Phosphorus is a macro nutrient that plants need to grow strong and healthy. In the cannabis world it has been long held that you need a boost of phosphorus in flower, but we have found that is lesser needed than we use to think.
That being said, at some point you will need to replenish depleted soils with a new source of phosphorous. The best way to know how much you need, and how depleted your soil is, is with a soil test.
When you get a soil test you can pay extra for recommendations. Places like A & L Laboratories, and Logan Labs will give you a recommendation based off of the results of your test. It is typically only a few extra dollars, and takes a lot of guess work out of the situation.
For the home gardener a local county extension office may be a better choice as their test may only run $10 or so. They won't come with a recommendation, but you will at least get a decent window into what is going on in the soil.
When you do get your test results back and more phosphorous is indeed needed, what are some of the best organic soil amendments to get the job done?
Organic Soil Amendments That Provide Phosphorus
Bone Meal – Bone meal is exactly what the name implies, crushed up bones. Usually bone meal is made from some sort of factory farmed animal whether it be cows, pigs, or the like. It is widely available in stores, and coops, and very reasonably priced. The down side to some organic gardeners is that factory farms seem to like to use hormones and antibiotics on their animals. This doesn't even address the possible issues with mistreating the animals, and how their lives are spent.
High Phosphorous Bat Guano – Bat guano has been used in the cannabis world as a High P amendment for what seems like forever. I personally use to use it for many years. It is very fast acting, not super expensive, and available online and in many grow stores. It usually has enough nutrients to last 21 to 28 days max. The down sides of bat guano is that when it is harvested the bats habit may be harmed or even destroyed. This could lead to displacing or even killing the bats just to harvest their poop. The other thing is it is usually in a powder form, and that powder can get in your lungs. Histoplasmosis is a lung disease you can get from inhaling bat poop, and the powder form makes it get deeper into your lungs. In recent years companies have begun to pay attention to how their harvesting techniques may effect the bats habitats, and the bats themselves.
Rock Phosphate – Rock Phosphate is a mined mineral that contains a large amount of phosphorous. It is not as readily available as some other forms of phosphorous, but long term it's not a bad option depending on your situation. The P in rock phosphate can take upwards of 12 months to become fully available, so keep this in mind when choosing it for your garden. There has been talk for many years that we are depleting our rock phosphate reserves, but it keeps getting produced year after year anyway. Rock Phosphate is available in most garden stores, and coop's across America.
Seabird Guano – Seabird guano will be similar to using bat guano. As far as I know there is not the same damaging effects to their habitat. It too is fast acting, and a very highly available source of phosphorous. For safety's sake I treat seabird guano like bat guano and choose not to use it though.
Fish Bone Meal – My all time favorite, fish bone Meal, is made of fish waste and contains a large amount of bones. The P won't be as available as say guano, but doesn't take near as long as rock phosphate. It usually will start to be available in a matter of weeks, and can last many months in your soil. I like the fact that is is a waste byproduct that would other wise be useless. The downside of fish bone meal is it's not widely available, and is a specialty product in most areas of the world.
Phosphorous Solubilizing Microbes – This will be newer to some gardeners, but sometimes your soil has enough phosphorous in it, but it's just not available to your plants. That's where microbes come into play. There are microbes that will mine phosphorous and make it available to your plants roots. This is a good place to start, and a key reason why it's important to pay attention to the microbial population in your soil.
Whether you are growing vegetables, flower, or even cannabis, having the right nutrient balance in your soil is key to a successful garden. Getting a soil test is always a good idea, and remember that balancing your soil can take months or even years depending on your situation. Be patient.