Intercropping for the home gardener

One of the oldest examples of agricultural coexistence is intercropping. Beans, corn, and squash—or rice/cereals and carp—or cereals, legumes, and camelina/mustard—these are all good examples. What is in common with these intercropping examples is nutrient cycling. Whether through N fixation by legumes, or P uptake by brassicas, the most limiting plant nutrient is added through the specific planting combination chosen. Then why does it seem as if intercropping is a thing of the past? While intercropping is often practiced in “developing” countries on small-holder farms, synthetic nitrogen, arguably the life force behind agricultural intensification, has effectively replaced intercropping and animals in the majority of industrial agricultural systems. 

Proponents of low-input agricultural systems are reconsidering traditional polycultures to facilitate natural mechanisms for nutrient input and protection of the primary crop. Additionally, different species can occupy distinct but spatially similar niches, which has the potential to produce more with less. Studies of intercropped species grown in industrial agricultural-type systems often focus on yield bonuses, but gloss over its broad application. Intercropping is suited to heterogenous environments where modern agriculture is either not possible or not practiced…Like in your backyard!

Industrialized agriculture has adopted the use of rotational systems, as this provides temporal crop diversity, and therefore, some economic flexibility. Rotations are also promoted to decrease weeds, disease, and insect build-up year to year. Common rotations are corn, soybean, and wheat (found in the Midwest) and wheat, barley, and pulses (here on the Palouse). Why are these rotations the standard? In some sense, the competitive production principle still applies. Crops grown in rotations will compete for resources spatially as well as temporally. However, the main reason for growing crops in rotation is to also rotate chemicals specific for certain weed species. Round-Up Ready®, imazamox, and 2-4-D are herbicide resistant technologies expanding to new crops.

Soil science

It can be argued that the agroecosystem is more the result of the soil than the terrestrial organisms which make use of it. A productive soil originates from the erosion of igneous rock, in some cases over millions of years. Biological action constitutes one of the more substantial sources of weathering parent materials. Agriculture can influence the weathering process, either by improving or degrading it.

Degradation can be observed through the erosion of arable soils and the over fertilization of crops, leading to ground and surface water contamination. Commercial organic operations can be equally culpable of over fertilization as conventional farms, because ammonia from urea can be just as soluble as that from manure. Site-specific management is necessary if we hope to mitigate widespread environmental contamination. The soil will also have to be seen as a biological system rather than reduced to fragmented components.

A ‘healthy’ soil is one with an abundance of worms, fungi, bacteria, and other subterranean creatures. Soil life is dependent upon plant and animal matter. The cycle of organic decomposition, immobilization by microbes, and mineralization provides nutrients for plant growth and feeds soil life. What organic materials are added, the soil’s parent material, and the climate all characterize the life of the soil.

Nitrogen fixation is probably the most important—but not the only—biological process, since nitrogen is often most limiting. The rhizobium, as well as other microbial symbionts and free-living (Azotobacter) microbes of legumes can fix N₂. The organic nitrogen released from decomposing legumes is not completely available to a succeeding crop until the organic molecules are mineralized into ammonium by specialized bacteria. The issue becomes when ammonia is converted to nitrate, because it is not readily adsorbed by most soils and leaches, or undergoes denitrification. The trick becomes balancing decomposition and microbial activity to facilitate adequate mineralization for plant growth, but not excessive as to promote loss—a definite challenge.

Phosphorus is probably the second nutrient most limiting to growth, and in tropical environments it can be more limiting than nitrogen. Phosphorous is typically insoluble and cannot be mineralized indefinitely like nitrogen can. Phosphorus limitation  makes our modern agricultural production systems unsustainable. The phosphorous inputs applied, from mined sources, are generally fixed before they are absorbed by agricultural crops. Certain bacteria (Bradyrhizobium) and fungi (Aspergillus-mycorrhizae) solubilize insoluble forms, cycling phosphorus. Again, the goal is to have a balance between fixed, microbial-immobilized phosphorus, and a sufficient ionic form for plant growth. Quantity and intensity will ultimately determine the buffering capacity of soil.

Organic matter

The complete decomposition of organisms can take thousands of years. Partially decomposed material makes up the organic matter or humus of soils. This soil fraction plays a key role in nutrient exchange and availability—the ‘slow’ and ‘fast’ components, respectively. We can use carbon:nitrogen ratios in the place of slow:fast fractions. C:N ratios less than 20 generate a net release of nitrogen, which is about the ratio of legume biomass. Sawdust on the other extreme would effectively tie up nitrogen with a C:N of 50:1. However, we should remember that the carbon component is integral for soil structure—why good compost is integral to sustainable agriculture.

Earthworms are reliable indicators of organic rich soils and create the physical structure of soil, most notable is porosity. They can be stimulated by reducing tillage, using cover cropping, and adding stabilized fertilizers like green manures and composts.

Pests

Pesticides are ubiquitous to agriculture. As we modified our landscape, we observed the effects certain actions had. From biological manipulations, to DDT, the effects are constantly reworked. While most of the historical pest outbreaks can be traced back to over production, pests are a reality that every gardener faces. Biological controls are, more often than not, more sustainable than the ‘chemical treadmill’ industrialized nations have been on. The basic concept of biocontrol revolves around cycles of pest/prey populations based on the carrying capacity of their food. The gardener must seek a stable situation where pest and prey oscillate below destructive levels. Diversification of the agroecosytem is the simplest way to stabilize pest populations. If effective, problems will be put into check automatically.

Biodiversity

We are currently experiencing a rate of extinction that is comparable to the Cretaceous extinction. However, little is known about the current number of species, as the majority of species diversity is composed of microbes. Biodiversity varies of course, with more found in the rainforest and less in the arctic tundra—but also within fragmented habitats and with agricultural intensity, biodiversity is negatively impacted. There is no doubt by sheer numbers of species, biodiversity is at greater risk in tropical than temperate zones.

So how can these seeds minimize that risk? Well I would suggest growing your food for one, and relying less on imported foodstuffs. It would also help to address the growing inequality found within the global food system to representatives in your government. These seeds offer a hands-on approach to crop diversity and activism. Where you go is up to you and the seed! You will be surprised that when you build it they will come. Seeds are power, knowledge is just the facilitator.

Now most of you know that your home gardens are much more diverse than their polar opposites—the industrial agricultural fields. Agricultural intensification inevitably depends on lower biodiversity and greater ‘control’ with agrochemicals, which substitute for functional biodiversity. So get growing, and get others growing, start a movement and change the world!