Pengaruh Agroforestri Terhadap Sifat Fisik Tanah

Tanah sangat penting artinya bagi usaha pertanian karena kehidupan dan perkembangan tumbuh-tumbuhan dan segala mahluk hidup di dunia sangat memerlukan tanah. Segala tumbuh-tumbuhan dan hasilnya yang sangat diperlukan bagi pertumbuhan dan perkembangan hidup manusia sepanjang masa akan sangat tergantung pada keadaan tanah selain iklim dan air. (Mulyani, 2008)

Tanah merupakan suatu lapisan kerak bumi yang tidak padu dengan ketebalan yang berbeda dengan bahan-bahan yang ada dibawahnya, yang juga tidak baku dalam warna, bangunan fisik, struktural, susunan kimia, sifat biologis, proses kimia atau reaksinya (Marbut, 1940). Menurut Buckman dan Brady (1969), tanah adalah suatu tubuh alam yang disebabkan oleh hancuran mineral dan percampuran mineral dengan bahan organik yang menutupi permukaan bumi dan mengandung sejumlah udara dan air, memberikan kekuatan mekanik, serta memberikan sebagian makanan kepada tumbuhan. Selanjutnya dikatakan bahwa tanah mineral terdiri atas empat komponen utama, yaitu bahan mineral, bahan organik, air dan udara.

Lapisan tanah atas adalah bagian yang paling cepat dan mudah terpengaruh oleh berbagai perubahan dan perlakuan. Kegiatan selama berlangsungnya proses alih-guna lahan segera mempengaruhi kondisi permukaan tanah. Penebangan hutan atau pepohonan mengakibatkan permukaan tanah menjadi terbuka, sehingga terkena sinar matahari dan pukulan air hujan secara langsung. Berbagai macam gangguan langsung juga menimpa permukaan tanah, seperti menahan beban akibat menjadi tumpuan lalu lintas kendaraan, binatang dan manusia dalam berbagai kegiatan seperti menebang dan mengangkut pohon, mengolah tanah, menanam dan seterusnya. (Widianto, 2003)

Dampak langsung dari berbagai kegiatan tersebut adalah menurunnya porositas tanah yang ditandai oleh peningkatan nilai berat isi. Tanah (umumnya lapisan atas) menjadi mampat karena ruangan pori berkurang (terutama ruang pori yang berukuran besar). Berkurangnya ruangan pori makro mengakibatkan penurunan infiltrasi (laju masuknya air ke dalam tanah), penurunan kapasitas menahan air dan kemampuan tanah untuk melewatkan air (daya hantar air). (Widianto, 2003)

Sistem agroforestri pada umumnya dapat mempertahankan sifat-sifat fisik lapisan tanah atas sebagaimana pada sistem hutan. Sistem agroforestri mampu mempertahankan sifat-sifat fisik tanah melalui:

• Menghasilkan seresah sehingga bisa menambahkan bahan organik tanah
• Meningkatkan kegiatan biologi tanah dan perakaran
• Mempertahankan dan meningkatkan ketersediaan air dalam lapisan perakaran

(Widianto, 2003)

Konsepsi agroforestri dirintis oleh suatu tim dari Canadian International Development Centre, yang bertugas untuk mengindentifikasi prioritas-prioritas pembangunan di bidang kehutanan di negara-negara berkembang dalam tahun 1970-an. Oleh tim ini dilaporkan bahwa hutan-hutan dinegara tersebut belum cukup dimanfaatkan. Penelitian yang dilakukan dibidang kehutananpun sebagian besar hanya ditujukan kepada dua aspek produksi kayu, yaitu eksploitasi secara selektif di hutan alam dan tanaman hutan secara terbatas.

Menurut International Council for Research in Agroforestry, mendefinisikan agro forestri sebagai berikut :

" Suatu sistem pengelolaan lahan dengan berasaskan kelestarian, yang meningkatkan hasil lahan secara keseluruhan, mengkombinasikan produksi tanamaan (termasuk tanaman pohon-pohonan) dan tanaman hutan dan/atau hewan secara bersamaan atau berurutan pada unit lahan yanag sama, dan menerapkan cara-cara pengelolaan yang sesuai dengan kebudayaan penduduk setempat". (King dan Chandler, 1978)

Menurut Lundgren (1982), definisi agroforestri seyogyanya menitikberatkan dua karakter pokok yang umum dipakai pada seluruh bentuk agroforestri yang membedakan dengan sistem penggunaan lahan lainnya:

1. Adanya pengkombinasian yang terencana/disengaja dalam satu bidang lahan antara tumbuhan berkayu (pepohonan), tanaman pertanian dan/atau ternak/hewan baik secara bersamaan (pembagian ruang) ataupun bergiliran (bergantian waktu);

2. Ada interaksi ekologis dan/atau ekonomis yang nyata/jelas, baik positif dan/atau negatif antara komponen-komponen sistem yang berkayu maupun tidak berkayu.

