Bone Meal for Phosphorus: When It Works

Bone meal for phosphorus works when a soil test shows low phosphate and soil pH is below about 7. In that setting it supplies phosphate slowly over months, with roots, flowers and fruit gaining most. In soil above about pH 7 most of its phosphorus stays unavailable, so the same amount gives little benefit.

How Bone Meal for Phosphorus Works in Slightly Acid Soil

Bone meal is ground bone, described on the calculator page as mostly phosphate. Apply it to raise phosphate, not to feed leafy growth or to supply potash.

Phosphorus supports root growth, flowering and fruiting, so a soil test that shows low phosphate gives you a reason to act, and adequate phosphate gives you a reason to stop. The calculator page states that bone meal works best in slightly acid soil, below pH 7. In alkaline soil the phosphate stays locked in forms roots cannot take up easily. In slightly acid soil it releases more readily, still slowly, over months, and cold soil slows release further.

What the Iowa State Analysis Means

The calculator page gives no numbered analysis for bone meal, only blood meal at 12 percent nitrogen. Any bone meal number here comes from a named outside source, not from the calculator.

Iowa State University Extension, in Selecting and Using Organic Fertilizers, lists steamed bone meal with an average analysis of 1 percent nitrogen, 11 percent phosphate and 0 percent potash, and 20 percent total phosphate and 24 percent calcium. Use the 11 percent figure for the calculator, not the 20 percent total phosphate figure, which is a different measure.

Read the bag before you calculate. Iowa State University Extension warns that its tables give general guidelines and do not represent a guaranteed analysis, and products differ between brands. If your bag shows a different phosphate percent, use the bag percent.

At 1 percent nitrogen, bone meal supplies very little nitrogen. In the main worked example below, 2.27 kg supplies only about 23 g of nitrogen. At 24 percent calcium, the same 2.27 kg supplies about 545 g of calcium.

Bone Meal Is a Range, Not One Number

No single bone meal analysis fits every bag. Across the sources read, phosphate as P2O5 runs from 11 percent to 27 percent, and nitrogen from 0.7 percent to 7 percent. Iowa State University Extension lists steamed bone meal at 11 percent phosphate and 1 percent nitrogen. Colorado State University Extension and the University of Connecticut list 15 percent phosphate and 3 percent nitrogen. Oregon State University lists raw bone meal at 15 to 27 percent phosphate and steamed bone meal at 18 to 24 percent phosphate and 0.7 to 7 percent nitrogen.

At 15 percent phosphate and 3 percent nitrogen, the same 250 g P2O5 target over 50 square metres needs 250 g divided by 0.15, which is 1666.7 g, or 1.67 kg. That is 0.60 kg less than 2.27 kg at 11 percent phosphate, and nitrogen supplied is 1666.7 g times 3 percent, which is 50 g. The answer moves with the analysis.

Colorado State University Extension, in GardenNotes 234, reports that phosphorus from bone meal is only available to plants in soils that have a pH below 7.0. A feed store label may state elemental phosphorus, not phosphate as P2O5. The phosphate number is 2.3 times the elemental phosphorus number, so multiply the elemental phosphorus percent by 2.3 before you enter it as P2O5 in the calculator. Entering it unchanged would make the calculator recommend more than twice the bone meal needed.

How the Calculator P2O5 Formula Works

The Organic Fertilizer Calculator uses a simple formula. Product equals nutrient target times area divided by nutrient percent. For phosphate, the nutrient percent is the bag phosphate percent, and P2O5 supplied equals product times P percent.

The three numbers on a bag are percents by weight of nitrogen, phosphate as P2O5, and potash as K2O. An 11 percent phosphate figure means 11 g of P2O5 in every 100 g of product.

Start with a phosphate target from a soil test or crop guide. The calculator page does not give per crop phosphate rates, and this blog does not invent any.

How Much to Apply for a Phosphate Target

Here is a worked example using the Iowa State 11 percent phosphate figure, with a target of 5 g of P2O5 per square metre over 50 square metres, as an example target only.

Phosphate needed is 5 g per square metre times 50 square metres, which is 250 g of P2O5. Product needed is 250 g divided by 0.11, which is 2272.7 g, or 2.27 kg of bone meal. The rate is 2272.7 g divided by 50 square metres, which is 45.5 g per square metre.

Nitrogen supplied is 2272.7 g times 1 percent, which is 22.7 g, rounded here to about 23 g. Phosphate supplied is 2272.7 g times 11 percent, which is 250 g, the target. Potash supplied is zero.

Use the 2.5 kg bag size from the calculator worked example. Divide 2.27 kg by 2.5 kg to get 0.91 bags, so buy 1 bag, rounded up. In imperial units, a target of 1 lb of P2O5 per 1000 sq ft over 500 sq ft needs 0.5 lb of P2O5, which is 4.55 lb of bone meal, rounded here to about 4.6 lb.

