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From page 431... ... Nutrient budgets and mass balances have been approached on various scales, ranging from experimental plots to estimates of global balances (i.e., Follett et al., 1987; Hauck and Tanji, 1982; Meisinger and Randall, 1991; Power, 1981; Thomas and Gilliam, 1978~. Nutrient budgets can be approached at various levels of detail and with varying degrees of completeness. Read the entire page →
From page 432... ... note: "These are also the sites where improved Ntnitrogen] -management practices will have the Greatest chance of improving groundwater quality." v ESTIMATION OF STATE, REGIONAL, AND NATIONAL BUDGETS The committee did not attempt to estimate complete mass balances of the nitrogen and phosphorus flux in U.S. Read the entire page →
From page 433... ... by legumes, and the nitrogen and phosphorus content of crop residues. Estimates of inputs were limited to primary sources since these nitrogen and phosphorus inputs can be directly affected by management and since it was desirable to limit the amount of computation required. Read the entire page →
From page 434... ... The nitrogen in irrigation water, generally available as nitrate, can be an important management consideration locally (see Chapter 6, and Schepers and Mosier, 1991~. It was not possible to characterize irrigation water nitrogen inputs for this analysis. Read the entire page →
From page 435... ... The proportion of the total nitrogen and phosphorus voided in manures that can be economically recovered varies depending on collection, storage, and application methods. Therefore, the manure nutrient production of each state was adjusted by Read the entire page →
From page 436... ... and are given in Tables A-2 and A-3. Only about one-third of the total nitrogen voided in manures was estunated as recoverable, while about one-half of the total phosphorus voided in manures was estimated as recoverable. Read the entire page →
From page 437... ... Livestock and Poultry Manure Production. Report ESCS-12. Read the entire page →
From page 438... ... LaRue and Patterson (1981) summarized published research and concluded there is not a single legume crop for which valid estimates of the nitrogen fixed in agricultural production were available. Read the entire page →
From page 439... ... To minimize environmental losses of nitrogen and to optimize crop yields, some estimate of the legume contribution to crop rotations must be made. To account for the combined effects of rotation and fixation, legume benefits are often estimated as a fertilizer nitrogen replacement value or a fertilizer nitrogen equivalence. Read the entire page →
From page 440... ... The amounts of nitrogen harvested in alfalfa hay and in soybean grain and residues were entered on the output side of the balance. The difference between the total alfalfa or soybean nitrogen input and the alfalfa or soybean nitrogen output was used as an estimate of the residual nitrogen replacement value potentially available to a succeeding crop. Read the entire page →
From page 441... ... Estimation of Outputs Only the primary desirable nutrient outputs were estimated by the committee, that is, the nitrogen and phosphorus taken up in the harvested crops and in crop residues. Other outputs include undesirable losses into the environment through ammonia volatilization, denitrification, soil erosion and runoff losses, and leaching losses. Read the entire page →
From page 442... ... These outputs are small relative to other outputs and typically have been implicitly included in nutrient-crop yield response models. Harvested Crops The desired nutrient output from an agricultural ecosystem is the nutrient taken up in the harvested crop. Read the entire page →
From page 443... ... Pp. 221-246 in Nitrogen Management and Ground Water Protection, R Read the entire page →
From page 444... ... Estimation of Balances The committee's estimates, as discussed earlier, are partial nutrient balances for harvested cropland. The balance, or residual term represents an estimate of the amount of nitrogen and phosphorus inputs that (1) Read the entire page →
From page 445... ... 445 oo En En a; ._ .= cn o U o V) Read the entire page →
From page 446... ... The committee's estimates of inputs are probably low, particularly the estimates for phosphorus and the estimates in the low and medium nitrogen scenarios, since no effort was made to account for the multiyear contributions or buildup of nitrogen and phosphorus over time. Standard assumptions were used by the committee to estimate nitrogen and phosphorus inputs from manures. Read the entire page →
From page 447... ... However, more than 35 percent of the total nitrogen in harvested crops is accounted for by legumes, which receive very little nitrogen fertilizer. Major commodities such as corn, cotton, potatoes, rice, and wheat account for more than 80 percent of the nitrogen applied in fertilizers, but the nitrogen harvested with these crops accounts for only 57 percent of the fertilizer nitrogen input. Read the entire page →

From page 431...

... Nutrient budgets and mass balances have been approached on various scales, ranging from experimental plots to estimates of global balances (i.e., Follett et al., 1987; Hauck and Tanji, 1982; Meisinger and Randall, 1991; Power, 1981; Thomas and Gilliam, 1978~. Nutrient budgets can be approached at various levels of detail and with varying degrees of completeness.