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Animal manure can be an important resource providing soil available N for plant needs, but determining the nutrient availability resulting from such amendments is difficult. A study was conducted to examine changes in electromagnetic induction (EMI) soil conductivity and available N levels during four growing seasons in relation to manure or compost application and use of a green winter cover crop. With simultaneous soil samples, a series of soil conductivity maps of a research cornfield were generated using a global positioning system (GPS) and EMI methods. The Clay Center, NE, site was treated during a 10-yr period with a winter wheat (Secale cereale L.) winter cover crop (+CC) and no-cover crop (-CC). The site was split for subtreatments of manure and compost at rates matching either the P or the N requirements of silage corn (Zea mays L.). Differences between the +CC and -CC treatments for values of N[O.sub.3]-N and water-filled pore space (WFPS), as estimated by apparent electrical conductivity ([EC.sub.a]), were compared for each year. Differences in profile weighted soil conductivity explained 79.5, 98.0, 93.4, and 98.4% of the variability due to N[O.sub.3]--N differences, and only 20.5, 2.0, 6.6, and 1.6% of the variability due to WFPS differences for years 2000, 2001, 2002, and 2003, respectively. Sequential measurement of profile-weighted soil electrical conductivity ([EC.sub.a]) was effective in identifying the dynamic changes in plant-available soil N, as affected by animal manure and anhydrous ammonia fertilizer treatments during four corn growing seasons.

Organic amendment (OA) addition is an agricultural practice that can greatly affect pesticide behavior in soil. The influence of three OAs, consisting of a liquid amendment (LOA), solid (SOA) humic amendment, and a solid urban waste (SUW), on sorption, persistence, and leaching potential of simazine (2-chloro-4,6-bis (ethylamino)-s-triazine) and 2,4-D (2,4-dichlorophenoxyacetic acid) in a sandy soil (P2) has been investigated. The OAs were added to P2 at a rate of 5% (w/w). Sorption was studied using the batch equilibration technique, degradation by incubations at 20 [degrees] C and -0.33 kPa moisture content, and leaching using handpacked soil columns. Simazine sorption increased in the order of P2 [less than or equal to] P2 + LOA < P2 + SUW < P2 + SOA. The herbicide, 2,4-D, did not sorb on P2 soil, but in amended soils sorption increased in the same order as simazine. The lower sorption of both herbicides in P2 + LOA as compared with P2 + SOA and P2 + SUW is attributed to higher dissolved organic matter (DOM) content of LOA. Simazine's half-life was reduced upon amendment. Degradation of 2,4-D followed a sigmoidal rate in P2, P2 + SOA, and P2 + SUW, but not in P2 + LOA, in which 2,4-D persistence was much higher. The higher stability of 2,4-D in P2 + LOA has been attributed to 2,4-D-dissoved organic C interactions. Leaching studies indicate that degradation affects movement to a higher extent than sorption, specially in the case of 2,4-D in P2 + LOA soil, in which the amounts leached are much higher than those in unamended soil. In the case of simazine and P2 + SUW soil, greater reduction in large-size conducting pores upon amendment, as revealed in Hg porosimetry studies, resulted in a greater reduction of leaching than that suggested from the small differences in sorption and degradation between P2 and P2 + SUW soil.