Fertilizer grades refer to the guaranteed percentages of plant nutrients. The ratio refers to the proportion of nitrogen (N), phosphate (P₂O₅) and potash (K₂O) in the fertilizer. For example, the grade is 5-10-5, and the ratio is 1-2-1. High analysis fertilizers are those with grades such as 20-20-20, 35-0-0 or 0-46-0, and are more economical to use on the basis of price per pound of nutrient.

Liquid starter fertilizers are materials that are completely water soluble and are high in phosphorus content such as 16-32-16, 10-52-17 or 9-45-15. These materials are used at the time of transplanting. Dry starter fertilizer can be banded at the time of seeding. The band is normally placed 2" below and 2" to the side of the seed. When using either dry or liquid starter fertilizer, follow label directions because an excess of starter could burn seeds and young seedlings. Starter fertilizer promotes early and rapid growth that leads to greater yields with certain crops such as tomato, pepper, and melon. See individual crops for rates.

Table 6, "Plant Nutrient Content of Various Fertilizer Sources" lists many fertilizers commonly used for vegetable cropping systems. Similar information for fertilizers and amendments commonly used in certified organic cropping systems are provided in the section entitled Guidelines for Organic Fertility Management (Table 10).

Table 6: Plant Nutrient Content of Various Fertilizer Sources (% by weight)

Fertilizer Source Material	Total Nitrogen N%	Available Phosphoric Acid P2O5 %	Water Soluble Potash K2O %	Combined Calcium Ca %	Combined Magnesium Mg %	Combined Sulfur S %
Ammonium sulfate	21	0	0	0	0	23
Ammonium nitrate	34	0	0	0	0	0
Anhydrous ammonia	82	0	0	0	0	0
Calcium nitrate	15	0	0	19	0	0
Calcium ammonium nitrate	27	0	0	0	0	0
Diammonium phosphate	18	46	0	0	0	0
Monoammonium phosphate	11	48	0	0	0	0
Epsom salts	0	0	0	0	10	13
Granulated Sulfur	0	0	0	0	0	90-92
Gypsum	0	0	0	19-23	0	15-18
Muriate of potash	0	0	60	0	0	0
Nitrate of potash	13	0	44	0	0	0
Nitrate of soda-potash	15	0	14	0	0	0
Nitrate of soda	16	0	0	0	0	0
Superphosphate	0	20	0	18-21	0	11
Sul-po-mag	0	0	22	0	11	23
Sulfate of potash	0	0	50	0	0	17
Triple superphosphate	0	44-46	0	13	0	0
Urea	45-46	0	0	0	0	0

 

 

Table 6a: Conversion Factors for Boron Nutrient Sources

Fertilizer Source Material	Boron content, %	Pounds of Material Required to Supply One Pound of Boron
Fertilizer Borate Granular1	14.30	7.0
Fertilizer Borate-48	14.91	6.7
Solubor	20.50	4.9
Fertilizer Borate-68	21.13	4.7

¹ Best for fertilizer blends.

 

 

Animal manure is an excellent source of nutrients and organic matter. Many of the nutrients in fresh livestock manure, especially nitrogen, are readily available.  Nutrient content varies by animal species, their diets and the form of their manure. There are times when readily available nitrogen is needed, but many people prefer to compost manure before field application (see Compost section below). Manure application rates are now regulated in many New England States (see Nutrient Management Regulations).

Nitrogen in manures and other waste products: The N content of manures is highly variable. Differences are due to the species of animal, the animal's age and diet, the moisture content of the manure, handling and storage, and the amount of bedding in the manure. The N fertilizer equivalent of a manure varies not only with the total N content of the manure, but also with the timing and method of manure application. Manure samples can be analyzed by the Universities of Maine and Vermont Laboratories. The values in Table 7 are based on analyses of Vermont manures as well as published data from other states. If specific manure analysis data is not available, growers should estimate N credits using these or other book values. The time elapsed between spreading and incorporation of manure is also important. About half of the N in dairy manure and three quarters of the N in poultry manure is in the form of ammonium (NH₄), which easily turns to ammonia gas (NH₃) and is volatilized (lost to the air). The longer that manure is left on the soil surface, and not incorporated, the greater NH₃ volatilization losses become (Table 7a). Broadcast application of slurry manure without incorporation should always be avoided because this method increases air contact and allows time for all ammonia to be lost. Research has shown that in reduced or no-till fields where manure must be surface applied without incorporation, ammonia can be best conserved if applied during cold temperatures, low wind speeds and especially to a growing cover. A growing cover also reduces manure run-off and leaching losses.  NOTE: Manure often contains human pathogens. Serious illness has occurred from eating produce where fresh manure was applied without an adequate waiting period (see Produce Safety).