The table below repeats the metric example for other areas at the same 5 g per square metre target, using the Iowa State 11 percent figure and the 2.5 kg bag size.

Area	P2O5 needed	Bone meal needed	Rate	Bags of 2.5 kg
10 square metres	50 g	0.45 kg	45.5 g per square metre	1
20 square metres	100 g	0.91 kg	45.5 g per square metre	1
50 square metres	250 g	2.27 kg	45.5 g per square metre	1
100 square metres	500 g	4.55 kg	45.5 g per square metre	2

Iowa State University Extension also lists average rates for steamed bone meal by soil fertility: 3 lb per 100 sq ft for low fertility, 2 lb for medium fertility, and 1 lb for adequate fertility. Those rates are a separate Iowa State guide, not a calculator phosphate target.

Slow Release Over Months: Timing Your Application

Bone meal is slow. The calculator page states that blood meal and fish meal act within a few weeks, while bone meal and alfalfa are slower, releasing over months.

Apply bone meal at planting time, or a week or more before planting, so release has started by the time roots need phosphate. For bulbs, mix it into the planting area. For established plants, work it into the top few centimetres around the root zone.

Do not expect a visible change in days. If a plant needs phosphate urgently, bone meal cannot meet that need on a short timetable. Adding extra bone meal does not speed release.

The Dog Warning and Safe Handling

Keep blood and bone meal away from dogs, which may dig for it. That warning comes from the calculator page.

Mix the meal into the soil rather than leaving it on the surface. Store closed bags where dogs cannot reach them. If a dog has eaten bone meal, contact a vet for advice.

When Bone Meal Is the Wrong Choice

Bone meal is the wrong choice in alkaline soil, where soil pH is above about 7 and the calculator page says its phosphorus is mostly unavailable. Applying more product does not fix that limit.

Bone meal is also the wrong choice when phosphate is already adequate. A soil test that shows adequate or high phosphate means the garden does not need more. Test first, and skip bone meal when the test says so.

Bone meal is the wrong choice for a fast phosphate need. If a plant needs help within days or a few weeks, a slow release meal is not the tool, because bone meal releases over months. Bone meal is also the wrong choice as a complete fertilizer. It supplies phosphate, a little nitrogen, and no potash.

Common Mistakes with Bone Meal for Phosphorus

The first mistake is applying without a soil test. Without a test you do not know whether phosphate is low, and you do not know the pH. The second mistake is ignoring the pH limit. Above about pH 7 the phosphorus is mostly unavailable, so check pH before you buy.

The third mistake is using the wrong percent in the formula, using the 20 percent total phosphate figure instead of the 11 percent bag phosphate figure. For the 50 square metre example, dividing 250 g by 0.20 gives 1.25 kg instead of 2.27 kg, and that 1.25 kg supplies only 137.5 g of P2O5, well short of the target.

The fourth mistake is treating bone meal as a nitrogen feed. At 1 percent nitrogen, it cannot meet a nitrogen target at a sensible rate. To supply 250 g of nitrogen at 1 percent would need 25 kg of bone meal. Use a nitrogen meal for nitrogen.

The fifth mistake is applying too late and expecting fast results. Bone meal releases over months, so a late application cannot feed early growth. Apply a week or more before planting.

The sixth mistake is leaving bone meal on the surface where dogs can find it. Mix it in, store it safely, and keep dogs away from treated beds.

Frequently Asked Questions

Q: Is bone meal for phosphorus a good choice for my garden?

A: Bone meal for phosphorus is a good choice when a soil test shows low phosphate and soil pH is below about 7. It supplies phosphate slowly over months. It is a poor choice in soil above about pH 7.

Q: How much bone meal do I need for a phosphate target?

A: Use the calculator formula. Product equals phosphate target times area divided by phosphate percent. With the Iowa State 11 percent figure, 5 g of P2O5 per square metre over 50 square metres needs 2.27 kg of bone meal.

Q: Why does bone meal not work in alkaline soil?

A: The calculator page states that bone meal phosphorus is mostly unavailable in soils above about pH 7, and that it works best in slightly acid soil. In alkaline soil the phosphate stays in forms roots cannot take up easily.

Q: How fast does bone meal release phosphate?

A: Bone meal is slow. The calculator page states that blood meal and fish meal act within a few weeks, while bone meal and alfalfa are slower, releasing over months. Apply it a week or more before planting.

Q: Is bone meal a nitrogen fertilizer?

A: No. The Iowa State average analysis for steamed bone meal is 1 percent nitrogen, 11 percent phosphate and 0 percent potash. In the 50 square metre example, 2.27 kg supplies only about 23 g of nitrogen.

Q: Is bone meal safe around dogs?

A: No, not without care. The calculator page warns to keep blood and bone meal away from dogs, which may dig for it. Mix bone meal into the soil, do not leave it on the surface, and store bags out of reach.