Predicting the content of soil mineral nitrogen based on the content of calcium chloride‐extractable nutrients

Sustainable management of nitrogen (N) in crop production requires a multifactorial assessment of the soil inorganic nitrogen pool (Nₘᵢₙ). It is assumed that the reliable prediction of the total Nₘᵢₙ content requires data on the content of mineral N forms (NO₃‐N, NH₄‐N), the contents of other extractable macronutrients and the soil pH. This hypothesis was tested during three growing seasons on a production farm in Górzno, Poland. The contents of 0.01 M CaCl₂‐extractable NO₃‐N, NH₄‐N, P, K, and Mg and the pH were measured in soil layers of 0–0.3, 0.3–0.6, and 0.6–0.9 m just prior to the start of spring vegetation of a given crop and immediately after its harvest (autumn). This study was conducted in 17 fields differing in cropping sequence (CS): 10 with oilseed rape (Brassica napus L.) (OSR‐CS) and seven with maize (Zea mays L.) (SM‐CS) as the dominant crops. Principal factor analysis (PFA) was applied to explore and interpret patterns in data sets defined by the changeability in the content of Nₘᵢₙ in association with variability in contents of other CaCl₂‐extractable nutrients. In spring, the first principal factor (PF1) for OSR‐CS was associated with phosphorus (P), whereas PF2 and PF3 were loaded by NO₃‐N and NH₄‐N, respectively. For SM‐CS, PF1 was loaded by both inorganic N forms, whereas PF2 and PF3 were loaded by potassium (K), magnesium (Mg), and P. In autumn, the dominance of P as the key variable associated with the PFs was stronger in both CSs compared with those in the spring. The prediction of Nₘᵢₙ, in spite of the moderate strength of the PFs (“r” coefficients), can be conducted based on the inorganic N content. In spring, the reliable prediction of Nₘᵢₙ for the OSR‐CS requires data on both N forms. In the SM‐CS, the content of NO₃‐N can be used as the sole Nₘᵢₙ predictor. In autumn, the variability in Nₘᵢₙ content can be explained based solely on the NH₄‐N content. This was also the main factor affecting the variability in other soil fertility characteristics, such as the contents of K and Mg and the soil pH.