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Natural Weed Control

By Philip A. Wheeler, Ph.D.

There are many ways for growers to implement non-toxic weed control methods on their farms. The most obvious is to take the chemical farming approach and find an organically-approved material to do the killing. Very strong vinegar has been the most marketed material. The important factor in vinegar formulas is to include a surfactant to strip away any waxy protective coating on the plant surface to allow the desiccation (drying out) of the plant. Salt provides the same mode of action and may be included in the formula.

Other modern mechanical approaches to weed control include flaming, cultivating, and smothering. Cultivators are a modern version of hoeing or hand pulling. Rotary hoes or spiked harrows are special adaptations of the cultivation approach. Using plastic films, whether biodegradable or not, is a form of smothering that is similar to mulching with any material. The cover denies sunlight to prevent growth.

Repeated cuttings of a perennial weed in a fallow field may weaken a plant over time by using up its stored energy. Farmers should also make every attempt to prevent the reseeding of an offending species. Treating isolated patches is worth the effort to keep them from spreading. If a field is overwhelmed to a point of not having an economic crop worth harvesting, be sure to take the whole field down before the weeds go to seed. Keep in mind that there are seeds in your fields that may have been there for years. Just lime or activate the calcium in your soil and watch clover appear in uncultivated ground, even if you haven’t seeded it since you bought the farm.

By the same token, there are all kinds of weed seeds that are just laying there, waiting for you to damage your soil to a point where they can spring out.  Weeds also wait for you to use a herbicide to suppress one of your current weeds, which has been keeping another weed you weren’t even aware of under control. Nature knows what she is doing!

Improving Soil for Weed Control

A more natural form of weed control is to activate the forces of nature by crop rotation, which allows the allopathic properties of each subsequent crop to affect the soil. Cover cropping is usually a more direct attempt to apply the specific beneficial properties of a given cover crop to the specific weed problem. Online sources of research are readily available to match specific cover crops to specific weeds. I have always recommended the use of a mixture of two or three plant species for cover cropping, and articles have recently appeared that report that mixtures of ten to fifteen species are providing even better weed suppression and soil health.

The allopathic effect of the economic crop can be amplified by cross-drilling small grains. During my organic farming days I used a little more than half the normal seeding rate and then drilled a north-south pattern followed by an east-west pattern. This increased the soil area impacted by the  economic crop to almost total coverage within the field. The results were spectacular, especially considering the previous corn crop had been infested with redroot pigweed that had gone to seed.

man on tractor
The primary condition that promotes broadleaf weeds is the ratio of available phosphorus to available potassium, as shown by a LaMotte soil test. The further you deviate from a 1:1 ratio, the stronger the broadleaf pressure.

Since all weed and grass germination is a function of soil and weather conditions, the most natural way to accomplish weed control is by soil mineralization and bioactivation.

The accumulation of knowledge from the last hundred years of research has provided us the standard Cation Exchange Capacity (CEC) model to aim for on our fields. You will find variations between labs and consultants, but don’t let that discourage you from taking tests and using them as a guide to begin balancing your soil for the purpose of production of nutrient-dense crops, with the added benefit of discouraging weed and grass pressure.

Achieving a soil that is so perfectly balanced and biologically active as to not have any weed or grass pressure is difficult, but not impossible. Dr. Rahm taught that the primary condition that promotes broadleaf weeds is the ratio of available phosphorus to available potassium, as shown by a LaMotte soil test. The further you deviate from a 1:1 ratio, the stronger the broadleaf pressure. In the 1970s, an associate of mine, Joe Scrimger, had a place on his farm where the usual herbicides totally failed to control the typical weeds he had in other fields. We performed LaMotte tests and compared the bad spot to a typical field. In his fields where the herbicide gave reasonable control, the ratio of P to K was 1:4. In the area where the herbicides failed, the ratio was 1:8. The weeds in that area were so healthy and strong that they could overcome the toxic effects of an herbicide applied at the usual rates.

I have regretfully discovered that excess wood ash, which is high in available potassium, is a sure way to have uncontrollable weeds in my farm garden. I have also noticed that areas receiving raw manure are prone to redroot pigweed and lambsquarter — aka, “fertility weeds.” It is now generally understood that broadleaf weeds thrive on diets of highly available nitrate and potassium. In standard NPK fertilizers, potassium is much more available than phosphorus due to its dependence on biological activity and its propensity to chemically bind with calcium. That bonding reduces the available calcium in your soil, increasing the pressure for Mother Nature to germinate a sour grass such as quack or crabgrass to bring calcium to the surface. In other words, NPK farming promotes weeds and grasses, increases your “rescue chemistry” costs, and lowers the value of your crop.

