It is all based on knowledge of what this or that plant does and needs. Take a typical white clover, typically white clovers will fix 150-200 pounds of nitrogen per acre per year. Legumes (like clovers) take in atmospheric nitrogen (N2) from the air and convert it to ammonia(NH3). To equal 150 pounds of N per acre, you would have to apply 15 bags of 10-10-10 per acre. Fertilizer analysis is in amounts of N-P205-K20. So in 100 pounds of 10-10-10 you only have 10 pounds Nitrogen, 4.4 pounds Phosphorous, and 8.3 pounds Potassium. The rest of the bag is just the carrier that does your soil no good. You are paying a lot for a little. To ensure your legumes are actually fixing nitrogen, just dig up a plant and inspect the roots. Look for nodules where the nitrogen fixing Rhizobium bacteria is and cut them open. If they are of a pink or reddish hue, then they are actively fixing nitrogen.
So a good cultural practice for adding nitrogen to the soil would be planting legumes on rotations. Take a 1 acre field and divide in two or whatever. Grow a legume on half for a few years, then rotate to the other half, meanwhile planting a nitrogen lover such as wheat or brassica ,etc in the half you just had the nitrogen fixer in. When you begin to notice signs that the N in that field is being depleted ( increased yellowing of plants, less growth, etc), then rotate back to a legume crop on that section. You could soil test every year to see how well your plan is working and when you need to rotate as well but knowing the symptoms is just easier to me.
Lets say you have a Phosphorous problem. Instead of adding synthetic P, grow some buckwheat. Buckwheat �mines� the soil for P that is already present and will bring up to where it can be available for future crops to use. In addition to that, the plant dying and decomposing will add organic matter to your soil.
Adding organic matter is one of the most important things. You achieve this through decomposition. So grow plants high in biomass such as buckwheat, millet, sorghum, annual weeds, etc, etc and allow them to die and decompose. Tilling in can be a good idea but you lose a lot of nutrients when you till through erosion by water and wind. Agriculture has gone away from the use of manure as a fertilizer due to the ease of synthetic fertilizers and that is just too bad. Manures add your essential elements to the soil and increase your organic matter. Build organic matter levels and you will increase your moisture retention, which of course is a great thing for us food plotters. As soon you till that soil, moisture starts being lost at a more rapid rate.
Unfortunately though, most people just want to go buy a bag or 10 of fertilizer and throw it out because it is so much more simpler than learning all of this and becoming a better steward of the soil and the land. There is a lot more to go on in regards to becoming a better steward of the soil but honestly it takes whole books to do it. I can add in what knowledge I have as necessary if anyone even cares.