Animal Manures

for Increasing Organic Matter and

Supplying Nutrients

The quickest way to rebuild a poor soil is to practice

dairy farming, growing forage crops, buying . . .

grain rich in protein, handling the manure properly,

and returning it to the soil promptly.

— J. L. HILLS, C. H. JONES, AND C. CUTLER, 1908

Once cheap fertilizers became widely available after World War II, many farmers, extension agents, and scientists looked down their noses at manure. People thought more about how to get rid of manure than how to put it to good use. In fact, some scientists tried to find out the absolute maximum amount of manure that could be applied to an acre without reducing crop yields. Some farmers who didn’t want to spread manure actually piled it next to a stream and hoped that next spring’s flood waters would wash it away. We now know that manure, like money, is better spread around than concentrated in a few places. The economic contribution of farm manures can be considerable.

The value of the nutrients in manure from a 70- cow dairy farm may exceed $7,000 per year; manure from a 50-sow farrow-to-finish operation is worth about $4,000; and manure from a 20,000- bird broiler operation is worth about $3,000. The other benefits to soil organic matter build-up, such as enhanced soil structure and better diversity and activity of soil organisms, may double the value of the manure. If you’re not getting the full fertility benefit from manures on your farm, you may be wasting money.

Animal manures can have very different properties, depending on the animal species, feed, bedding, and manure- storage practices. The amounts of nutrients in the manure that become available to crops also depend on what time of year the manure is applied and how quickly it is worked into the soil. In addition, the influence of manure on soil organ matter and plant growth is influenced by soil type. In other words, it’s impossible to give blanket manure application recommendations. They need to be tailored for every situation.

We’ll deal mainly with dairy cow manure, because there’s more information about its use on cropland. We’ll also offer general information about the characteristics and uses of some other animal manures.

MANURE HANDLING SYSTEMS

Solid versus Liquid

The type of barn on the farmstead frequently determines how manure is handled on a dairy farm. Dairy-cow manure containing a fair amount of bedding, usually around 13 to 20 percent dry matter, is spread as a solid. This is most common on farms where cows are kept in individual stanchions. Liquid manure-handling systems are common where animals are kept in a “free stall” barn with little bedding. Liquid manure is usually in the range of from 2 to 10 percent dry matter (90 percent or more water). Manures with characteristics between solid and liquid are usually referred to as semi-solid or slurry, depending on the method of handling.

Composting manures is becoming an increasingly popular option for farmers. By composting manure you help stabilize nutrients, have a smaller amount of material to spread, and have a more pleasant material to spread (and if neighbors have complained about manure odors, that might be a big plus). Although it’s easier to compost manure that has been handled as a solid, some farmers are separating the solids from liquid manure and then irrigating with the liquid and composting the solids.

Storage of Manure

Researchers have been investigating how best to store manure to reduce the problems that come with year-round manure spreading. Storage allows the farmer to apply manure when it’s best for the crop and during appropriate weather conditions. This reduces nutrient loss from the manure caused by water runoff from the field. However, significant losses of nutrients from stored manure also may occur. One study found that, during the year, dairy manure stored in uncovered piles lost 3 percent of the solids, 10 percent of the nitrogen, 3 percent of the phosphorus, and 20 percent of the potassium. Covered piles or well-contained liquid systems, which tend to form a crust on the surface, do a better job of conserving the nutrients and solids than unprotected piles. Poultry manure, with its high amount of ammonium, may lose 50 percent of its nitrogen during storage as ammonia gas volatilizes, unless precautions are taken to conserve nitrogen.

CHEMICAL CHARACTERISTICS

OF MANURES

A high percentage of the nutrients in feeds passes right through animals and ends up in their manure. Over 70 percent of the nitrogen, 60 percent of the phosphorus, and 80 percent of the potassium in feeds may be available in manures for use on cropland. In addition to the nitrogen, phosphorus, and potassium contributions given in table 9.1, manures also contain significant amounts of other nutrients, such as calcium, magnesium, and sulfur. In regions where the micronutrient zinc tends to be deficient, there is rarely any deficiency on soils receiving regular manure applications.

Differences in feeds, mineral supplements, bedding materials, and storage systems make manure analysis quite variable. Yet, as long as feeding, bedding, and storage practices remain unchanged on a given farm, manure characteristics will be similar from year to year. The major difference among all the manures is that poultry manure is significantly higher in nitrogen and phosphorus than the other manure types. This is partially due to the difference in feeds given poultry versus other farm animals.

The relatively high percentage of dry matter in poultry manure is also partially responsible for the higher analyses of certain nutrients, when expressed on a wet ton basis.

It is possible to take the guesswork out of estimating manure characteristics; most soil-testing laboratories will now analyze manure. Manure analysis should become a routine part of the soil fertility management program on animal- based farms.

( Building Soil For Better Crops - Fred Magdoff and Harold van Es)