The Limitations of Poor-Quality Compost

by | May 1, 2023 | 0 comments

Pick up any gardening magazine or book, and you are sure to soon come across the use of compost. Composting is a basic principle of gardening. In the natural cycle of things, life grows from death; the decomposition of once-living things provides fuel for new living things. There are many ways and methods of composting, but, in general, composting refers to the action of organic material decomposing. Compost is what remains after partial decomposition.

I will confess that at one time I had a rather negative view of the compost-centric focus of many organic/natural gardening publications. This was due to real-life experience in the early days of my agriculture consulting work. I saw a negative pattern emerge in many home gardens. It was the worst in gardens on livestock farms.

This problem I so frequently saw, I came to refer to as “constipated” soil. It is the result of too much raw manure or poor-quality compost (referred to in the industry as SAM, stacked aged manure). SAM can be a good product but only if aged for three or four years.

The organisms, mostly bacteria, that flourish during the early stages of decomposition are not necessarily the same organisms that dominate a healthy soil. It is perfectly fine and normal to have some decomposition happening in gardens on an ongoing basis. Where we create problems is the application of too much soluble nutrition in the form of partially decomposed plant material. For the ultimate in health and quality, plants need to be able to dominate the biological life in the soil. Ideally, the crops that we plant dominate the soil with their root exudates and feed the biology that most benefits them. When there are too many soluble nutrients in a soil, plants don’t have to work for their nutrition, and we have a similar situation to a child turned loose in a candy store with no adult control of their diet. In both cases, the result is likely to be ill health.

The point is, do not confuse well-made compost with organic material in lesser stages of decomposition. All decomposed organic material has value, but don’t over-apply soluble nutrients in the form of manure or vegetation that is not well on its way to being soil.

Mulch is, by definition, not very soluble and doesn’t cause the same problem. Finely-chopped green grass clippings could be over-applied to the point of harming a healthy soil biology. That is more of a concern from an anaerobic potential of tightly packed grass blocking oxygen from the soil. Sawdust, on the other hand, will pull so much nitrogen from the soil as it starts to decompose, that very little else will grow well surrounded by sawdust. In general, organic mulches in the form of dry grass, straw, shredded newspaper, bark, and larger wood chips do no harm. These high-carbon mulches do not provide a great deal of soluble nutrition, and the plants we are growing can continue to dominate the soil biological community.

Even SAM will usually not harm healthy soil function if kept on top of the ground and used as mulch. The constipation problem I talked about earlier becomes more of a problem when these partially decomposed organic materials are tilled in. The problem with stacked aged manure as a mulch is that, until it is well-aged, it likely will not provide a mulch but will instead grow a healthy crop of weeds.

Soil constipation is a problem widespread enough that I want to address it further here. Next month we will look in more detail at making and using compost.

You likely have a soil constipation problem in a garden where a soil test shows generous levels of nutrients but some crops struggle with a lot of disease and insect pressure. Another sign of a constipation issue is heavy broadleaf weed pressure, especially pig weed, galinsoga, and lambsquarters. The same disease and insect issues can also come from a nutritionally deficient soil, although, in that case, the weeds are more likely to be predominately grassy species.

The cure for soil constipation is simple but may take some time, depending on how severe the soluble nutrient overload is. To deal with this problem, the soluble nutrients need to be complexed into stable carbon or soil clay.

In most cases, I don’t suggest adding clay to a garden spot. Most soils naturally have sufficient clay, and many places have more than is ideal for growing vegetables. What needs to happen for clay to hold more nutrients is for it to expand or flocculate, increasing the surface area and nutrient-holding capacity. Clay flocculation comes about through the action of biology in the soil, especially from various types of fungi that produce glomalin. Glomalin is a glue that binds soil particles together. Soil aggregates or clumps formed by glomalin have a lot more surface area and nutrient-holding capacity than the same soil particles that are compacted together with no extra structure.

A constipated soil almost always lacks fungal activity. The two most common reasons for lack of fungal activity in gardens is too much tillage and/or the application of too much soluble nutrition. Tillage is harmful to fungal activity because it mechanically breaks it apart and sets it back. Many species of fungi grow in long root-like hairs. When these are broken, some sections will die.

Too much manure or other soluble nutrition prevents many beneficial species of fungi from growing because they’re dependent on plants for nutrition. When plants are growing in the proverbial “candy store,” they don’t have the need to cooperate with fungi. I don’t think we really understand why or by what mechanism plants do not feed fungi when growing with plenty of soluble nutrition. We just know from observation that this is the case.

The fastest way to turn around a constipated garden area is to grow as much biomass as possible. Large heavy-feeding crops such as corn, and cover crop species like sorghum-sudan will be the most beneficial. In most cases, a constipated soil will not be providing enough nitrogen to maximize the growth of these large crops. We’ll need to supplement additional nitrogen to optimize growth and the drawdown of other soluble nutrients, particularly phosphorus and potassium.

Obviously, we want to use forms of nitrogen that do not contain significant amounts of other nutrients. Organically, this would be available as blood meal, or a derivative of soybean meal. Look for an organic product with a fertilizer analysis of 8-0-0 up to as much as 20-0-0. If you’re not concerned about being strictly organic, foliar feeding with a diluted chemical form of nitrogen, either ammonium sulfate or urea, is also very effective. I don’t recommend using chemical nitrogen as a soil application because it sets back the production of organic nitrogen in the soil, particularly if you have any legumes growing in your cover crop mix. Used as a foliar spray on the growing crop avoids this negative effect. Be careful to keep your dilution low enough to not burn the foliage.

Moderate applications of sugar in the form of table sugar or any type of molasses can also help loosen up a constipated soil. I suggest a maximum application rate of no more than 1 pound per 1000 ft.² two or three times a year. The application of sugar mimics plant root exudates and can boost the population of bacteria that most directly feed fungi. Sugar also helps speed the breakdown of any organic residue and partially digested material that is contributing to sluggish microbial action in your soil.

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