Are you using more compost or manure than you should?

“Putting on too much compost is expensive and it pollutes.” - Klaas Martens. Organic Grain Farmer. Finger Lakes, New York.

We all know that using organic nutrients such as compost and manure have many benefits. They can increase organic matter, improve soil structure and build a reserve of nutrients in the soil. But the difficulty with using compost and manure is they release nutrients slowly at rates that we cannot always predict. Often the ratios of nitrogen to phosphorus and potassium don’t match what our plants need.

Recently I learned that there can be too much of a good thing. I spoke last week to Klaas Martens an organic grain farmer in the Finger Lakes region of New York. Working with Thomas Bjorkman, a researcher from Cornell, they applied a poultry based compost at different rates in order to determine if they could predict the best compost applications using traditional soil test recommendations. They found that compost applied at rates based on the soil test, did not increase yields or increase organic matter. But, weeds grew better! Where compost was applied at double the recommended rate, weeds were twice as tall (Bjorkman, 2008).

Over application severely impacts the environment. Agriculture account s for 73% of nitrogen and 64% of phosphorus in the Chesapeake Bay. Excess nitrogen easily leaches into ground water and into streams. Phosphorus binds tightly to soil particles that are washed into streams. These nutrients encourage growth of algae and aquatic weeds that use up the oxygen dissolved in the water, making it impossible for fish and other organisms that need this oxygen to survive.
Even the best organic farmers commonly over apply. Researchers surveying organic farms in the Northeast found that vegetable farms that relied on compost often had surplus nutrients, as much as 180-200 lb P and N/ acre – yr in excess (Drinkwater et. al. 2005).

It is easy to apply too much. We often apply at a rate of 1” or 2”s which is easier than calculating how many tons we are applying. Researchers at Penn State did just this, applying 1” or 2” of dairy based compost compared to inorganic fertilizer before peppers. They found that adding just 1” of compost released 441 lbs N/acre compared to the 100 lbs N/acre the peppers needed. This was considering that compost generally only releases 15% of its N per year. Another 2,450 lbs of N/ acre was still tied up in soil organic matter (Sanchez, 2008).

Don’t guess, soil test. Test your compost and soil. If your soil nutrients are too high (over 320 P2O5, 335 K2O, 2505 CaO, 490 MgO lbs/ acre), compost may not be the best choice this year. You will know your soil test is saying you are to high when the bars go into the “exceeds crop needs” section of the table.

Don’t guess, compost test. There is no such thing as the average compost. The average nitrogen content of 126 dairy manure based compost samples analyzed at Penn State was 1.45% nitrogen. But when we looked at three individual dairy manure compost samples from the same farm, the amount of nitrogen applied in 20 ton/ acre of compost ranged from 510, to 415 to 842 lb/ acre. Phosphorus ranged from 88, to 146 to 200.

If your soil test is okay calculate how much N,P,K is available based on your compost test. The compost you apply will be decomposed by bacteria and fungi that live in the soil which release the nutrients slowly over time. Use simple calculations available in Penn State publications “Managing Organic Nutrient Sources” to evaluate how much compost to apply to meet your crop needs. We have also put together a flow chart to help you decide which calculations to do for your farm. Ask for the “Organic Nutrient Sources Decision Making Tree.”