How much compost is too much? Compost, chicken manure and the phosphorus problem

Once a bed's Olsen P is in range, more than a few millimetres of compost a year is too much. Compost spread thickly enough to supply a crop's nitrogen carries about 17 times the phosphorus and nearly 7 times the potassium the crop takes away, and the surplus stays in the soil. Use compost for organic matter, at a rate set by phosphorus, and get nitrogen from a nitrogen input.

Why compost is a poor nitrogen source

None of this is an argument against compost. It supplies organic matter that improves soil quality, which mineral fertilisers do not (Reid and Morton 2019). The problem is asking it to do a fertiliser's job.

Plant & Food Research tested two municipal composts from Timaru and Christchurch, both made to NZS 4454 (Horrocks et al. 2016). They were about 50% dry matter and, on a dry basis, held 21 to 25 g of nitrogen, 4.2 to 4.6 g of phosphorus and 12.7 to 14 g of potassium per kilogram. Taking the midpoints and halving for moisture, a tonne as delivered holds about 11.5 kg N, 2.2 kg P and 6.7 kg K.

Very little of that nitrogen is available. In the field trials only 12% was released in the first year, most of it mineral nitrogen already present at spreading, then 2.3%, 1.0% and 0.5% in the following three years. Oregon State University gives a similar first year range, from nil to about 10% for stable composts (Sullivan et al. 2018). The phosphorus behaved differently: much of it was plant available, and Olsen P one year later had risen by 0.15 mg/L for every tonne of compost (fresh weight) per hectare. The authors concluded that compost alone will not meet the nitrogen requirement of organic crops in New Zealand.

The sums for a lettuce crop

1. Crop need. For a 30 t/ha lettuce crop on soil with an Available N test of 40 kg/ha, Reid and Morton (2019) recommend 100 kg N/ha. That is 10 g of nitrogen per square metre.
2. Available nitrogen in compost. 11.5 kg N per tonne × 12% = 1.38 kg per tonne.
3. Compost required. 100 ÷ 1.38 = 72.5 t/ha, or 7.25 kg per square metre. Screened compost at 50% moisture weighs about 600 kg per cubic metre (Sullivan et al. 2018), so that is about 12 litres per square metre: a 12 mm layer.
4. Phosphorus and potassium that arrive with it. 72.5 × 2.2 = 159 kg P/ha. 72.5 × 6.7 = 486 kg K/ha.
5. What the crop removes. 30 t/ha of lettuce takes away 9.3 kg P and 73 kg K per hectare (Reid and Morton 2019).
6. Surplus. 159 ÷ 9.3 is about 17 times removal for phosphorus. 486 ÷ 73 is nearly 7 times for potassium.

At 0.15 Olsen units per tonne, one 72.5 t/ha application would lift Olsen P by about 11 units on the silt loam in that trial. A 25 mm layer is 150 t/ha: 1,725 kg of total nitrogen, 330 kg P and 1,005 kg K per hectare. Gardeners do apply this much: a survey of 142 gardeners and urban farmers in Minnesota, with compost the most common amendment, found median inputs of 300 kg P/ha and 2.5% of it recovered in harvest (Small et al. 2019).

Chicken, sheep, cattle and horse manure

Manures release nitrogen faster, but the imbalance remains. This table repeats the sum for 100 kg of first season nitrogen per hectare, using the midpoint of each availability range.

The New South Wales analysis is from Griffiths (1998): 75% dry matter with 2.6% N, 1.8% P and 1.0% K in the dry matter. The other rows and all the manure availability ranges are from Bary, Cogger and Sullivan (2016), converted from pounds per ton. Every workable option delivers about 10 to 17 times the phosphorus the lettuce removes. Horse manure with woody bedding can tie nitrogen up, so it is a soil builder only. Both guides say it plainly: poultry litter supplies too much phosphorus for its nitrogen, and manures applied to meet nitrogen need usually oversupply phosphorus and potassium.

Fresh poultry litter can also burn plants. The New South Wales samples averaged pH 8.1 and an electrical conductivity of 6.8 dS/m, with about a quarter of the nitrogen as ammonia (Griffiths 1998). Soluble salts injure seedlings and transplants first, and ammonia can harm sensitive plants (Sullivan et al. 2018). Litter sold as composted is often incompletely composted and releases nitrogen like fresh litter. Fresh manure can carry Salmonella and E. coli, and the United States organic rule is 120 days between spreading it and harvesting salad greens or root vegetables (Bary et al. 2016).

What high Olsen P and potassium do

Most soils bind phosphorus (Griffiths 1998), so the surplus accumulates and shows up as a rising Olsen P. Three things follow.

• No yield return. Reid and Morton (2019) advise no maintenance phosphorus above Olsen P 40 for sweetcorn, 50 for cabbage, broccoli and cauliflower, 55 for carrots and 70 for lettuce and potatoes.
• Mycorrhizae are suppressed. In petunia, high phosphate supply repressed the plant's symbiosis genes and then root colonisation by mycorrhizal fungi (Breuillin et al. 2010). That was laboratory work with no Olsen P threshold attached. The nearest New Zealand field guidance is for sweetcorn, where Olsen P above about 30 may discourage the mycorrhizal symbioses that help zinc uptake (Reid and Morton 2019). Our soil science clip on phosphorus covers the mechanism.
• More is lost to water. The risk of loss to groundwater and run-off is greatest from soils with high soil test values (Reid and Morton 2019), and Horrocks et al. warn that this may set the upper limit for compost rates.

Potassium is more forgiving: many crops tolerate high levels, though some suffer imbalances (Bary et al. 2016). When magnesium falls below potassium on a soil test, magnesium uptake can be suppressed (Hill Laboratories n.d.).

