also moves easily from older plant tissue to new plant tissue. This is one way farmers can tell if their crops are deficient in phosphorus; they can look at the bottom (older) parts of the plant for signs of poor health.
Although phosphorus is common in the soil, most of it is inherently unavailable to plants. Only about four pounds of phosphorus per acre is an available form in most soil types. Farmers have to make up the rest of a crop’s phosphorus needs with fertilizer. Studies by crop fertility experts indicate that phosphorus deficiency may be more responsible for below-optimum crop yields around the world than other nutrient deficiencies or plant health problems.
As plants mature, phosphorus moves into the seed and fruit, helping the cells divide and grow in a regular and healthy manner. Cell enlargement and division directly affect crop size and weight (yield) and crop quality in the form of its shape and appearance, especially in the fresh produce market for fruits, vegetables, and ornamental plants.
As with nitrogen, phosphorus is lost from the soil through plant harvest and can be lost by surface water erosion. Unlike nitrogen, however, phosphorus moves very little in the soil. This makes placement of P absolutely critical. Research has shown that plant roots come into contact with only about three percent of the soil particles in the volume of soil in which the plant grows. The roots may never contact soil particles as little as an inch away from the seed or root.
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