The fertilizer was added while the window was open, allowing air currents to interfere.

You: ‘Dr Smith the window is open!’

Dr. Smith moves to stand beside the open window near your lab bench. “Good observation!  Drafts, even gentle ones, can affect the performance of an analytical balance. This is particularly true when we keep the doors of the balance open during weighing. Air currents create micro-fluctuations, causing the reading to drift or plateau at an inaccurate weight. Closing the window or shutting the balance doors until a stable reading is achieved usually resolves this.”

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