Before the 19th century, it was believed that plants got their nutrients from decaying organic matter, otherwise known as humus. In the 19th century, a new theory by the German chemist [[Justus von Liebig]] held that plants only needed a few essential minerals such as calcium, phosphorus and potassium, which they absorbed through their roots. (This led to the use of bird guano from the South Pacific Islands as fertiliser.) What happened to the belief that plants got their nourishment from humus (organic soil matter)? First, there was the discovery of photosynthesis, which showed that plants got their carbon from the air. Then it was found out that humus was insoluble, so it could not be absorbed through the roots of the plants. It wasn't until the 1930s that the humus theory was revived (though still neglected), when Sir Albert Howard, an English agronomist working on commercial plantations in India, observed that composting contributed to healthy crops and bountiful harvests. He formulated the Law of Return, which argued for the importance of returning organic material to the soil. He also suspected that it was mycorrhizal fungi which gathered nutrients and fed them to the roots of the plants. (Mycorrhizal fungi are those which live in simbiosis with plant roots.) --- Source: [[Growing a Revolution Bringing Our Soil Back to Life]]