AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Winter 2004, No. 5
Animal manures are lower in total phosphorus concentration...and portion that is water-soluble phosphorus (0.4 to 5% water-soluble)...compared to commercial fertilizers, yet manures may also deliver significant amounts of water-soluble phosphorus when applied at relatively high rates.
In most soils, when phosphorus is applied at agronomic rates, it moves very little: only a few inches from the depth of application or tillage within 5 to 10 years, or more. Excessive transport of phosphorus from land surfaces can pose a significant water quality risk to nearby streams. Surface runoff in grasslands may be dominated by soluble inorganic phosphorus. In tilled (plowed, disced, cultivated) fields, more of the surface runoff loss of phosphorus may be associated with soil erosion.
Natural nutrient loss of phosphorus to surface waters before human settlement was estimated by the National Research Council at about 0.4 lb/A/year. The loss of phosphorus from farm fields, forest lands, and lawn and turf areas depends on the phosphorus content of the soil, the amount of crop residues and their decomposition rate, tillage intensity and timing, soil slope, crop and soil management factors, rainfall (frequency, intensity, and duration), and other environmental factors…including the rate of erosion.
Maximum crop uptake of applied phosphorus and minimum loss to water resources can be encouraged in several ways.
• Prevent or correct soil compaction to enhance water infiltration and crop use.
• Provide other essential nutrients in appropriate balance with phosphorus.
• Manage irrigation water effectively to maximize crop use and to minimize runoff.
• Use agronomically appropriate rates, timing, and placement of phosphorus.
• Avoid surface applications of phosphorus sources (including manure) when there is significant risk of runoff-producing rains, especially where applied phosphorus will not be soil-incorporated (such as in no-till cropping, pastures and hay meadows, turf and lawns, and in established forests and forest plantations).
• Band P applications beneath the soil surface (where practical and economically feasible). This application method can reduce the risk of runoff loss during storm events. However, care must be taken to ensure that grooves caused by placement equipment do not run up-slope and down-slope in a way that could aggravate or accelerate channelized flow of water. Channelized flow in phosphorus application grooves could potentially increase phosphorus runoff loss.
• Establish and maintain vegetative buffers and riparian areas. Keep stream banks vegetated and stabilized.
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