Garden Guide: Adding organic matter to the home garden

Apr. 17, 2013 @ 04:59 AM

Lee County is fairly unique in that the Sandhills region of the state, characterized by deep, sandy soils, meets the Piedmont, characterized by clay soils. Sometimes this conversion happens in our own backyard!   

There are areas that have less topsoil due to erosion or little to none in new housing developments, where developers have stripped away the topsoil prior to building. Though we have topsoil, many of our soils are not black or brown in color, which I’ve found people associate with topsoil.

Soil is formed from organic materials and minerals derived from weathering rock. The mineral constituents of soil are broken down into three main particle sizes: sand, silt and clay. Sand is the largest particle size (you can see a grain of sand with your naked eye) and clay is the smallest (have you ever been able to distinguish a particle of clay?). The proportions of these three particles make up the soil texture.   

Why is this important? The texture and the structure of the soil determines how the soil will “behave.” Will the soil hold nutrients, will it drain freely, etc.?

So how do you correct and improve a soil? Regardless of whether you have a more clay soil or a more sandy soil, the answer is to add organic matter. Organic matter is residue that comes from a once-living or living organism and is often in a stage of of decomposition.   

Organic matter (OM) helps soil particles bind into aggregates, which improves nutrient holding capacity. In addition, OM improves water infiltration in clay soils and improves water retention in sandy soils. OM also adds nutrients to the soil.

In the spring and fall, OM should be added in a 2-3 inch layer and incorporated into the soil. Be careful when choosing your source of OM, since normal applications can increase nutrients and soluble salts to a level toxic to plants. Food compost and animal manure can be very high in soluble salts.   

Sources of OM include animal manure, compost, and cover crops or green manure.

When using animal manure, be very careful about using fresh manure, which can contain human pathogens such as E. coli, Listeria and Salmonella. If possible, choose aged or composted manure instead. If you do use fresh manure, apply it no less than 120 days (4 months) prior to the harvest of the crop.   

The nutrient content of manure varies based on the species of animal, the bedding, the storage of the material, and the age, as well as other factors. A few cautions when using animal manure: 1. fresh manure or poorly composted manure may contain viable weed seeds, 2. the manure may contain residual herbicides that can be dangerous to garden plants (ask your supplier what herbicides have been used on his/her pasture and hay), and 3. over application of manure may over fertilize plants and leach nitrates into the water supply. Nitrates can have serious health effects on infants, pregnant women and other susceptible humans, as well as animals.

Compost is decomposed organic matter and another good source of OM. The term compost is not regulated, so be on the lookout for poor quality products. Compost that is hot, smells like ammonia or the materials are readily identifiable has not gone through the complete compost process.   

Using cover crops when your garden is not in production can also add OM back to the soil without increasing soluble salt levels. A cover crop is a grass and/or legume “mat” established in September or October to prevent soil erosion and weed establishment. A legume is a plant in the bean family such as crimson clover, hairy vetch or Austrian winter peas. Legumes are unique in that bacteria that associate with the plant can convert atmospheric nitrogen into a plant-friendly nutrient (think free fertilizer!). Grasses that can be used for cover crops include annual ryegrass, rye and wheat. You can establish cover crops (with different plant species) in the summer; however, most people are using their gardens during this time.

Cover crops must be tilled into the soil at least one month prior to planting the garden. This prevents the cover crop from seeding and starts the decomposition process in the soil.   

Working with the soil in Lee County can be challenging. By using organic matter, you can improve your soil for plant growth. For more information on organic matter, call our Center at (919) 775-5624.

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