Determination of Critical Phosphorus Dilution Curve Based on Capsule Dry Matter for Flax in Northwest China

One of the cores of flax (Linum usitatissimum L.) production is to precisely measure the requirement of phosphorus (P) fertilization for optimizing seed yield, grower profits, P-use efficiency, and reducing environmental risk. Therefore, critical P concentration (Pc) was proposed as a suitable analytical tool to assess the flax P nutrition status. Four field experiments, with five P applications (0, 40, 80, 120, 160 kg P2O5 ha(-1)) and four cultivars (Longyaza 1, Longya 14, Lunxuan 2, and Dingya 22) were conducted from the 2017 to 2019 seasons. The capsule Pc dilution curve based on capsule dry matter (CDM) was described by Pc = 2.84 x CDM-0.22 (R-2 = 0.87, p < 0.01), CDM ranging from 0.60 to 4.17 t ha(-1). The P nutrition index (PNI) exhibited a significant positive relationship with P application rate. In addition, the relative seed yield was closely related to PNI. Those results validate that the capsule Pc dilution curve can be an alternative and more rapid tool to diagnose flax P status to support P fertilization precise decisions during the reproductive growth of flax in northwest China.