Ash Content of Organic Fertilizers

By Ken Quandt, Regional Sales Manager

In recent years, one of the rumors about using organic fertilizers is that the regular use of organic products would eventually plug up the pore spaces in porous root zones, such as those found in USGA type greens and sand based athletic fields. This would either severely limit the infiltration rate of a particular soil or seal it off completely over time. Naturally, anyone who had invested in building a green or athletic field with a porous root zone would not want to take such a risk.

Sadly, depending upon what type of organic fertilizer is used, there is some truth in these rumors. The culprit is usually ash. Ash is defined as what remains after an organic fertilizer is subjected to very high heat to remove the actual organic portion of the material. This ash would then be comprised of inorganic minerals that are extremely fine textured. In turf, such a burning off process never takes place. Instead, the soil microbes break down the organic material as well as they can and leave behind the ash plus anything they cannot digest, such as lignin. Now we have not only the ash content to worry about, but also the organic portion that cannot be digested.

Many manure type organic fertilizers contain up to 50% ash by weight. This ash is from the sawdust, wood shavings, and straw used as bedding for the animals that produced the manure. Since it impossible to separate the bedding from the manure, the entire material is composted and made into fertilizer. In most instances, the manufacturer would not want to separate the bedding from the manure because a major portion of the nutrients claimed on the bag, especially potassium, are derived from the bedding material.

Sewage sludge fertilizers present a slightly different problem. While these types of products do not contain any ash from wood shavings and straw, they do contain ash from other sources, plus what amounts to extremely fine sand. Some universities have reported finding sewage sludge fertilizer that contained between 30% and 50% of extremely fine sand and ash by weight. It is anyone’s guess as to what this extremely fine sand and ash can do to the infiltration rate of a porous root zone over time if such fertilizers are applied regularly.

Nature Safe has a very low ash content and is very easily digested by the soil microbes without leaving lots of indigestible portions behind. Most of the ash found in Nature Safe comes from the sulfate of potash or langbeinite that is used as a potassium source. We know from long years of experience and from extensive field trials that sulfate of potash does not plug up pore spaces. Our 10-2-8, for instance, has an ash content of a little over 23%. Our 12-2-0, which is essentially 10-2-8 with the potash removed, has an ash content of under 9%. Once the sulfate of potash is accounted for, none of the Nature Safe products would contain more than 9% or 10% ash.

The fact that most sewage sludge and manure type fertilizers have very low Nitrogen contents compounds the problem. Typically, this type of fertilizer contains 5% or less Nitrogen. Therefore, if a manure product with 5% N and 45% ash were used to apply one pound of N per thousand square feet, you would have to use twice as many pounds of fertilizer to get the same amount of N that would be available in 10-2-8. Therefore, you would be applying nearly 10 times as much ash with the manure product as you would with the Nature Safe 10-2-8.

(20 lbs. 5-2-4 Sustane with 45% ash = 1lb. N and 9 lbs. ash.)

(16.67 lbs. 6-2-0 Milorganite with 27% ash = 1 lb. N and 4.5 lbs. ash.)

(10 lbs. Nature Safe 10-2-8 with 9% actual ash = 1 lb. N and 0.9 lbs. ash)

It is pretty easy to see why Nature Safe would be the organic product of choice, even if you did not take into account all of the other tremendous advantages that it offers over sludge and manure type products.