How to tell from a Hill Laboratories report

• Olsen P. Hill's medium range for horticultural soils is 30 to 80 mg/L (Hill Laboratories n.d.). Above 80 is high, and above the crop figures listed earlier no more phosphorus is needed.
• Potassium. The horticultural range is 0.5 to 1.0 me/100 g, or 3 to 6% of base saturation. Magnesium should ideally be twice potassium.
• Organic matter. Ask for this test. High Olsen P and potassium together with high organic matter is what the sums above predict after years of generous compost or manure.
• Soluble salts. Worth adding for tunnel houses and covered beds, where heavy feeding lets salts build up (Hill Laboratories n.d.).

Sample 6 to 12 months after the last compost application (Sullivan et al. 2018).

What to do

1. Test before you spread. See where to test your soil.
2. Olsen P below 30: compost is a sound way to build phosphorus and potassium. Up to 10 mm a year (6 kg per square metre) supplies 132 kg P and 402 kg K per hectare. Retest in a year.
3. Olsen P in range: hold compost to about 3 mm a year, which is 3 litres or 1.8 kg per square metre. That is 18 t/ha carrying 40 kg P and 121 kg K, the upper end of what two vegetable crops remove (lettuce 9, summer cabbage 13, carrots 25 kg P/ha; Reid and Morton 2019). This depth is our working rule for compost of this analysis. Composts typically range from 0.3 to 0.9% P in dry matter (Sullivan et al. 2018), so ask for the analysis and redo the sum.
4. Olsen P high: stop compost and manure and let the figure fall. Grow cover crops for organic matter instead (Bary et al. 2016).
5. Buy nitrogen as nitrogen. Seacliff's is mussel meal. Its product page lists 8.60% N, 0.85% P and 1.10% K, so 116 g per square metre carries 10 g of nitrogen with 1 g of phosphorus (10 kg P/ha, about what the lettuce removes). Not all of that nitrogen will release in the first season, and we have no measured figure [LAB RESULT NEEDED: independent N, P, K and sodium analysis and first season nitrogen release for Seacliff Mussel Meal]. The listing also shows 3.6% sodium, so it is not something to pile on either. Legume cover crops fix nitrogen and supply no phosphorus or potassium (Bary et al. 2016).
6. Count everything compost-like. Seacliff sells worm castings and compost-based mixes. They supply organic matter and biology, and phosphorus and potassium too [LAB RESULT NEEDED: N, P, K and moisture for Seacliff Worm Castings], so they belong inside the same allowance. The same applies to recycled living soil in indoor beds for medical cannabis, where little or nothing leaches.
7. Treat chicken manure as a phosphorus fertiliser. Use it composted, on beds that test low, and never fresh on salad crops.
8. Ask for the standard. NZS 4454:2005 sets compositional requirements, test methods, contaminant limits and microbiological requirements for composts (Standards New Zealand 2005). It is a product standard, not an application rate.

The soil planner reads a Hill Laboratories report, lets you enter a compost or manure analysis and rate, and subtracts the phosphorus and potassium it carries before recommending any more of either. The fertiliser calculator gives a per-crop recipe in grams per square metre built on mussel meal. It does not read your soil test, so on a bed that tests high, use the mussel meal line and leave out the BioPhos and potassium sulphate.

References

• Bary, A., Cogger, C. and Sullivan, D. (2016) Fertilizing with manure and other organic amendments. PNW 533. Pullman: Washington State University Extension. Available at: https://pubs.extension.wsu.edu/product/fertilizing-with-manure/
• Breuillin, F. et al. (2010) 'Phosphate systemically inhibits development of arbuscular mycorrhiza in Petunia hybrida and represses genes involved in mycorrhizal functioning', The Plant Journal, 64(6), pp. 1002 to 1017. https://doi.org/10.1111/j.1365-313X.2010.04385.x
• Griffiths, N. (1998) Best practice guidelines for using poultry litter on pastures. Agnote DPI-212, 2nd edn. NSW Agriculture. Available at: https://www.epa.wa.gov.au/sites/default/files/PER_documentation/A1412_R1083_PER_Appendix%202%20-%20litter%20on%20pasture.pdf
• Hill Laboratories (n.d.) Technical note: Soil tests and interpretation. KB item 3196, version 7. Hamilton: Hill Laboratories. Available at: https://www.hill-labs.co.nz/media/djdbzzhl/3196_technical-note-soil-tests-and-interpretation.pdf
• Horrocks, A., Curtin, D., Tregurtha, C. and Meenken, E. (2016) 'Municipal compost as a nutrient source for organic crop production in New Zealand', Agronomy, 6(2), 35. https://doi.org/10.3390/agronomy6020035
• Reid, J.B. and Morton, J.D. (2019) Nutrient management for vegetable crops in New Zealand. Wellington: Horticulture New Zealand. Available at: https://www.fertiliser.org.nz/files/site/Nutrient-Management-for-Vegetable-Crops-in-NZ.pdf
• Small, G., Shrestha, P., Metson, G.S., Polsky, K., Jimenez, I. and Kay, A. (2019) 'Excess phosphorus from compost applications in urban gardens creates potential pollution hotspots', Environmental Research Communications, 1(9), 091007. https://doi.org/10.1088/2515-7620/ab3b8c
• Standards New Zealand (2005) NZS 4454:2005 Composts, soil conditioners and mulches. Wellington: Standards New Zealand. Available at: https://www.standards.govt.nz/shop/nzs-44542005
• Sullivan, D.M., Bary, A.I., Miller, R.O. and Brewer, L.J. (2018) Interpreting compost analyses. EM 9217. Corvallis: Oregon State University Extension Service. Available at: https://extension.oregonstate.edu/catalog/em-9217-interpreting-compost-analyses