. Efficient use of nitrogen on crop land in the Northeast . ± .14 .16 .18 .20 22 .24 ABSORBANCE .26 .28 .30 .32 Figure 12. Nitrogen supplying capacity of soil vs. ultraviolet absorption of soil extract. ( Fox, Pennsylvania) The double bonds in organic matter absorb UV light near 260 nm; the absorption by the extract is a mea- sure of the organic matter content of the extract (Rao 1967). Further, the concentration of N in the organic matter is assumed to be quite constant from soil to soil (Bremner 1949). In support of this assumption the coefficient between the UV absorption at 260 nm and
. Efficient use of nitrogen on crop land in the Northeast . ± .14 .16 .18 .20 22 .24 ABSORBANCE .26 .28 .30 .32 Figure 12. Nitrogen supplying capacity of soil vs. ultraviolet absorption of soil extract. ( Fox, Pennsylvania) The double bonds in organic matter absorb UV light near 260 nm; the absorption by the extract is a mea- sure of the organic matter content of the extract (Rao 1967). Further, the concentration of N in the organic matter is assumed to be quite constant from soil to soil (Bremner 1949). In support of this assumption the coefficient between the UV absorption at 260 nm and the N concentration of the M NaHCO:j ex- tracts was Therefore, it follows that if the N con- tent of the extract was correlated with the N supply- ing capability of soil, the UV absorption by the ex- tract would also be correlated with this capability. The equation, Nj = Np - N, E , proposed by Stan- ford (1973) illustrates how this test could be used to predict N fertilizer requirements of a crop of corn. Assumptions to be made for a com crop follow: Np (kg/ha) = X (maximum grain yield ex- pected, metric ton/ha, HoO) N, (kg/ha) = 773 x (UV abs at 260 nm of m NaHCOa extract) - 74 E = Nf (kg/ha), maximum = 20 X (maximum grain yield expected, metric ton/ha, HoO) minimum = 15 in starter fertilizer The Np factor was calculated from the ratio of the grain yield to total crop N at the lowest N rate that produced maximum yields in the N response experi- ments (Fox & Piekielek 1978a). The equation for N^ was derived from the regression between the UV ab- sorbance of the M N9HCO3 soil extract and the N supplying capability of the soil shown in Figure 12. An E of was fairly representative of the fer- tilizer N efficiency that can be expected at N fer- tilizer rates adequate for maximum yields with good crop and fertilizer management. Using the above equation and the data from the 1977 corn yield response to N fertilization experiments, t
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