Liebig’s Law of Diminishing Returns

When people first get into growing vegetables it is pretty common to have considerable initial success using the conventional method of importing nutrients in the form of manure and mulch and irrigating their beds extensively. Even if they manage to stick to it and keep up the momentum through the unavoidable labour and regular disappointments a common pattern emerges where the yields get a bit less each year, pests and diseases appear that weren’t a problem to begin and refuse to leave, and gradually the balance of input required versus yield returned sinks to the point where a rational person would give up.

Productivity and health of crops depends on the availability of a range of different plant nutrients, including “macronutrients” like nitrogen, phosphorus and potassium used in fairly large amounts along with a longer list of “micronutrients”, often called trace elements, used in smaller amounts. Production from any particular crop is limited by whichever nutrient is in shortest supply, regardless of how much excess of other nutrients is present. This is like how a cake recipe calls for one cup of flour, two eggs and a half cup of sugar. If any one ingredient like the eggs run out then no more cakes can be made, regardless of how much flour and sugar is left. This is often presented as Liebig’s law of the minimum, commonly shown as a barrel where each stave represents the amount of a different nutrient. The yield represents the amount of water the barrel can hold, with the shortest stave determining when the barrel is as full as possible.

For simplicity I am using a simple bar graph to represent the same idea.

When you first start off with native soil the profile might look like something in the first graph. The vertical axis represents the percent of the optimum amount of each nutrient present and available. The values on each nutrient here are just put together to illustrate the broad point I am making. This soil is fairly rich in trace elements but is mostly limited by nitrogen, so if you tried to grow a crop on this the nitrogen limitation would limit it to about 30% of the maximum.

The second graph shows the effect of adding imported manure, shown as orange sections added on top of the original amounts in the native soil. Nitrogen has been greatly increased, along with lesser amounts of other nutrients. The yield potential is now much higher, around 70%. The crop is now limited by magnesium instead of nitrogen. When the crop is removed the second graph is cut off below the 70% line as all those nutrients are harvested, leaving the soil profile in the third graph (now all in blue). The soil is now depleted in magnesium, the limiting nutrient in the last cropping cycle. Surely we can keep things going by just adding the same amount of manure that we used last time?

Graph 4 below shows the result of this. Now the soil is limited by potassium, magnesium and trace elements sulfur, manganese and zinc to similar degrees, giving an optimum yield of only about 20% of maximum. All sorts of new plant health problems will emerge growing in this strangely unbalanced soil.

The crop has a large amount of nitrogen and copper as well but a low level of zinc. Many trace elements and minerals act as antagonists, where a high level of one makes a low level of the other more problematic. High levels of phosphorus tends to make many trace elements insoluble and unavailable to plants, even if present in sufficient amounts. Reliance of industrial agriculture on large amounts of synthetic phosphorus may be behind dropping trace element levels in western diets. All other nutrients have a complex web of antagonistic interactions (http://www.agritech.tnau.ac.in/agriculture/agri_min_nutri_antagonism.html).

In modern industrial farming this issue is managed by regular testing of soil and crop nutrient levels, followed by applying individual nutrients in carefully calculated doses since adding too much is very easy and often worse than deficiency. This practice is not practical for home gardeners due to the high cost of repeated testing and technical expertise required.

For the home gardener options are more limited. Crop rotation helps to some degree since different crop species require slightly different levels of each nutrient. Varying the fertility inputs can help as well but the nutrient content of different batches of even the same input can vary wildly so you are still flying blind. Both of these approaches merely slow down the problem in the absence of clear information about which imbalances are occurring. This only leaves one strategy in the home gardener’s arsenal against deranged soil: fallow. Traditional pre-industrial farming relied on long periods of resting ground between crops, often 5-10 years of allowing the land to return to weedy pasture that was periodically grazed by livestock. Doing so allowed plant species that are able to exploit imbalances in the soil to dominate, allowing livestock in turn to remove that biomass from the space. This timeframe allow also allowed leaching of excess minerals into the subsoil, coupled with weedy species mining deficient minerals from the same subsoil.

In a home garden situation this means that beds need to be rested more often than they are cultivated intensively. Pasture and livestock is out of the repertoire of most small space gardeners, but green manure crops and biomass banks are definitely not. Green manures must be as diverse as possible, ideally including weedy locally adapted species from as many different plant families as possible. For longer fallows these can be followed up with biomass crops that can produce mulch that can be used in other locations. Taking this approach means the garden has less total area under crops, which theoretically means less produce. The alternative is pushing the entire space to maximum productivity in the short term by importing massive amounts of fertility can ruin the whole space for any crops for a good number of years. Producing more of your own biomass does mean greatly reducing the need for imported fertility and associated costs, meaning the space is more likely to be a net profit. Well managed green manure and biomass beds also require much less time to maintain, reducing the ongoing labour requirements (consisting predominantly of harvesting and processing in a successful garden in my opinion).

Overall the take home message for small space gardeners is to go easy on your land and yourself. Small spaces can be pushed to be extremely productive in the short term with massive inputs but tend to go off the rails in time. Instead maybe consider reducing your reliance on inputs and create a system that can sustain itself (and you) to a greater degree.

Busy workers shifting excess nutrients elsewhere.