Another way of verifying that you are fertilizing for weeds is to measure the Brix of the weeds and of the cash crop. If the Brix of the weed is higher than the economic crop, then obviously the weed is healthier/stronger than the economic crop. The logical conclusion is that your fertilizer program favored the weeds rather than the crop you were trying to harvest and sell. If you were intending to make corn silage, it could ironically turn out that the feed value of your silage would be higher with more high-Brix broadleaf weeds than with less.

Oats cover crop for weed control
The most natural way to accomplish weed control is by soil mineralization and bioactivation.

Organic and sustainable growers can fall victim to the same phenomenon as conventional NPK farmers when they apply raw or partially-composted manures, organically approved Chilean Nitrate (NaNO3), or any other soluble nitrate, sodium, or potassium source. Even if they have applied a natural phosphate, failure to insure mycorrhizal inoculation and general bioactivation can lead Mother Nature to think that the ratios are still off.

Most of the weed problems in farming are surface problems. Each time the soil is disturbed, new weed seeds come to the surface, rays of sunlight strike them, and they germinate if the other soil signals tell them it is their turn to come out. This relation to sunlight exposure led to another weed control approach: cultivating or planting at night with hooded or red headlights to avoid the sunlight-triggering response.

Because of GMOs, we now also have to add the factor of herbicide resistance. According to the March 2013 issue of The Furrow, Arkansas farmers had to hand hoe up to 52 percent of the state’s cotton acreage at a cost of $30 per acre.

Weed Control: Results in the Field

I first learned of the successful use of liquid calcium and molasses for surface weed control in the early eighties. At about the same time I learned that oats were a good accumulator of phosphorus and that winter-killed oat crop residue provided both allopathic suppression as well as additional bio-available phosphorus. After writing about this in my newsletter, a Pennsylvania farmer reported a very successful and weed-free, no-till corn crop by planting a heavy fall cover of bin-run oat seed, letting it winter-kill, and then no-till drilling his corn seed into the residue. He also added the liquid calcium and molasses spray right after planting. The net effect was a weed-free field of organic corn.

The next report we received was from a strawberry grower. For those of you who have come by our booth at an Acres U.S.A. conference, the sequential pictures show the amazing total suppression of weeds and grasses in what amounts to open soil between the rows of strawberries for an entire season. The grower planted a heavy crop of fall oats in Pennsylvania. Naturally, they winter-killed. After tilling them in, he planted his rows of strawberries. Following the correct procedure, he sprayed a liquid calcium and molasses mixture at a slightly reduced rate. Since this was a U-pick operation, he repeated his calcium-molasses spray at about one-quarter rate every few weeks as he was getting soil disturbance from customers. The net effect was almost perfect weed control without the use of any mulch, any mechanical removal of weeds, or any toxic spray.

Scores of growers have since tried this concept on every imaginable crop. At first the two-gallon liquid calcium part of the formula was applied as non-organic chelated calcium nitrate or organic chelated calcium chloride, along with the two gallons of molasses. The molasses is critical to the formula as it feeds/activates the phosphatase bacteria that break the bonds of tied-up phosphorus. A few years ago, two alternative sources of calcium came on the market: limestone ground to five hundred mesh and Tennessee Brown phosphate that contains both calcium and phosphorus ground to five hundred mesh. Both are available as liquid suspensions (Premium Cal 33 and Phos Cal 22) for easy mixing and blending.

The latest test of the formula came in 2013 on a California organic rice farm. A Lundberg Farms contract grower used these procedures and materials to achieve control of water grass on ninety-two acres of short-grain rice. As the last disturbance of soil was leveling, they applied the weed suppression formula of two gallons of molasses and forty-two ounces per acre of Phos Cal 22 (we had recommended sixty-four ounces per acre, but the final rate was more of a factor of, “we have this much product and this much acreage to cover”) in twenty gallons of water applied by ground rig. They took the additional step of using drop nozzles to insure a uniform coating on the soil. A few days later, they flooded the field and then seeded by air. The results were excellent.

The local flying service reported that the organic field had equal or better control than the chemical fields in the area. The yield was five thousand pounds per acre, at a weed control cost of less than $20 per acre!

This article appeared in the March 2014 issue of Acres U.S.A.

Philip Wheeler is a crop consultant and lecturer who has worked with growers all over the world. He can be contacted at Crop Services International Inc. or 616-246-7933. The Non-Toxic Farming Handbook, by Philip A. Wheeler and Ronald B. Ward, is available from the Acres U.S.A. bookstore.