Page 23 - December Newsletter

Nitrogen availabilityWhat is the crop availability of N shortly following poultry litter application?In the case of N, it is important to consider that this nutrient is primarily in the organic form in poultry litter (upto 75-80% organic N). Organic N needs to be mineralized before becoming available to crops. A fraction of this or- ganic N may become part of the soil organic matter pool and unavailable to crops in the short term.Field and laboratory studies suggest the fraction of total nitrogen that becomes plant available the first year ofapplication is approximately 45-55%, which includes both the inorganic N in the manure and a percentage of the or- ganic N. This value varies depending upon components in the litter and the method of handling and application. For example, poultry litter that contains a large fraction of bedding material will tend to have lower N availability in the year of application. Reduction in N availability may also occur when litter is aged and has undergone some compost- ing. Nitrogen lost from the volatile ammonium fraction at the time of application on the soil surface can also reduce plant available N. Ammonium volatilization is typically higher during windy and warm days. Incorporation of litter im- mediately after application will reduce volatilization and potential nutrient loss by water runoff in case of a rainfall event, in addition to reducing the odor of the litter.If the manure is applied to pastures, the percentage of N utilized by the forage in the first year will depend onwhether the pasture consists of cool-season or warm-season grasses. For cool-season grasses, such as fescue pasture, N utilization will likely be less than 50% in the first year. Most of the growth in cool-season pasture occurs early in the year. The microbial community will not mineralize as much N early in the spring as later in the summer. Fall applications may result in better N utilization for fescue than winter or spring applications. For warm-season grasses, such as bermudagrass pasture, nitrogen utilization from manure will likely be close to 50%. In both cases, producers should base application rates on the P and K needs of the grass and supplement additional N fertilizer to meet the N needs of the grass.Phosphorus and Potassium AvailabilityWhen manure is applied to the soil, what percentage of the phosphorus and potassium is available to the crop during the first year?A large fraction of the P in manure is considered to be plant-available immediately after application. The frac- tion that is not plant available shortly after application will become available over time.Estimated values of P availability are from 50 to 100%. This range accounts for variation in sampling andanalysis and for P requirements with different soil test levels. Use the lower end of the range of P availability values (50%) for soils testing “Very Low” and “Low” (below 20 ppm). In these situations, large yield loss could occur if insuf- ficient P is applied and soil P buildup is desirable.On the other hand, use 100% availability when manure is applied to maintain soil test P in the “Optimum” soil test category and when the probability of a yield response is small.Several studies have shown that manure P is a valuable resource that is comparable to inorganic fertilizer Pfor crop production. These two P sources are similarly effective when the manure P concentration is known, and the manure is applied properly. Nevertheless, excessive application of manure P (e.g., at rates sufficient to meet the crop’s N needs) often results in excessive soil P buildup, resulting in a higher risk of surface water contamination.This problem of excessive P buildup in the long term can be minimized by:• Applying manure to meet the P needs of the crop and using inorganic sources of fertilizer to complement nitro-gen needs,• Constantly monitoring soil test P levels, and• Using the P-index to assess the potential impact of P buildup on water quality.Producers should consider actual P application rates and not just tons per acre of manure being applied. Uniformapplication of manure at precise rates can also be difficult. Careful calibration of manure applicators is needed. If these aspects are not considered, the efficiency of manure P compared with inorganic fertilizer P may be re-duced. Careful management pays off.Availability of potassium is usually near 100% with proper application. Poultry litter can also provide sig- nificant amounts of secondary and micronutrients.