Every gardener and farmer knows that replacing nutrients is key to successful crops year after year, so you and your lecturer are correct to focus on this.
We have used organic practices in our garden for many years, and our crop yields increase each year (for the most part). Last year was our best ever, and today as I look out across the garden, this year will also be successful. We have never put a drop of chemical fertilizer into the soil. Nutrients are restored by several methods – the use of mulch which conditions the soil and adds nutrients as it breaks down, the addition of manure when available, some use of canola meal, and most importantly, using green manure between crop rotations. One fourth of our garden is planted in green manure at any given time.

Phosphorus is mined and shipped in the form of rock phosphate, which is essential to plant growth. The global supply of phosphorus is diminishing, and some farmers are looking for alternatives. In Germany there is a movement to process urine for its phosphorus content. We use rock phosphorus, but we also keep a pee bucket and dilute the urine 50/50 with water, then add it to our beds. This provides additional phosphorus.