Most organic fertility programs start with soil building practices that increase organic matter with cover crops, compost and fertilizer materials that mineralize slowly over time so that there is an adequate and steady supply of nutrients. There are times when you may need to boost the soil fertility to stimulate plant growth when existing programs aren’t providing what the plants may need. Some reasons:
- Soils are cool.
- The crop that you are growing is simply not finding enough minerals needed for sufficient growth through the existing supply in the soil.
- Compost or cover crops simply didn’t have sufficient nutrients in it to supply the crop that you want to grow.
- Routine soil and tissue tests show that you are deficient in nutrients for your crop and you need to take action to remedy any problems.
It is not Just the Fertilizers
In organically farmed systems, you need to have a healthy functioning microbiome in the soil as the microorganisms generally have to convert existing plant and animal residues into useable nutrients through microbial digestion. Generally, most organic fertilizers do not leach readily as the nutrients are bound into more complex organic molecules that are broken down by microbes. Because of this, the soil needs to be favorable for microbial activity. Things that are required:
- Favorable pH: microbes grow best when the soil is near pH 7 as this is near ideal for the mineralization of most plant nutrients. Acid soils need to be amended with calcium (such as lime) and alkaline soils need to be acidified with sulfur. These actions may take time. It is useful to have amendment materials blended into your compost if possible to simplify application and there are no doubt synergies to mineralizing and releasing both calcium and sulfur into the soil that you are applying the material to.
- Moisture: microbes don’t grow in dry soil. You have to time your compost applications and incorporation of cover crops at a time of the year when there is sufficient moisture in the soil to allow microbes to do their work. Fall applications of compost prior to working up soil for seeding winter cover crops is very favorable for starting the nutrient cycling process. If you are going to turn under cover crops in the spring, it is best to do so when the cover crops are flowering and there is still moisture in the soil. If you are applying concentrated organic fertilizers, you also want to make sure that they end up in an area that will have sufficient moisture to mineralize and work.
- Warmth: Microbes won’t grow well in cold soil. Most decomposers need temperatures of at least 60 degrees F to grow. Applying concentrated fertilizers to cold soil won’t work effectively until the soil is around 70 degrees F.
- Air: While the soil microbes need moisture, they won’t perform very well under water logged conditions when there is no oxygen in the soils. Under these conditions, denitrification of organic fertilizers can occur releasing NO 2 which is a potent greenhouse gas. If nitrates are present just ahead of a long irrigation, they may get leached right through the root zone, ending up in the water table where they are causing contamination. It is important to irrigate following fertilization in a way that provides for the needs of the plant but doesn’t leach nutrients beyond the root zone.
Strategies for Fertilizing with Concentrated Organic Fertilizers
- Foliar sprays: These materials are expensive and usually are used at key times like flowering, pre-veraison or other periods where you think you need a boost of some key nutrients. They are usually water soluble. Boron, potassium and calcium are often used, frequently applied with wettable sulfur sprays around bloom time on grapes to help set a crop. There are numerous proprietary mixes that many growers put on as a kind of insurance policy. They can make a difference in fruit set, particularly if you are farming on low fertility soils. Long term, roots are much better adapted to absorbing nutrients and working through the soil is the most cost effective way to fertilize. Fish protein applied to the leaves also can be a quick way to green up foliage but use it early in the season so that there is no residual tastes or smells.
- Concentrated dry (and usually pelletized) fertilizers: Almost all of the nitrogen fertilizers are plant and animal residue based. Most have pretty low analysis of under 10 percent nitrogen and less that 1 percent phosphorus or potassium. They are functionally time release fertilizers since they need to be digested by microbes and then will eventually release nitrate ions for uptake. Depending on the source, this can take from 2 to 12 weeks. Plant sources include alfalfa meal and cotton seed meal. Feather meal, bone meal and other animal by products are used, often mixed with plant derived fertilizers as well. To work effectively they need to be placed into the soil. If the vineyard is drip irrigated, a shallow hole should be made beneath drippers, the fertilizer placed in it and then covered back up. Another effective way is put the fertilizer beneath the dripper and then throw a shovel full of compost or mulch over it. If you leave it uncovered, it is likely to be a pretty ineffective application and a great treat for passing wild life since any animal proteins are attractive to everything from birds to dogs.
- Fertigation: Most organic fertilizers are not truly soluble and have to form a fine suspension in the irrigation water in the drip tubes during fertigation and then pass through the emitters. A few key points: make sure the material is suited for this application method. It should be able to pass through a 200 mesh screen. Also, inject the material before your irrigation system’s filters to avoid accidental clogging. Start the system up and run water into it 10 or 15 minutes to pressurize everything and get water flowing. Then inject the material required, running for enough time after the material is injected to send it out through the end of the system completely followed with enough water to get it into the soil. This may require as much as an hour or more after injection depending on how your system is set up. Besides nitrogen, solution grade potassium fertilizers can be used (potassium sulfate) and usually extra water is required to insure that all of the material goes through the system. Be careful with any phosphorus materials—they can permanently clog your system if you have high calcium water. Best to use soil applied materials for that task. If you are making stock solutions, use everything up as organic fertilizers will become a microbial soup if left sitting around and could be a health risk if you are applying it to anything that might be consumed uncooked.
Concentrated organic fertilizers are expensive so they are best viewed as a supplement to standard soil organic fertility programs based on compost and cover cropping. If your plants need an additional nutritional boost at key times such as bloom and fruit growth, it is totally appropriate to consider using some additional nutrients to improve growth. Keep records, always leave a few spots unfertilized to see if the material that you applied actually made a difference.