Soil Amendment Rate Converter

How it works

Soil test reports and product labels give rates per unit of area: pounds per 100 or 1,000 square feet for gardens and lawns, pounds or tons per acre and kilograms per hectare for fields. The calculator turns the rate you enter into pounds per square foot, multiplies by your area, and divides by the bag weight, rounding up to whole bags.

It also restates the rate in every other common unit, so a figure from a farm-scale recommendation in tons per acre can be read as pounds per 1,000 square feet, and the reverse. An acre is 43,560 square feet, so pounds per 1,000 square feet × 43.56 = pounds per acre, the factor UGA's Soil Test Handbook (Special Bulletin 62) gives; one ton per acre is 2,000 ÷ 43.56 ≈ 45.9 lb per 1,000 square feet; and one kilogram per hectare is 0.892 lb per acre.

Compost, topsoil and other bulk amendments are usually specified as a depth instead. Area × depth gives the volume: square feet × inches ÷ 12 is cubic feet, and dividing by 27 gives cubic yards, so cubic yards = square feet × inches ÷ 324.

Formula

lb per sq ft    = rate (as lb) ÷ rate area (as sq ft)
pounds needed   = lb per sq ft × area (sq ft)
bags            = pounds needed ÷ bag weight, rounded up
lb per acre     = lb per 1,000 sq ft × 43.56
kg per hectare  = lb per acre × 0.4536 ÷ 0.4047 (≈ × 1.121)
depth mode: cu yd = area (sq ft) × depth (in) ÷ 324

Example

A rate of 50 lb per 1,000 sq ft on a 400 sq ft bed is 50 ÷ 1,000 × 400 = 20 lb, or 9.1 kg; in 40 lb bags that is 1 bag. The same rate is 5 lb per 100 sq ft, 2,178 lb per acre and 2,441.2 kg per hectare.

Spreading compost 2 in deep over 100 sq ft takes 100 × 2 ÷ 324 = 0.62 cu yd (16.7 cu ft), or 17 bags of 1 cu ft.

Assumptions and limitations

• The rate is applied evenly over the whole area entered. Rates on soil test reports are for the area tested; product labels set their own rates, and for fertilizers and pesticides the label is the legal and authoritative instruction.
• Rates are taken as the weight of the product as sold, not of a nutrient in it. To convert a nutrient rate (for example lb of nitrogen) into product, divide by the nutrient percentage on the label first.
• Bag weights and volumes are nominal. Bags are rounded up to whole bags; no allowance is made for spillage.
• Depth mode assumes a uniform layer on a flat surface and does not allow for settling or compaction of the material after spreading.
• This is a unit conversion, not a recommendation. The rate entered is taken as given; it normally comes from a soil test report, the product label or extension guidance.

Frequently asked questions