Ryan Foxley spreading manure with his team in April, 2010, on his farm outside Arlington, WA. He uses true horsepower for his various on-farm power needs, and those horses provide the additional benefit of manure and compost for the fields. Ryan is spreading with an 80-year-old New Idea spreader. Click here to read his Littlefield Blog, or check out his recent columns in the Small Farmer’s Journal.
Now that we know something about how to make compost, and what compost can do for the soil, the next question is: how and when to apply it? The short answer is: it depends. Let’s take a look at the variables that go into determining that answer.
The first question of how best to apply compost is relatively simple. Most folks apply it directly to the soils, either during the pre-planting stage for crops, or right after cutting/grazing for hay and pasture crops. For small growing areas, finished compost can be wheeled about in carts or wheelbarrows and either shoveled or dumped into growing areas, then spread more evenly with a rake. For larger areas, some folks will use either a front end loader or powered carts to deliver larger volumes, then spread the compost either by hand or with a scraper attachment for a tractor. For larger areas growers can use either a wagon to bring the compost out to the field, then shovel it out a bit at a time, or they can use a manure spreader, which both carries the compost out to the field, and spreads it behind the tractor as the combo crosses the field. All these options give growers quite a few options according to their own particular needs and areas.
The question of how much to spread is a bit more involved. We can take the specific or general route. The specific route involves testing both the soils and the compost to see what the former needs, and what the latter has to offer. That testing is a very good idea when first working with either new land or a new composting setup. We simply cannot tell by appearance, by touch or by circumstance what’s really going on with either soil or compost until we’ve had a tremendous amount of practice. If working with a new parcel or a new compost operation, we haven’t had that practice yet. Hence the testing.
Testing labs are relatively straightforward to work with, but each of them has their own particular criteria. I’ve listed some regional testing labs at the end of this blog, but you may also want to contact your nearest Extension office or Conservation District to get a list of the labs they recommend. Typically test samples are relatively easy to gather and submit, but the cost will vary according to how much testing is done on the samples. Many labs will handle both soil testing and compost testing, but be sure to confirm that before submitting any samples. Most labs have sample submission guidelines either posted on their website, and/or they can send those guidelines upon request. I’ve provided some links at the end of this blog for how to best gather samples, not only in terms of actually gathering the samples but also in terms of how to cost effectively choose the type and number of tests to run.
Once the testing is done, the labs will provide a printout of all the criteria found in the submitted samples, along with recommendations for how to remedy any deficiencies or surpluses. Here is where we move even further away from conventional agricultural treatment and deeper into the realm of sustainable agriculture. Why is that? Because this is the moment when we consider not only this year’s soil fertility needs, but also the soil’s fertility needs for years and even decades to come. As we’ve already seen, some soil nutrients such as nitrogen are extremely mobile, moving through the soil via water, and/or evaporating into the air. We’ve also seen how other nutrients may be present in the soil, but are inaccessible to plants due to chemical or physical bonds with each other and/or with the inorganic components of the soil itself. Those who are primarily interested only in feeding this year’s crop have become very adept at using quick-release or seasonal-release formulations for nitrogen, phosphorus and other nutrients to give this year’s crop exactly what it needs, exactly when it needs it. From one point of view that’s extremely cost-effective. But from a long-term point of view, over the passage of time that only creates a soil that has no reserve of fertility. Additionally, many micro-nutrients are never measured, let alone added. So the soils end the season more impoverished than they began. That might not add up to much change after a few years. But the longer that pattern holds, the more impoverished the soils become and the harder it will be to get them back into being generally fertile again. Some soils are so bereft of both nutrients and organic matter that they are nearly lifeless. Other civilizations have worn out their soils in this manner, and their national wealth suffered as a result. We may want to take a long hard look at where that pattern is leading us.
Conversely, the application of compost can be both cost-effective and provide an investment not only in this year’s production, but also improve soils for years to come. Why is that? Recall that compost is not merely providing nutrients; it also provides a rich web of once-living materials which are in various states of decay. Those materials are accompanied by the microorganisms that are busy consuming those materials and freeing up the elements which built them. That process only starts in the compost pile; the actual lifespan of that process can be measured in years. So as those hungry little beasties continue their consumption, new elements are continuously released into the soils, renewing the soil’s reserve for those nutrients. Furthermore, the very presence of those microorganisms will also improve the recycling of whatever other materials are also present - crop residues, urines and manures applied via grazing, and even additional non-compost soil amendments. Hence the term “slow release fertility”, because those nutrients are released slowly but surely over the passage of time, regardless of what is actually being grown or how the soils are being cultivated. Applying compost, then, is truly an investment not only in this year’s growth, but even moreso in future growth.