Previous manure applications: Up to 50% of the total N in cow manure is available to crops in the year of application. Between 5% and 10% of the total N applied is released the year after the manure is added. Smaller amounts are furnished in subsequent years. The quantity of N released the year after a single application of 20 tons per acre of cow manure is small (about 15 lb N per acre). However, in cases where manure has been applied at high rates (30-40 tons per acre) for several years, the N furnished from previous manure increases substantially. The buildup of a soil's capacity to supply N resulting from previous applications of manure has important consequences for efficient N management, including: 1) The amount of fertilizer N needed for the crop decreases annually; and 2) If all the crop's N needs are being supplied by manure, the amount of manure needed decreases yearly.

With poultry manure (as compared with manure from cattle) a higher percentage of the total N in the manure is converted to plant-available forms in the year of application. Consequently, there is relatively less carry-over of N to crops in succeeding years. This does not mean, however, that there is never any carry-over of N from poultry manure applications. If excessive rates of poultry manure (or commercial N fertilizers) are used, high levels of residual inorganic N, including nitrate (NO₃), may accumulate in soil. High levels of soil nitrate in the fall, winter and spring have the potential to pollute groundwater and coastal seawater.

Table 7: Nitrogen Credits from Manure Applied Before Planting

Type of manure	Dry Matter	Total N	NH4-N	Organic N	P2O5	K2O
		------------------------- lbs/1,000 gallons  -----------------
Dairy, liquid	<5%	12-16	4.9	7.3	4.8	15.1
Dairy, slurry	5%-10%	22.3	7.6	14.7	8.9	22.0
		---------------------------  lbs/ton  -----------------------
Dairy, semi-solid	10%-20%	8.5	1.8	6.7	4.1	6.1
Dairy, solid	>20%	5-12	1.4	10.9	8.1	10.0
Beef (paved lot)	29%	14	5	9	9	13
Swine (hoop barn)	40%	26	6	20	15	18
Sheep	25%	23	n/a	n/a	8	20
Poultry, layer	41%	16-37	18	19	55	32
Poultry, broiler	69%	75	15	60	27	33
Horse	20%	12	n/a	n/a	5	9

Adapted from Nutrient Recommendations for Field Crops in Vermont (2018). Dairy manure values are from Vermont samples analyzed by University of Maine, 2012-2016, others are adapted from University of Nebraska-Lincoln NebGuide G 1335 and Penn State Agronomy Guide (2016). Values do not include bedded pack. Manures vary greatly, so obtaining a manure analysis is always best practice. n/a = data not available.

Table 7a: Availability of ammonium nitrogen from spring or summer applied manure (% fertilizer N equivalent)

	Cattle1 Thin (<5% DM)	Cattle MEdium (5%-10% DM)	Cattle Semi-SOlid (>10% DM)	Cattle Solid (>20% DM)	Poultry Solid (>20% DM)
Time to incorporation by tillage or rain	-------------- % NH4 - N available to crop -----------
Immediate	95	95	90	95	95
< 8 hrs	80	70	60	80	90
1 day	70	55	40	60	85
2 days	65	50	30	45	80
3-4 days	65	45	23	35	70
5-7 days	60	40	25	25	60
>7 days, or not incorporated	60	40	20	10	50

 

Composting livestock manure and other organic matter stabilizes the nutrients by partially decomposing the materials.  Nutrients are releaed more slowly from finished compost than from fresh livestock manure.  Compost is considered mature (i.e., finished) when most of the easily decomposed components of the material have been broken down and biological activity has slowed. At this time, the pile returns to ambient temperature, and it does not reheat when mixed or turned. The composting process results in a dark-brown material in which the initial constituents are no longer recognizable and further degradation is not noticeable. The length of time needed to achieve finished compost will vary with many factors and can range from a couple of weeks to over a year.

Application of unfinished compost could affect plant growth adversely because the compost-making microbes may compete with the crop for nitrogen. Applying compost at least one week before transplanting or seeding a crop will allow a margin of safety in case the compost is immature. Immature composts made from nitrogen-rich feedstock are also often high in ammonium, which can change to ammonia gas and be toxic to plant growth. High ammonium concentrations are not typically a problem if the compost is field applied, but if compost will be used in a greenhouse mix, it is important that it be low in ammonium. 

Vegetable growers can make compost on the farm although most don’t have enough raw materials to satisfy their needs. Some bring in additional materials such as manure or municipal yard wastes to compost on-site. Others purchase compost from commercial composters. 

Compost as a nutrient source. Finished compost is a dilute fertilizer, typically having an analysis of about (1-1-1 N-P₂O₅-K₂O), but the analysis can vary greatly depending on the types of materials used to make the compost and how they were composted. Composts should be analyzed for their available N, total N, P₂O₅, and K₂O content before application to agriculture fields.

Carbon to Nitrogen Ratio. The recommended C:N ratio for finished compost is 15-18:1. The C:N ratio plays a crucial role in the availability of nitrogen in any organic material added to the soil. If the C:N ratio is much above 30:1 microorganisms will immobilize (i.e., consume and make unavailable for plant uptake) soil nitrogen. This soil nitrogen will remain unavailable until the carbonaceous material is consumed by the bacteria.

Table 8: Typical Carbon-to-Nitrogen Ratios

Material	C:N RATIO
Legume hay	15-19:1
Non-legume hay	24-41:1
Corn stalks	42:1
Oat straw	70:1
Rye straw	82:1
Cow manure	18:1
Finished compost	17-20:1
Agricultural soils	8-14:1
Hardwood sawdust	500:1

 

Nitrogen. The majority (usually over 90%) of the nitrogen in finished compost has been incorporated into organic compounds that are resistant to decomposition. Rough estimates are that only 10%-30% of the nitrogen in these organic compounds will become available in the first season following application. Some of the remaining nitrogen will become available in subsequent years and at much slower rates than in the first year. Repeated annual applications of compost at high rates above 400 pounds of nitrogen per acre can result in excessive amounts of nitrate in the soil.

Phosphorus. There is not much research information published about the availability of phosphorus from compost. The few papers published show that composts made primarily from manures supply phosphorus over the growing season at 70%-100% of the availability of triple superphosphate fertilizer. The amount of organic amendments that can be added without building up excessive phosphorus depends primarily on: 1) the existing soil test P level of the field; and 2) the P₂O₅ content of the amendment. Table 9 shows the effect of both soil test P categories and the P₂O₅ concentration of an organic amendment on the suggested maximum amount of material to apply. If these rates of amendments are applied every year, analyze the soil for extractable P annually to ensure that soil test P has not risen to excessive levels. Additional compost applications to soil that tests optimum for P could increase P to above optimum levels. If a soil test shows an above optimum P level, avoid compost applications until P returns to the optimum range. 

Table 9. Maximum Compost or Organic Amendment Application and total P₂O₅ per Soil Test Category and P₂O₅ Concentration¹

	Soil test phosphorus (P) Category
Compost/organic amendment P2O5 content	Very Low/Low		Optimum		Above optimum
% P2O5 (dry wt.)	P2O5 (lb/acre)	Compost (tons/acre)	P2O5 (lb/acre)	Compost (tons/acre)
Low (0.1%-0.5%)2	330	120	82	30	No application
Medium (0.5%-1.5%)	330	30	55	5	No application
High (1.5%-3.0%)	330	15	No application		No application

¹ Assumes moisture content of the compost or organic amendment of 45%.

² Average rates used to calculate amounts of P₂O₅ applied for various rates of compost applications.

Potassium. Potassium in finished compost is much more available for plant uptake than nitrogen because potassium is not incorporated into organic matter. However, some of the potassium can be leached from the compost because it is water soluble. In one study, potassium levels were reduced by 25% when finished compost was left uncovered in the open over a winter.

Soluble Salts. In general, soluble salts are not a concern from additions of composts to field soil. However, soluble salts can be a serious problem when using compost in greenhouse mixes. Incorporation of 40 tons per acre of compost in the top 6 inches of field soil would be a ratio of 50 parts soil to one part of compost. Compost used in the preparation of greenhouse media will make up a much greater percentage of the whole mix and therefore will have a greater influence on all aspects of fertility, including soluble salts. It is important to have composts tested for salt levels. Electrical conductivity (EC) is a measure of salt level, and compost used in greenhouse mixes should have EC < 1 mmhos/cm.

Compost and pH. The pH of finished compost is usually slightly alkaline. In general, composts will not raise soil pH to undesirably alkaline levels because of the low total alkalinity of composts. However, caution should be taken if the compost has been “stabilized” with the addition of lime (thus increasing the total alkalinity) or with heavy applications to certain crops such as potatoes, for which the soil pH should be about 5.2. Heavy applications can cause increases in soil pH that might last for a growing season.

Heavy Metals and Trace Elements. The danger of heavy metals in some composts has received much attention. At one time, some heavy metals in some composts were high enough to be toxic to plants (copper, nickel, zinc) or of concern to human health (cadmium). There have been documented cases where elements such as boron have been raised to toxic levels with repeated applications of compost. These composts with high metals or boron were made from materials with high concentrations of these elements. Governmental regulations control the materials that may be used in composts for applications to farmland. None of these toxicity problems are likely to occur with compost that has been made from farm manures or crop residues or with the commercially available composts of today.

Herbicide Residues in Compost. There are broadleaf herbicides registered for use on turfgrass, pastures, and hay crops that retain activity in the manure of animals that have fed upon them, as well as through the composting process of crop residues from areas treated with these herbicides. There have been many cases where vegetable growers have unknowingly purchased organic amendments such as manure and composts that are contaminated with herbicides and have damaged vegetable crops. If you purchase organic amendments, you should be aware of this possibility and get assurance that herbicides are not present in the manures and composts that you purchase. 

Have Compost Analyzed. No compost should be applied to field soil or used in greenhouse mixes without testing for nutrient content. If the compost will be used in greenhouse mixes, it should also be tested for maturity. Some soil test labs will test compost. Check to be sure the lab analyzes compost before submitting samples, and make sure to have it tested as a compost sample, not as field soil.

Take Soil Test After Applying Compost. A good way to evaluate the effect of compost on the fertility of a soil is to obtain a soil test after applying compost. It is best to wait six to eight weeks after application before testing the soil to allow the compost and soil to equilibrate. 

There are many different types of commercially available soil amendments and plant nutrient sources on the market today.   While some products contain detectable quantities of nutrients that become available to plants in the near term, other products may instead increase availability of existing plant nutrients in the soil. Many have not been well tested in controlled studies.  There are many  categories of such products available for purchase. Some, but not all, have been approved for organic production.

Organic Residuals

Organic by-products of industrial processes fall into this category. Note that the use of the word “organic” in this case refers to the nature of the material itself, e.g. derived from biological sources. It does not necessarily indicate acceptability for certified organic production. Materials included in this category include processed slaughterhouse wastes, leather processing waste, biosolids, papermill sludge, and composts. In general, as these products decompose, plant nutrients are released. Many are sold with the nutrient analysis content listed, which is generally very low on a “percent-by-weight” basis. The greater benefits are usually for soil conditioning, and in some cases, liming activity. Not all of these products are acceptable for certified organic production, and acceptability for use in food production should be verified.

Foliar Amendments

Foliar feeding has become a more common practice among some vegetable farmers. Many products are now available on the market, for use in both conventional and certified organic production. Foliar feeding is not recommended as a major source of nutrients for a growing crop, but it can be used for supplemental feeding under certain circumstances. Such circumstances include: 1) when soils are cold and N and P mineralization rates are low; 2) at the onset of nutrient deficiency symptoms in rapidly growing plants (verified by properly conducted leaf tissue testing); and 3) during periods of high nutrient demand, especially fruiting. Even so, nutrient deficiencies often result from indirect causes, such as water issues, soil compaction, pH, root diseases or even macronutrient (N, P or K) deficiencies that can be limiting micronutrient uptake or availability. Addressing these issues is likely a more long-term, as well as time- and cost-effective way to ensure crop micronutrient needs are met.

New England soils are glacial in origin and are considered “young.” For this reason, our soils are not typically lacking in micronutrients. In soils with pH greater than 7, metal cations become less available to plant roots, and plants may show signs of deficiency. Most soils in New England are acidic, requiring periodic lime applications. Where soils are alkaline, the best way to correct deficiencies of Zn, Mn, Fe and Cu may be to apply foliar sprays of these nutrients in chelated form. Certified organic growers should ensure that they are using forms allowed under organic certification. In some cases, it may be necessary to lower soil pH using products such as elemental sulfur, aluminum sulfate or ammonium sulfate.

Plant Biostimulants, Biofertilizers, Microbial Biostimulants, Microbe-containing Bio-products

A biostimulant is a substance or microorganism (or mixture of one or more of these) applied with the intent of enhancing a crop’s nutrient efficiency, abiotic stress tolerance and/or quality traits, regardless of the material’s nutrient content. There are now hundreds of commercially available products that fall into these categories. This does not include products labeled for pest control purposes, however, which fall under strict EPA guidelines mandating EPA registration.

One category of these products is familiar to most: various strains of species of Rhizobium inoculants for legumes. Research has consistently shown the benefit of legume inoculation to realize full nitrogen fixation potential of legumes, provided that the plant and bacterial species are properly matched.

There has been a proliferation of mycorrhizal fungus inoculant products. These fungi are symbiotic with many crop plants (excluding brassicas and a few others) and extensive research has shown their beneficial effects on plant nutrition, growth, and stress reduction in field, nursery pot and greenhouse conditions. The fungi live inside plant roots, where they obtain a carbohydrate energy source from plants. In turn, the fungal mycelia transfer water and mineral nutrients to plants, which they can extract from the soil volume more efficiently. In this way, the fungi “extend” the rhizosphere that surrounds plant roots. Unfortunately, real-world test results of these products are not readily available. It is unknown at this time whether inoculation has short or long-term economic impact in annual vegetable production.

There are numerous other soil microbial inoculant mixtures available from commercial suppliers. Peer-reviewed research with many of these organisms has shown some positive potential. They are intended to influence crop plants’ rhizosphere, promoting potential availability of mineral nutrients already present in the soil, sometimes by stimulating plant responses to stresses or diseases. They do not, however, directly supply nutrient elements to plants. There may well be a promising future for microbial inoculants, particularly if it means reduced fertilizer input, but product effectiveness has not been well documented at this stage.

Compost Tea

The legal definition of compost tea from the National Organic Program is “A water extract of compost produced to transfer microbial biomass, fine particulate organic matter, and soluble chemical components into an aqueous phase, intending to maintain or increase the living, beneficial microorganisms extracted from the compost.” Microbial species content is highly variable, depending on the source of compost and the “brewing” conditions. Compost teas have a very low analysis of plant nutrients. Although they are widely produced and used on farms of various scales, research evidence of their efficacy is inconsistent at best. Benefits of using commercial microbial inoculants are variable, and using compost tea is even less dependable despite its widespread popularity. If you do plan to use compost tea, care must be taken to avoid cultivating bacteria harmful to human health (e.g., start with finished compost, use potable water, and avoid using additives like molasses).

Humates, Humic Acids, Humic Substances

These materials are made of very large and complex molecules. Most commercial products are extracted from peat or soft brown coal deposits of lignite. Extraction processes and treatments vary widely, so it is difficult to make comparisons between various products on the market. Humic materials contain only small amounts of plant nutrients, thus are not considered fertilizers. Their usage has been promoted by some to provide physical, chemical, and biological benefits to soils. These materials have been studied for over 50 years, mainly in controlled settings, with mixed results from laboratory and greenhouse studies; some resoundingly positive reports, many neutral, and a few detrimental. Under field conditions there are few documented positive effects from their usage. Naturally occurring compounds in soil organic matter effectively perform the same functions, such as chelation of micronutrient metals, and possibly producing plant hormonal effects. Nevertheless, there are many commercial products available, and little consistency among them.

Seaweed Extract Products

Seaweeds have been applied to agricultural land for at least a few thousand years and until recently, their primary benefit was considered to be similar to that of other organic amendments, releasing nutrients through decomposition. It was discovered over 50 years ago that seaweed nutrient content was too low to directly boost soil test nutrient levels and that other growth stimulating mechanisms must be involved. Seaweed has been proposed to have several different effects on the root zone environment and on plants themselves.

Though seaweed extracts are used in crop production in large quantities world-wide, there is surprisingly little published research on their use and effectiveness in field settings. One of the more common claims is alleviation of the effects of environmental stresses, such as temperature and moisture extremes. Subtle effects are difficult to measure in the field alongside many other possible factors. Therefore, when used during typical conditions, their effects are hard to detect.