Nutrient Management Research Database

Based on a qualitative rating system of different orchard floor management systems (Hogue and Neilsen, 1987), mulches compare better for soil and crop quality than do herbicides. Mulches were rated as very beneficial for so il organic matter, phosphorus and potassium, and soil moisture, and for regulating tree vigor and providing good yields. Where mulches are weakest is in providing soil nitrogen, and this is where including a legume cover crop or incorporating a “green manure” crop into an organic orchard soil can be beneficial. In addition, composts, animal manures, and other organically approved materials containing nitrogen can be applied. Cultivation or tillage is generally a less desirable op tion in managing orchard floors because it is detrimental to soil organic matter and reduces soil moisture.

Unlike more chemically intensive conventional apple production, organic growers should understand and work with the biological processes within the orchard soil, because they have fewer and more expensive commercial options for nutrient management. Since the soil is the one natural resource that distinguishes organic from conventional farms, improving the soil is the key to successful organic farming, and orchard floor and nutrient management practices are the tools available to accomplish this goal. Specific orchard floor and nutrient management decisions depend on the unique conditions of soil and climate at the orchard site. Of course, conventional growers are free to adopt some of the same practices available to organic growers in order to benefit the quality of their soils as well. Because little research has been conducted to understand the interactions between orchard floor and nutrient management and biological processes in the soil of organic orchards, it is necessary for organic growers to test organic management practices and products themselves on small plots within their orchards. In time, as more research is conducted, we will gain a better understanding of how organic orchard floor and nutrient management practices affect the soil and then us e them to improve orchard productivity and fruit quality of organic apple orchards

This paper outlines the important considerations in organically managed apple orchards.

Sections include:

• Nitrogen Cycle
• Nutrient Loss and Gain
• Effects of Floor Management
• Biological Nitrogen Fixation
• Soil Nitrogen Transformations
• Soil Physical Properties
• Approved organic fertilizers and amendments

Nitrogen Cycle

The nitrogen and carbon cycles are the two most critical cycles for growers to consider. This paper focuses on the N cycle, as it controls plant available N.

Most N on earth is in the form of nitrogen gas, a simple compound consisting of two nitrogen atoms. Before this becomes plant available it must be 'fixed' by microbes that live in the roots of legumes.

Organic forms of N, whether from plant residue or organic amendments, must first be mineralized before becoming plant available.

Estimated Losses and Gains of Nutrient from Apple Orchards

In a mature 'Delicious' apple orchard with a yield of 300 bu/ac, only 19 lbs N/ac per year was lost in harvested fruit. A total of 77% of N lost from the was returned to the soil.

Over 95% of calcium lost from the tree was retained in the soil system.

However, 44% of potassium was permanently removed the the system in harvested fruit. This came to a total of 51 lbs K/ac for one year.

Floor Management Effects

Straw mulches, cover crops, deep cultivation can influence tree root density. A study in an orchard growing 'Empire' and 'Jonagold' apples found tree root density highest under 6" of straw mulch. A crown vetch cover cropped reduced tree root density, as did deep cultivation and use of glyphosate.

Biological Nitrogen Fixation

Legume cover crops, or 'green manure,' can supply more than 60 lbs N/ac. The use of over crops demands careful management to avoid excess fertilization.

Controlling the Release and Immobilization of Nitrogen in the Soil

Nitrogen held in soil organic matter can be released as plant available N (known as mineralization) while plant available N can also be converted into organic forms not available to plants (known as immobilization).

The ratio of carbon to nitrogen (C:N ratio) determines whether mineralization or immobilization will occur. Organic amendments such as legumes, with relatively low C:N ratios of around 15:1 to 20:1 will release N held in soil organic matter. Grasses with C:N ratios of 40:1 to 80:1, on the other hand, result in N being tied up in organic forms that plants cannot use.

Manure has a particularly low C:N ratio, and can sometimes result in raid N release than injures plant roots. Sawdust has a particularly high C:N ratio and can immobilize N for long periods of time.

Organic material left on the soil surface can provide a steady supply of N without causing immobilization.

Soil Physical Properties

Organic floor management can improve soil bulk density, water infiltration rates, and soil structure when compared to the use of pre-emergent herbicides.

Approved Organic Fertilizers and Amendments

More details about the standards of USDA organic certification can be found at https://www.ams.usda.gov/grades-standards/organic-standards.