So we return now to the question of how much compost to apply. If soil and/or compost testing was done, a good starting point would be to figure out which of the macronutrients (typically nitrogen) is in greatest need, and apply a measured amount of compost to meet that need. However, that recommendation comes with a very big cautionary note. Because no one soil is perfectly balanced, and because any given compost comes with a very wide variety of nutrients, it is unlikely that a single application of compost will meet every criteria needed by the soil. More than likely, the compost will provide too much of X and not enough of Y nutrient. Additionally, compost by itself may not be sufficient; the soil may need other amendments to change undesirable characteristics. A prime example would be if the soils are slightly to severely acidic due to moderate to heavy rainfall. Sure compost will help that, but you’d have to apply mountains of compost to make the necessary changes when a minor application of lime would accomplish the same task. As wonderful as compost is, it can’t do everything. Particularly if your soils are acidic, but generally if your soils need something which the compost can’t provide, don’t be afraid to supplement the compost with other ingredients. Another example would be if fields need a tremendous amount of nitrogen. The temptation would be to pour on the compost, but that might introduce other surpluses or deficiencies in other nutrients. Bottom line, many folks want to apply enough compost that their manure piles go away and/or their compost bins are empty. The better yardstick is to apply as much compost to the soils as is beneficial, but use the soil’s needs as the guide. If you have compost left over, consider that money in the bank, or even a product to be sold. More compost on the soil than what is needed is wasteful.
So far we’ve talked about working from lab results for both the soil and the compost. What if you don’t have those lab reports? There are generalized recommendations for how much to apply, but they vary a lot. For croplands where the compost is going to be worked into the soil prior to planting, many regional authorities recommend applying either a concentrated band alongside row crops, or a blanket over whole beds varying from ½ inch to 3 inches deep. Most authorities warn against applying deep volumes of compost, because of both the risk of nutrient runoff and diminishing returns. Those greater depths would only be appropriate in cases of extremely sandy soils in desperate need of organic content. In those cases, incorporation prior to planting would be ideal.
For pasture lands where the compost will not be worked into the soil, the recommendation is for no more than ½ inch of compost. This helps ensure that the application won’t smother the plants underneath. Most authorities recommend applying compost in late spring after the soils have come up in temperature, so that the microorganisms will have good environmental conditions in which to live, and as much growing season as possible before the return of cold weather.
If either the soil report or general conditions warrant heavier applications, the general recommendation is to stick to that ½ inch depth, but make several applications spread out through the growing season. Mid spring, summer and early fall would be three good times to spread compost and still reap some benefit from doing so. Compost should generally not be applied after early autumn, because there’s too little growing season left to make use of the more mobile nutrients, and those nutrients would then be washed away with winter precipitation. One exception would be to apply and work in compost just head of an autumn planting or flush of new growth, such as what happens when autumn rains drive new growth on pastures. Compost can be very well timed at that late date, if the nitrogen and other mobile nutrients can be taken up by that flurry of new activity before winter sets in.
Before we leave this topic of how much and how often, let’s look for a moment at each compost application’s longevity. As we mentioned above, the total composting process actually takes years to play out. So to extend that thought, applying a single load of compost during Year 1 would actually benefit the soil for at least several years to come, right? It turns out that’s exactly what happens. Both traditional wisdom and recent experimental data confirms that a single application of compost will provide measurable benefits for 3-5 years. Furthermore, quite a bit of research indicates that the biggest flush of soil improvement won’t happen during the first year, but instead will take place in the second and even third year after application. This again has some nice implications for the long-term health of the soils, and the grower’s wallet. If managing fields for long-term sustainable growth, growers don’t need to apply compost every single year. They can get by with applying every other year or even every third year, and still keep the momentum going from past improvements. For particularly worn out or infertile soils, compost applications might be warranted once a year for the first 3-5 years or so, but after that the growers could still drop back to applying compost once every few years. Not only does this mean fewer trips across the garden or landscape, but it provides some scheduling opportunities for other amendments.
For instance, we have issues here with acidic soils due to our high rainfall. I know of some growers who use a three-year model, such that they apply lime the first year, compost the second year, then rock dust or other amendments the third year. Or perhaps they weave a cover crop into the mix at some point and push the amendment schedule out even further. Then they repeat the cycle. That means they don’t have to provide all these amendments to every field, every year. Gardeners can do the same thing with different garden areas or growing beds. We’ve used something like this for a number of years here, where we’ll apply lime one year, compost next year, wood ashes from our woodstove the third year, raked up leaves the fourth year, then alfalfa mulch or cover crops the following year. The possible variations are nearly endless, and are limited only by the materials on hand, the state of the soil at any given time, the planting and cultivation schedule and the production goals for any given planting bed or field. The main point to be taken from this is not to drive yourself crazy with the possibilities, but rather to consider compost as one valuable tool within a toolbox of other soil fertility options. Use them all, in varying ways and times, to round out your soil fertility needs.
OK, now that we’ve covered the basics of compost creation, function and application, next time we’ll start to get into some of the myths and facts about compost details. In between these long blog entries I might toss in some non-composting items just as a bit of a break. It’s been a deep dive into a worthy topic. Thanks for touring the topic with me. In the meantime, here is some additional reading you can do if you haven’t quite gotten your fill of All Things Compost:
Soil testing labs and sampling technique recommendations:
Compost application amounts and timing recommendations:
Additional composting resources: